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FEBRUARY 2025 ISSN 1030-2662 02 9 771030 266001 $ 00* NZ $1390 The VERY BEST DIY Projects! 13 INC GST INC GST These plus more in this month’s magazine: Programmable Frequency DIVIDER COUNTER 300Hz – 77MHz input frequency range | 85,000 division ratios | USB programmable High-Bandwidth Differential Probe Battery-powered, ideal for use with oscilloscopes Measure signals up to ±400V from Earth 100:1 or 10:1 ranges 30MHz/25MHz bandwidth IR Remote Control Keyfob with NFC programmaing Compact keyfob case which can attach to a keyring Sends up to three IR commands in NEC, Sony or Philips RC5/RC6 formats. Commands can be changed via NFC. www.jaycar.com.au Contents Vol.38, No.02 February 2025 10 Open-Source Software There is a vast array of quality open-source software available, much of which is also free. We detail the different kinds of software available for various jobs such as video, image & audio editing, productivity, CAD etc. By Dr David Maddison, VK3DSM Computer software 24 Mini UPS Module This mini uninterruptible power supply (UPS) module provides continuous 9-12V DC <at> 10W. It’s suitable for smaller devices like a WiFi router, and it only requires a single lithium-ion cell connected to its 50 x 20mm PCB. By Jim Rowe Using electronic modules 40 Antenna Analysis, Part 1 Learn how antennas work and how to design matching circuits for them. In this first part, we cover the fundamentals, reactance, Smith charts and other related topics. By Roderick Wall, VK3YC Radio antennas 72 Precision Electronics, Part 4 In this fourth installment in the series, we look at how to extend the current measurement range of our current-sense circuit. We can do that by switching between two or more shunt resistors. By Andrew Levido Electronic design 32 High-Bandwidth Differential Probe This high-bandwidth and high-voltage Differential Probe is great to use with a ‘scope. With a max common-mode and differential-mode voltage of ±400V DC and rechargeable battery, it’s perfect for your test bench. By Andrew Levido Test equipment project 58 The PicoMite 2 The newest MMBasic interpreter for the Raspberry Pi Pico 2 and “W” variant is a comprehensive programming environment. It’s not only faster with more memory, it also has built-in support for HDMI video, USB keyboards, mice etc. By Geoff Graham & Peter Mather Raspberry Pi Pico project 64 IR Remote Control Keyfob This infrared (IR) remote control is one of the smallest we’ve made yet. It has three buttons that can be programmed wirelessly via NFC (near-field communications) and it works with many of our other projects. By Tim Blythman Remote control project 80 Programmable Frequency Divider This Divider/Counter reduces the frequency of an incoming signal by a factor of 3 to 21,327,000 and can be configured via USB. The input frequency can be 300Hz to 77MHz and is powered from 5-12V DC <at> 20mA. By Nicholas Vinen Test equipment project Project Page 32 High-Bandwidth Differential Probe Antenna Analysis and Optimisation Part 1: Page 40 PicoMite 2 Page 58 for the Raspberry Pi Pico 2 2 Editorial Viewpoint 5 Mailbag 31 Subscriptions 46 Circuit Notebook 48 Mini Projects 71 Silicon Chip Kits 88 Serviceman’s Log 94 Online Shop 95 Vintage Radio 100 Silicon Chip Kits 101 Ask Silicon Chip 103 Market Centre 104 Advertising Index 104 Notes & Errata 1. IR repeater 1. Power control for vehicle accessories 2. Power supply transformer tap switching 1. Wireless flashing LEDs 2. Transistor tester TRF-One AM radio by Dr Hugo Holden SILICON SILIC CHIP www.siliconchip.com.au Publisher/Editor Nicholas Vinen Technical Editor John Clarke – B.E.(Elec.) Technical Staff Bao Smith – B.Sc. Tim Blythman – B.E., B.Sc. Advertising Enquiries (02) 9939 3295 adverts<at>siliconchip.com.au Regular Contributors Allan Linton-Smith Dave Thompson David Maddison – B.App.Sc. (Hons 1), PhD, Grad.Dip.Entr.Innov. Geoff Graham Associate Professor Graham Parslow Dr Hugo Holden – B.H.B, MB.ChB., FRANZCO Ian Batty – M.Ed. Phil Prosser – B.Sc., B.E.(Elec.) Cartoonist Louis Decrevel loueee.com Founding Editor (retired) Leo Simpson – B.Bus., FAICD Silicon Chip is published 12 times a year by Silicon Chip Publications Pty Ltd. ACN 626 922 870. ABN 20 880 526 923. All material is copyright ©. No part of this publication may be reproduced without the written consent of the publisher. Subscription rates (Australia only) 6 issues (6 months): $70 12 issues (1 year): $130 24 issues (2 years): $245 Online subscription (Worldwide) 6 issues (6 months): $52.50 12 issues (1 year): $100 24 issues (2 years): $190 For overseas rates, see our website or email silicon<at>siliconchip.com.au * recommended & maximum price only Editorial office: Unit 1 (up ramp), 234 Harbord Rd, Brookvale, NSW 2100. Postal address: PO Box 194, Matraville, NSW 2036. Phone: (02) 9939 3295. ISSN: 1030-2662 Printing and Distribution: 9 Kendall Street, Granville NSW 2142 2 Silicon Chip Editorial Viewpoint Staying on Windows 10 Microsoft wants everyone who uses Windows 10 to switch to Windows 11, but I don’t want to for several reasons. Even after Microsoft’s official support ends in October 2025, there are ways to keep Windows 10 secure so that you can still use it if you want to. Don’t be bullied into “upgrading” if you don’t want to. First, let’s quickly consider why you might not want to switch to Windows 11. The first is if it doesn’t support your computer hardware. Frankly, I think many of Microsoft’s hardware requirements for Windows 11 are ridiculous. The only logical explanation I can come up with for them is that they also sell hardware and they want you to throw away a perfectly usable computer and spend more money to get a new one. I don’t know about you, but I find that kind of forced obsolescence quite offensive; I like to continue using hardware as long as it still works well enough. I could switch on TPM in the BIOS and install Windows 11 but I don’t want to for the following reasons. I consider having to sign into your own computer using a Microsoft account to be an invasion of privacy. I want to be able to use my computer ‘offline’, as a self-contained device, not as some part of Microsoft’s network where they collect data on me. I’m willing to pay for software like Windows, but only if I can own it, and if I have to sign into an account to use it, I don’t consider that ownership. Another reason is that I don’t want some of the new “features” like Windows CoPilot or their other AI nonsense built into my operating system. We should be able to decide what software we want to run on our computers, not have it forced down our throats. There’s also the fact that Windows 10 does everything I need, so why would I want to switch to something new? Say you want to stay on Windows 10 for some or all of those reasons. What do you do? Microsoft are offering Extended Security Updates for three years but they are expensive, at $95 + $190 + $380 = $665 per computer over those three years. There must be better options. Another one is to switch to Windows 10 Enterprise LTSC 2021, which is supported until 2027. However, that isn’t very far away anymore. I think there is a better way. I am going to sign up for the 0patch service (https://0patch.com) for Windows 10, which promises to address any significant security vulnerabilities that are discovered for €24.95 (about $41) per machine per year. There is a free tier, but I think it’s worth paying for the Pro version (for us, at least). I think the price is reasonable, and they install the patches in the background, while you’re using your computer, so you don’t need to reboot for patches any more (yay!). They are promising to provide these patches for at least five more years. I suspect it will be longer, as many people like me will want to remain on Windows 10 for as long as possible. I suppose some people could be concerned about giving the 0patch software full access to their computer’s memory. However, many programs require that, like anti-virus software, the infamous CrowdStrike and even many games these days (for ‘anti-cheat’). I guess it comes down to who you trust. My fingers are crossed that 0patch are as trustworthy as, say, Microsoft. Why don’t I switch to Linux? I actually use it quite extensively, but I need a Windows computer to run important software that is not yet available on Linux (and emulating it gives a poor result). If you just need a computer for email, web browsing, writing documents and such, Linux is a great option. Cover background image: https://unsplash.com/photos/aerial-view-of-ocean-qztBRIrU1FM Australia's electronics magazine by Nicholas Vinen siliconchip.com.au 4 Silicon Chip Australia's electronics magazine siliconchip.com.au MAILBAG your feedback Letters and emails should contain complete name, address and daytime phone number. Letters to the Editor are submitted on the condition that Silicon Chip Publications Pty Ltd has the right to edit, reproduce in electronic form, and communicate these letters. This also applies to submissions to “Ask Silicon Chip”, “Circuit Notebook” and “Serviceman’s Log”. Hydraulic computers do exist In your November Editorial, you speculated on the possibility of hydraulic computers. They have existed for quite some time. Until fairly recently, all automatic automotive gearboxes employed a hydraulic computer. It was contained in the flat box-like enclosure slung on the bottom of the gearboxes. You can read about them on Wikipedia at https://w.wiki/ CM$e and https://w.wiki/3prj, while there is a YouTube video on the MONIAC at https://youtu.be/rAZavOcEnLg The document at siliconchip.au/link/ac3f gives a brief history of hydraulic computers. There are high-resolution photos of the components of a ‘valve body’ at siliconchip. au/link/ac3g George, via email. Comment: thanks for the relevant links. Hydraulic computers in automatic transmissions are certainly an interesting and common application of the principles. We would have mentioned our article on Fluidics from the August 2019 issue (siliconchip.au/Article/11762) in the editorial, but there wasn’t enough room. It covered valve bodies, MONIAC and more. Building Fuzz Face clones I enjoyed reading Brandon Speedie’s Vintage Electronics article about the classic Fuzz Face guitar effects pedal (December 2024; siliconchip.au/Article/17321). I have restored (but mostly just revived) a few Australian 1960s transistor radios. Several have featured in Vintage articles in Silicon Chip magazine. Often, when I tried to look up the specs of vintage germanium transistors, I was ‘highjacked’ by websites talking about their use in guitar pedals. Their discussion of germanium versus silicon transistor pedals mirrors the valve versus transistor amplifiers debate. Maybe this explains why germanium transistors are expensive on the second-hand market. ourPCB LOCAL SERVICE <at> OVERSEAS PRICES AUSTRALIA PCB Manufacturing Full Turnkey Assembly Wiring Harnesses Solder Paste Stencils small or large volume orders premium-grade wiring low cost PCB assembly laser-cut and electropolished Instant Online Buying of Prototype PCBs www.ourpcb.com.au siliconchip.com.au Australia's electronics magazine 0417 264 974 February 2025  5 “999p”, for example, nothing appears on the PC nor the LCD screen! The only way out of this is to reboot the meter. Lou Amadio, Figtree, NSW. Comment: we re-tested the final version of the software on an Arduino Uno and were able to update the calibration via the Serial Monitor. We suspect that your unit has not locked up but is simply waiting for input. We have our terminal’s line ending set to CR/LF (although the code should work with CR only) and got the following after typing “C<Enter>” (because the serial monitor only sends on <Enter>) after the menu is displayed: Typing “c<Enter>”, LC Meter prints: C selected. Enter a value: I decided to build a Fuzz Face clone for my guitar-­playing son and followed the transistor selection article by R.G. Keen entitled “Picking transistors for FF clones” at www. geofex.com I tested a range of transistors that I had accumulated over the last 60 years and a few from radios that were beyond repair; I would never cannibalise a good radio! The gain and leakage varied greatly between ‘identical’ transistors. Maybe it was a particular combination of these that Hendrix was unknowingly looking for. Keen suggests that many guitarists prefer a gain of 70-100 for the first transistor and 90-130 for the second. My germanium PNP “Fuzz Face” was very smooth, with gentle distortion that could be wound up if needed. As Brandon Speedie explained, the negative peaks are cut before the positive. This can be seen in the accompanying oscilloscope trace photo that I took a few years ago when I built the circuit. As you can see, it is not a ‘hard’ clipping. From memory, I used a pair of AS128 transistors, which were AWV Australian-made equivalents of the AC128. I also built a second, NPN germanium transistor based Fuzz Face, which sounded slightly different. I have no musical ability at all and can’t play a single note or chord on a guitar, but when I just strummed it, I commented that it sounded like “Southern USA” music. My son’s reply was “Yes, swamp rock”. I had never heard of swamp rock as a musical genre. It is the slightly muddy Creedence Clearwater Revival sound. Maybe germanium transistors have left their mark on musical history, just as electric guitars, tape recorders, valves, 45s, 78s and even mechanical horn speaker technology did earlier. Dave Dobeson, Berowra Heights, NSW. Wide-Range LC Meter not responding to calibration I have built the Arduino-based Wide Range LC Meter from the June 2018 issue (siliconchip.au/Article/11099). The meter works fine, and I am able to test capacitors and inductors. However, I have a problem with calibration, at least on the small capacitance range. When measuring 0.5% precision capacitors in the pF range, the meter reads more than 10% high. I tried to set a new value for C2, but the Arduino Serial Monitor does not allow me to edit the capacitance value. I can enter calibration mode with C and C again to select C2 for a new value, but that is as far as I get. When I type 6 Silicon Chip Typing “1n<Enter>”, LC Meter prints: 0.0000000010000 C2 changed to 1000.0000pF Note that neither the “c” nor the “1n” is not echoed on the Serial Monitor. We suspect the apparent lock-up is because the LC Meter is waiting for user input that it is not getting (ie, it doesn’t run tests during setup). That could be down to simply line-ending settings on the serial monitor. Your high readings for low-value pF should be rectified by running the “G” stray capacitance calculation or setting the stray capacitance manually. We suspect you haven’t gotten that far yet. Lou Amadio later replied that changing the line ending solved this problem. The Exteek C28 has too much delay for instrumental use Thanks for the review of the Exteek C28 Bluetooth audio transmitter/receiver (September 2024 issue; siliconchip.au/ Article/16569). Allan lists several possible applications, and it sounds like a brilliant device for some of them, but I’m curious about the actual latency. Any significant latency is critical for applications such as using TV wireless headphones, and to “turn musical instruments from wired to wireless”. Musicians can generally cope with 5-6ms of latency; singers need less, typically under 4ms, especially if using headphones. There is also a psycho-acoustic phenomenon where, if you listen from to a loudspeaker say six metres away, the delay is somewhere around 20ms and nobody even notices. But if you put that 20ms delay into a pair of headphones, and your brain doesn’t have the visual cue of the distance, it becomes noticeable. Allan hasn’t specifically mentioned the latency, but he says, “I had to delay analyser measurements by 500ms to ensure accuracy”. For many applications, the ultimate S/N ratio and THD figure won’t affect typical uses, but the latency can be critical. I wanted to figure this out, so I bought some myself (they are cheap) and tested them. I set up a signal generator and fed it into one channel of my oscilloscope directly, with the other channel over the wireless link. The latency measured around 450ms. So, they are very useful for some applications, but Allan’s article mentioned musical instruments or remote TV viewing etc several times. These items are completely unworkable for those applications. Australia's electronics magazine siliconchip.com.au FREE Download Now! Mac, Windows and Linux Edit and color correct using the same software used by Hollywood, for free! Creative Color Correction DaVinci Resolve is Hollywood’s most popular software! Now it’s easy to create feature film quality videos by using professional color correction, editing, audio and visual effects. Because DaVinci Resolve is free, you’re not locked into a cloud license so you won’t lose your work if you stop paying a monthly fee. There’s no monthly fee, no embedded ads and no user tracking. 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Professional Editing DaVinci Resolve 19 ............................................................... Free DaVinci Resolve Micro Color Panel .............Only $809 DaVinci Resolve is perfect for editing sales or training videos! The familiar track layout makes it easy to learn, while being powerful enough for professional DaVinci Resolve’s color page is Hollywood’s most advanced color corrector and has been used on more feature films and television shows than any other system! It has exciting new features to make it easier to get amazing results, even while learning the more advanced color correction tools. There’s editors. You also get a library full of hundreds of titles, transitions and effects that you can add and animate! Plus, DaVinci Resolve is used on high end work, so you are learning advanced skills used in TV and film. www.blackmagicdesign.com/au siliconchip.com.au Australia's electronics magazine Learn the basics for free then get more creative control with our accessories! Learn More! February 2025  7 NO SUBSCRIPTIONS • NO ADS • NO USER TRACKING • NO AI TRAINING Anyway, I think you make an amazing magazine, but the suggestion of using these devices for musical instruments is simply unworkable. Claire Baker, Adelaide, SA. Comment: you are right; we got Allan to test this, and he also found the delay to be around half a second. As you say, that makes them totally useless for live instruments. He was unfortunately misled by the manufacturer’s delay specification, which was stated as 7µs. That is clearly a fantasy and shows why we must take manufacturer-­supplied specifications with a bag of salt. We did some testing ourselves by connecting one of these devices to an Android smartphone and playing videos, and there was no noticeable delay. We believe this is because the phone can determine the delay and compensate by also delaying the video by the same amount. However, if you were to use two of these devices to form a wireless audio link from a TV to headphones, delay compensation would not be possible and the result would be unwatchable. Raspberry Pi 5 vs Rock 4C+ What an interesting article on the Rock Model 4C+ (April 2024). It is strange you did not compare it with the Raspberry Pi 5 (1GiB RAM) available at the time, the 8GiB RAM (Pi 5) came out much the same time your article was needed for the publishers to get the April edition. I have been trying to replace my old eight-year-old Windows 10 laptop that has multiple hardware problems (faulty screen, keyboard, battery, WiFi stopped, lack of memory, hard-drive getting choked). So I agree when you say in summary (page 61), “We don’t think it’s ready to replace a desktop computer completely, but…” (what a pity, that is what I was after). My thinking is, as hardware fails or needs upgrading, it will be cheaper to replace the items instead of the cost of laptop repair or laptop replacement. I turned the laptop off for the last time a few weeks ago. Since you have used LibreOffice, I would describe the spell checker as: if you need to use it, then your spelling is not so good. A better choice would be to use the internet search engines to get correct spelling. The program “Mousepad” in (Raspberry Pi 5 Debian), similar to Microsoft “notepad”, has absolutely brilliant spelling suggestions. You have to turn it from the Edit → Preferences → Plugins menu. I have used the Raspberry Pi 1 & 3 and the first Raspberry Pi 5 only had 1GiB of RAM. Even if it was not used for a few hours, it would lock up for no obvious reason. The 8GiB RAM version is better and would lock up only if I aggravated (or overworked) it. Eric Richards, Auckland, New Zealand. Comment: we ordered a Raspberry Pi 5 4GiB version for testing (we don’t think there was a 1GiB version). It was hard to get a Pi 5 initially and ours arrived after the April 2024 issue had gone to press, so it was too late to add to the Rock 4C+ review. We published our Pi 5 review in the July 2024 issue (siliconchip.au/Article/16323). We have found the LibreOffice spell checker to be reasonably good but you can also use an online grammar checker tool like https://languagetool.org or https://prowritingaid. com for more thorough checking. Both have paid and free options. SC 8 Silicon Chip Australia's electronics magazine siliconchip.com.au siliconchip.com.au Australia's electronics magazine February 2025  9 GAMES IMAGES OPERATING SYSTEMS LLVM AI Tensor Flow DATA DOSBox GNU Thunderbird VLC 7-Zip FFmpeg VIDEO SPICE DEV ARCHIVES GIMp 3D OpenPGP FreeCAD Notepad++ Open-Source Software Open Street Maps Blender PRODUCTIVITY AV1 SIMS FreeDV EMAIL OPUS By Dr David Maddison, VK3DSM Firefox Linux Audacity CHROMIUM WINE AUDIO LibreOffice WEB PCB You may have used free/open-source software in the past, but you might not be aware of the variety and quality of free software available. You could also be wondering: why would anyone go to the huge effort of creating software, only to give it away for free? I n our article on Repairable Electronics, we described open-source hardware, that is, hardware where the plans and parts are all freely available (July 2024 issue; siliconchip.au/Article/16320). We also briefly discussed open-source software in that article, because of the way it relates to the hardware. This article will provide a lot more detail on that subject. Part of the inspiration for this article is the wide variety of excellent free and open-source software that’s available. Many people think that they need to pay for software to get something that’s useful and works well. While it’s true that some free/opensource software can be ‘unfinished’, much of it these days is actually very good with decent stability, many features and possibly a very polished user interface. While modern commercial software can be very capable, compared to earlier software, it can be quite ‘bloated’ (taking up a lot of CPU, memory and disk space), buggy, insecure, concerns about privacy (eg, spying on users), or can force users to create online accounts. Because of this, many people today are looking to alternatives. The high cost of much commercial software is now also a concern, especially as some of it is no longer available for purchase. You may have to subscribe to it, at a cost that can increase rapidly and unpredictably. In some cases, this can mean paying 10 Silicon Chip more in one year than you used to pay for a piece of software outright (ie, that you could use more-or-less indefinitely). Due to the poor testing of much commercial software, many end users don’t like being unpaid testers. Software bugs cost a lot of time and money, as does endlessly upgrading hardware to cope with the demands of often inefficient and bloated software. One big advantage of using opensource software is that the source code can be audited by third parties to ensure that it doesn’t do anything nefarious and it isn’t full of security flaws. Another reason to use open-source software or operating systems is that they may support older versions of hardware than commercial versions of software. For example, many people find their perfectly good and relatively new printers or scanners become obsolete with new operating system upgrades. The latest version of Linux will run happily on 10-year-old hardware. The same cannot be said for the latest version of Windows, which often won’t even install on a computer that’s just a few years old! Another great reason to use opensource software is that it often has cross-platform support, meaning it will typically run on Windows, Linux or macOS. That makes it more universal and also means that you can decide Australia's electronics magazine to change operating systems (eg, from Windows to Linux or macOS) and continue to use the same software. The interfaces are usually even similar across platforms. With open-source software, if it doesn’t support a platform you use, since you have access to the source you may even be able to ‘port’ it to a different operating system. Having said that, it usually isn’t a trivial process. For all of the above reasons, a social movement has developed for people to voluntarily get involved in the production and distribution of free software. It isn’t necessarily inferior in terms of features to commercial software, either. FOSS One alternative to traditional commercial software is so-called “free and open-source software” or FOSS. FOSS is software that is distributed “under a license that grants the right to use, modify, and distribute the software, modified or not, to everyone free of charge”. The mere availability of source code does not necessarily mean software is FOSS unless the other conditions are met. FOSS is a broad-ranging term for software that is mostly distributed under the terms of licenses from either the Free Software Foundation (www. fsf.org) or the Open Source Initiative (https://opensource.org/osd). siliconchip.com.au These organisations have slightly different philosophies. The Free Software Foundation The Free Software Foundation defines four essential freedoms of free (FOSS) software, originally developed by Richard Stallman: 1 The freedom to run the program as you wish, for any purpose. 2 The freedom to study how the program works, and change it so it does your computing as you wish. Access to the source code is a precondition for this. 3 The freedom to redistribute copies so you can help others. 4 The freedom to distribute copies of your modified versions to others. By doing this, you can give the whole community a chance to benefit from your changes. Access to the source code is also a precondition for this. You can read a collection of Stallman’s essays on open-source principles at www.gnu.org/doc/fsfs3-hardcover.pdf Open Source Initiative software The Open Source Initiative defines open-source software according to the ideas of Bruce Perens as requiring the following: 1 Free distribution. 2 The source code must be freely available and not obfuscated in any way. 3 Derived works must be allowed. 4 Integrity of the author’s source code must be maintained, with limitations on modifying it or indicating when it is. 5 No discrimination against people or groups. 6 No restrictions on where or how the software is used. 7 The same license applies to all people to whom the software is distributed. 8 The software license applies to all products derived from a particular software distribution. 9 The license may not restrict what software is distributed along with a particular operating system ‘distribution’. 10 The license must be technology neutral. Examples of software licenses from the Open Source Initiative are Apache License 2.0, BSD 3-Clause and BSD 2-Clause Licenses, all versions of the GPL (GNU General Public License), siliconchip.com.au all versions of the LGPL (GNU Lesser General Public License) and Mozilla Public License 2.0 (used for Firefox and Thunderbird, among others). Licensing Both FOSS software and Open Source Initiative software are issued under license agreements, although this doesn’t generally involve any physical paperwork. It is automatic when you download the software, perhaps after agreeing to its terms and conditions. Common open-source licenses used by various organisations include the Apache License, BSD License, GNU General Public License, GNU Lesser Public License, MIT license and the Mozilla Public License. There are two broad categories of license for free and open-source software: permissive and copyleft (see below). Permissive licenses generally come from academia and have minimal restrictions. Copyleft licenses come from the free software movement and typically require distribution of the software and derivative works with attribution and source code. Both types usually have a warranty disclaimer (then again, so does most commercial software). Copyleft Copyleft is a concept of granting certain rights for use of copyrighted works such as sharing, modifying, copying or redistributing them. Author attribution is required and is usually incorporated in the source code files along with full license conditions. Copyleft allows people to freely use the copyrighted product, but does not allow them to own it or earn royalties from it. Naming confusion & ideological differences Both the FOSS and Open Source Initiative have the words “open source” in their names, which leads to some confusion between the two approaches, although this is of little practical consequence. There are important differences between the views of the Free Software Foundation and the Open Source Initiative. A basic difference is that FOSS software is always free, but Open Source is not necessarily so (but usually is). There may be copyright issues or distribution restrictions of various kinds. See www.gnu.org/philosophy/ open-source-misses-the-point.html for more on this. In terms of the practical differences to users of these two forms of software, there are few differences to be concerned with. They are mostly ideological and lie with the proponents of the two movements. There may be costs While the software we talk about is generally free, there may be a cost if it is distributed by a commercial organisation who that offers technical or other support. For example, WordPress is free software, but there are companies that charge for hosting and/or technical support for it. Similarly, there are versions of Linux such as Red Hat Enterprise Linux (RHEL) that cost money. They Fig.1: examples of FOSS software running on Fedora Linux with the KDE Plasma desktop environment: Firefox, Dolphin file manager, VLC media player, LibreOffice Writer, GIMP and KCalc. Source: https://w.wiki/BsLi Australia's electronics magazine February 2025  11 may be used on supercomputers or in major commercial or government enterprises. There can also be charges for some other large enterprise software installations where support by commercial organisations is offered. Public domain software Public domain software was popular from the 1950s to the 1990s. It still exists, but has been mostly replaced by FOSS and Open Source licenses. Software that has been placed into the public domain has no ownership, licensure, or any other restriction placed on it whatsoever. It became mostly obsolete due to changes in copyright laws in the United States and elsewhere with the implementation of the Berne Convention, which meant that all original works are by default copyright protected and required an explicit waiver to enter into the public domain. Freeware Freeware is software that is distributed without charge, but unlike FOSS or Open Source software, the source code is not typically available. Freeware can be full-featured, or it might be from a commercial source, as a type of “sampler” to encourage purchase of a more capable version of the software (eg, see the DaVinci Resolve entry below). It may come with restrictions on the way it’s used. Shareware Shareware is proprietary software that either has a trial version available, or has limited functionality. It might be supported by advertisements or a purchase of a more capable or less restricted version. It might display some mark in the output, such as a watermark or logo. Source code is usually not available. In the rest of the article, we will not necessarily distinguish between FOSS and Open Source software or other types of free software, although we will try to mention which category each entry falls under. ● The Brazilian government, which moved from Windows to Linux. ● Austria, which uses OpenOffice products and Linux. ● The German armed forces, which use Matrix for internal communications. Examples of free and/or open-source software Naturally, it would be impossible to list or review all available software. The following will hopefully give you an idea of the fantastic variety of free and open-source software that’s available. We’ll break down the different types of software into five categories: ● General software, that will be of interest to most readers ● Engineering & mathematical software, that we expect will also be useful to many of our readers ● Operating systems ● Development/back-end software, which will be most interesting to those who are more into computers and software development General software We’ll start things off by covering open-source software available for most common day-to-day tasks such as document editing, web browsing, email etc. Productivity software LibreOffice (www.libreoffice.org & Fig.2) is a free and open-source set of productivity programs including a word processor (Writer), a spreadsheet program (Calc), a presentation program (Impress), a drawing program (Draw), a database access program (Base), an equation editor (Math) and a charting module. It is a ‘fork’ of Apache Open Office, but LibreOffice is more actively maintained and has a few more features. We use LibreOffice extensively as it provides all the features we need with an easy-to-use interface at no cost. LibreOffice Calc is the spreadsheet program that comes as part of LibreOffice. It supports 1,048,576 rows and 16,384 columns. It can read and write Microsoft Excel files, except those parts (if any) that contain Microsoft proprietary Visual Basic for Applications (VBA), which may have to be rewritten in Apache OpenOffice Basic. Notepad++ (https://notepad-plusplus.org) is a free & open-source text editor program that’s intended to be similar to but much more powerful than Microsoft’s Notepad app that comes with Windows. OnlyOffice (www.onlyoffice.com) is a collaborative online office suite that includes document, spreadsheet and presentation editors, plus a PDF creator, editor and form filler. Scribus (https://sourceforge.net/ projects/scribus) is free, open-source desktop publishing software. If you want to publish a book or magazine, it might be a good place to start. Sumatra PDF (www.sumatrapdf reader.org) is a lightweight, opensource PDF reader. Being lightweight, it is much faster to load and use than programs like Adobe Acrobat. Adoption by governments Various governments worldwide have adopted free and open-source software. Examples include: ● Massachusetts, USA, which has adopted the OpenDocument standard. ● The US White House, which uses Linux and Drupal on its web servers. 12 Silicon Chip Fig.2: the LibreOffice Calc spreadsheet program. Source: www.libreoffice.org/ discover/screenshots Australia's electronics magazine siliconchip.com.au Web Browsers Brave (https://brave.com & Fig.3) is a free and open-source browser released under the Mozilla Public License. It is privacy focused with a strong level of privacy protection, and blocks most ads and website trackers with its default settings. Optional ads can be turned on, which earn users “Basic Attention Tokens” that can be used as a cryptocurrency currency token (based on Ethereum) or to make donations to various websites and creators. Chromium (www.chromium.org) is the open-source web browser that Google Chrome, Microsoft Edge, Samsung Internet and Opera are based on. Firefox (www.mozilla.org/firefox) is a privacy-focused free and open software browser that runs on Windows, macOS, Linux, Android and iOS. It automatically blocks most ad trackers. It also works with Google products such as Gmail and docs and offers a “Facebook container” extension (https://addons.mozilla.org/addon/ facebook-container) to stop Facebook tracking you around the web. Screenshots can also be made from within the browser. Firefox is the successor of one of the original web browsers, Netscape Navigator, introduced in 1994. There are also many privacy-focused forks of Firefox such as Librewolf and GNU IceCat. Tor browser (www.torproject.org) has a slogan that goes, “You have a right to SEARCH without being followed”. It is strongly privacy focused and is designed for safe and anonymous web browsing. It operates over the Tor overlay network, itself built with free and opensource software, designed for anonymous communication via ‘onion routing’ through a network of volunteer-­ operated relays which create random paths for your internet data. This is all encrypted, making tracing and tracking of personal data very difficult for hostile parties like malicious hackers. Its main disadvantage is said to be its slow browsing speed due to the nature of the volunteer-operated onion routing it uses. Communications & email Matrix (https://matrix.org) is a communications protocol to provide secure, decentralised instant messaging, Voice over IP (VoIP) signalling and Internet of Things (IoT) siliconchip.com.au Fig.3 (upper): the Brave web browser on several devices. Source: https://brave. com/static-assets/images/optimized/features/images/Browser-2-1.png Fig.4 (lower): a sample screen of the Mozilla Thunderbird email client. Source: www.thunderbird.net communications, including bridging together existing communications. It is used by the French Government and the German Armed Forces, among others. Mozilla Thunderbird (Fig.4 & www. thunderbird.net) is a free and opensource email client and personal information manager. It also has newsgroup integration, a news feed, a calendar (“Lightning”) and an instant messaging client. It will run on Windows, macOS, FreeBSD and Linux. It supports all common email standards such as POP, IMAP, LDAP, S/ Australia's electronics magazine MIME and OpenPGP. The mail file format it uses is MBOX with MSF (Mail Summary File) but emails can be exported in EML format and others such as text, CSV, PDF and HTML. Drawing, painting, animation & image manipulation Blender (www.blender.org & Fig.5) is a well-regarded 3D graphics program that runs on Linux, macOS, Windows and other operating systems. It is suitable for making animated films, 3D art, creating 3D-printed models, motion graphics, visual effects and other uses. February 2025  13 It has become an industry standard program of sorts. Darktable (www.darktable.org) is an open-source digital photography workflow application for that runs on Windows, Linux or macOS. It can also integrate with GIMP (see below). Inkscape (https://inkscape.org) is a vector graphics editor for Linux, macOS and Windows, similar to Adobe Illustrator or CorelDRAW. GIMP (GNU Image Manipulation Program, www.gimp.org & Fig.6) is a free and open-source image manipulation program for Linux, macOS and Windows. It is considered by many to be a substitute for Adobe Photoshop (it can perform many similar functions). It can be used for image manipulation, image editing, free-form drawing, conversion of different image file formats and other tasks. It can also be enhanced using third-party plugins and the use of scripting. A new major version, GIMP 3.0, is planned to be launched soon and includes many improvements, such as non-­ destructive editing, that solve complaints by people who are used to using similar features in Photoshop. There is a good video on the new features at https://youtu.be/1HoZjHn8gVU Krita (https://krita.org/en/ & Fig.7) is a free and open-source graphics manipulation program for raster graphic art and 2D animation that runs on Windows, macOS, Linux, Android, ChromeOS and Haiku. Some people prefer Krita over GIMP as an alternative to Photoshop. Audio, codecs, transcoders & media players Ardour (https://ardour.org) is a (mostly) free and open-source digital audio workstation (DAW) as used by recording engineers and music producers. It’s similar to commercial music production programs you might have heard of, like Ableton Live or Cubase. Audacity (www.audacityteam.org & Fig.8) is a free and open-source audio editor and recorder said to be the world’s most popular program of its type. It works on Windows, macOS and Linux and supports all major audio formats. There are many thirdparty plugins available for it. Codec is short for coder/decoder. It is a piece of software that is involved in digitising, compressing, decompressing, storing or decoding audio or video data and is also used for streaming. Many codecs are proprietary and/ or patented, but many free and opensource codecs have been developed, such as the following: ● AV1 (https://aomedia.org/av1/) is a video codec developed as a royalty-­ free and open-source alternative to HEVC (H.265). ● Codec 2 is a speech codec for low-bandwidth applications at 700-3200bits/s. ● MP3 was developed by Fraunhofer Fig.5: Blender is a 3D modelling suite, but also doubles as a video editor. Source: https://docs.blender. org/manual/en/latest/ getting_started/ about/index.html Fig.6 (below): a sample screen of the GIMP image editing software. (1) main toolbox, (2) tool options, (3) image editing window, (4) brushes, patterns, fonts and history, (5) layers, channels and paths. Fig.7: an example below of artwork made using Krita. Source: https:// krita.org/en/ 14 Silicon Chip Australia's electronics magazine siliconchip.com.au IIS (siliconchip.au/link/ac2n) and originally required licensing fees. Since the patents expired worldwide by 2017 it is now free and open-source. ● Ogg Vorbis (https://xiph.org/ vorbis) is a free, open-source alternative to MP3. They also published FLAC, a popular lossless audio codec, plus two video codecs, Theora and Daala. ● OpenH264 (www.openh264.org) is an open-source implementation of the standard H.264 video compression system by Cisco Systems. ● Opus (https://opus-codec.org) for audio compression, including speech. ● uvg266 is an open-source H.266 video encoder (https://github.com/ ultravideo/uvg266). ● x264 and x265 are a free & opensource video encoder for H.264 & H.265 respectively. ● VPX (www.webmproject.org/ tools) is a free & open-source implementation of the WebM video codec. FFmpeg is another important opensource project that combines numerous open-source codecs and related software to create a cross-platform video & audio recording/converting/ streaming/playback library. It is used by many open-source media players. Handbrake (https://handbrake.fr) is a popular open-source video encoding and transcoding tool. It runs on Windows, macOS or Linux and can convert from just about any video format to any other. The media player mpv (https:// mpv.io & Fig.9) has an opaque control scheme, but provides a lot of control for users who want to tinker. VLC (www.videolan.org/vlc) is a multimedia player that can play an enormous variety of media file formats, discs, webcams, devices and video and audio streams and comes with the necessary codecs for most applications. It runs on platforms such as Android, Linux, iOS, macOS, Unix and Windows. Video editing, streaming & capture DaVinci Resolve is not open-source but it does have a free version (www. blackmagicdesign.com/products/ davinciresolve). You have probably seen it advertised in this magazine; it was developed by Australian company Blackmagic Design (based in Melbourne). We have used it and think it is excellent. There is a paid version that would be great for professional siliconchip.com.au Fig.8 (above): a screenshot of Audacity showing spectrograms of an audio clip. Source: https://w.wiki/BsLk Fig.9 (right): a sample screenshot of the mpv media player playing Casablanca, which is in the public domain. Outside of a few basic controls on the bottom bar, everything else requires hotkeys to use. Fig.10: a sample screen from OBS Studio, an open-source video streaming platform. Australia's electronics magazine February 2025  15 Fig.11 (left): a sample screenshot of the video editor Shotcut. Source: www.shotcut.org Fig.12: a sample screen of the 7-Zip compression/archiving program. use; the free version is suitable for a range of tasks from beginners to advanced users. OBS Studio (https://obsproject.com & Fig.10) is free and open-source software for video recording and livestreaming. It runs on Linux, macOS and Windows. It can capture images and video from sources such as the computer screen, windows, images, text, browser windows, webcams, capture cards and others. It is widely used by streamers on platforms like YouTube & Twitch. SimpleScreenRecorder is a Linux screen recorder program to record the operation of programs and games (siliconchip.au/link/ac2o). ShareX (https://getsharex.com) is a free and open-source program for screen capture and sharing of the output to other users. It has been likened to a superior replacement for the Windows Snipping Tool. Shotcut (www.shotcut.org & Fig.11) is a free and open-source cross-­ platform video editor. It runs on Linux, macOS and Windows. It offers numerous features and supports a wide variety of formats. Among many uses, it could, for example, be used to make YouTube videos. Even though the final version of VirtualDub (www.virtualdub.org) was released in 2013, it is still a popular video processing and stream capture program with hundreds of third-party plugins written for it. Forks (additional developments branches) have been produced for VirtualDub, such as VirtualDub2 (www.virtualdub2.com). Compression and archiving 7-Zip (www.7-zip.org & Fig.12) is a free and open-source file compression 16 Silicon Chip and archiving tool that achieves greater compression than standard ZIP archives (although it also supports the ZIP format). It was developed by Igor Pavlov and first released in 1999. There is a Windows graphical (GUI) version, plus a command-line version for Linux and macOS. It supports the following formats: 7z (its own format), GZIP, XZ, BZIP2, WIM, ZIP and TAR. It can also unpack (but not pack) files in APFS, AR, ARJ, CAB, CHM, CPIO, CramFS, DMG, EXT, FAT, GPT, HFS, IHEX, ISO, LZH, LZMA, MBR, MSI, NSIS, NTFS, QCOW2, RAR, RPM, SquashFS, UDF, UEFI, VDI, VHD, VHDX, VMDK, XAR and Z formats. Files can also be encrypted. The 7z format uses LZMA and LZMA2 compression, and files have a self-extracting capability. Cross-platform software Wine (www.winehq.org) is a compatibility layer for POSIX-compliant operating systems like Linux, macOS and BSD to enable Windows applications to run on them. One of its advantages is that it will run early Windows programs as far back as Windows 3.1, which will probably not run on current versions of Windows, so it is a way to continue to use legacy programs that may not have a current equivalent. WINE, along with associated tools like Proton, allow many Windows games to run on Linux. This has made it quite a popular gaming platform; for example, the Steam Deck portable gaming system runs Linux and has access to thousands of games, many of which were only designed for Windows. Australia's electronics magazine File transfer software FileZilla (https://filezilla-project. org) is a free and open-source file transfer application for Windows, Linux and macOS. It supports the FTP and FTPS (FTP over SSL/TLS) protocols and can connect to SFTP servers. There is also FileZilla Server for creating FTP/FTPS servers. FreeFileSync (https://freefilesync. org) is a freeware program for folder comparison and synchronisation. It’s useful for creating backup copies of files or synchronising sets of working files between different locations. It is open source and available for Linux, macOS and Windows. Donors get access to a version of the program with some additional features. We use it and find it quite good. LocalSend (https://localsend.org) is an open-source, cross-platform file sharing system, including support for transferring files between mobile devices and computers using Bluetooth. NextCloud (https://nextcloud.com) is a content collaboration program that provides functions like Google Drive and similar when used with office suites like integrated Collabora Online or OnlyOffice. ProjectSend (www.projectsend.org) is a private web-based file sharing program that runs from a server. Encryption software Cryptomator (https://cryptomator. org) is used for encrypting cloud drives from the client’s side. That way, if the data on the cloud server is compromised, it is still safe as only the client holds the encryption key. It is available for Android, Linux, iOS, macOS and Windows. siliconchip.com.au Fig.13: part of the Open Street Map map of Melbourne. Note how even buildings are shown. GnuPG (GNU Privacy Guard, https:// gnupg.org) is an encryption suite that uses the OpenPGP standard (see below). KeePass (https://keepass.info) is a free and open-source password manager purely for Windows. There are also popular cross-platform forks of it such as KeePassXC. OpenPGP (www.openpgp.org) is said to be the most widely used email encryption standard. It is defined by the OpenPGP Working Group of the Internet Engineering Task Force (IETF). It is available for Android, iOS, Linux, macOS and Windows. VeraCrypt (www.veracrypt.fr/en/ Home.html) is for on-the-fly encryption, to create a virtual encrypted disk that works like a regular disk although it is actually a file. In addition, it can encrypt actual disks. Mapping and navigation software Open Street Maps (OSM, www. openstreetmap.org & Fig.13) is a geographic database from the OpenStreetMap Foundation published under an Open Database License. It can be used as a mapping app on mobile phones and is particularly useful in the absence of phone coverage, as the map database is held within the device. It can also be used online. Data is provided by a community of users, and anyone can become a contributor. Fonts and typefaces There are many fonts and typefaces that are open-source and can be used freely without charge or restrictions (see Fig.14). These are available from various sources, such as: https://github.com/showcases/fonts https://open-foundry.com https://fonts.google.com Virtual machine software DOSBox (www.dosbox.com) is a DOS emulator for running old software on modern systems. Proxmox VE (www.proxmox.com), QEMU (www.qemu.org), VirtualBox (https://www.virtualbox.org) by Oracle and Xen (https://xenproject.org) are all popular, free and open-source virtualisation systems that let you run multiple operating systems on a single computer simultaneously. Virus and anti-malware ClamAV (www.clamav.net) is an open-source antivirus engine for scanning emails for trojans, viruses and malware. Other open-source anti-­virus packages exist but nothing full-­featured, likely due to the effort required to constantly monitor for new viruses and malware, develop antidotes for them and to update antivirus files. Video games There are some open-source computer games, such as SuperTuxKart, Mindustry, OpenTTD, UFO: Alien Invasion and OpenXcom. Two we have tried are: OpenTTD (www.openttd.org & Fig.15) is an open-source game based on the commercial game Transport Tycoon Deluxe. Like OpenXcom (below), optional improvements, graphics, music and add-ons have been contributed to enhance the game. While the original Transport Tycoon Fig.14: an example of a typeface called Chunk, reminiscent of old American West woodcut typography. Source: https://github.com/ theleagueof/chunk Fig.15 (right): a screengrab from version 1.9 of OpenTTD. Source: www.openttd.org/screenshots/1.9coldice_3 siliconchip.com.au Australia's electronics magazine February 2025  17 Deluxe only ran under Windows or DOS, OpenTTD can be played on Windows, macOS, Linux and Android. OpenXcom (https://openxcom.org) is an open-source clone of the 1994 DOS game X-COM: UFO Defence (also known as UFO: Enemy Unknown). It is widely regarded as one of the best turn-based strategy computer games of all time. OpenXcom require a copy of one of the original games (XCOM or Terror From the Deep) to run but is a modern Windows program with many bugfixes and improvements over the original. It has the same ‘look and feel’ as the original but is more fun due to many ‘quality of life’ improvements that have been implemented in the spirit of the original. The Battle for Wesnoth (www. wesnoth.org) is another open-source strategy game. application for Linux, macOS and Windows. GNU Octave (https://octave.org/ index.html & Fig.16) is an alternative to MATLAB and mostly compatible with it. It runs on Linux, macOS, BSD and Windows. Gnuplot (http://gnuplot.info & Fig.17) is a free and open-source program to produce 2D and 3D plots of functions, data and data fits. It runs on Linux, macOS, Windows and other systems. It was first released in 1986 and is still under active development. Despite being free and open-source, its source code is copyrighted and distribution of a modified version is not permitted. Such restrictions are permitted under Open Source Initiative licenses. Despite the name, it is unrelated to the GNU Project. Apart from working as a stand-alone plotting program, it is used as a plotting engine by a number of other packages and websites. ParaView (www.paraview.org) is a versatile multi-platform scientific visualisation program developed by Sandia National Laboratories, Kitware Inc and Los Alamos National Laboratory. R (www.r-project.org) is a language and environment for statistical computing and graphics made by GNU. It is similar to the S language and environment that was developed at Bell Laboratories. Miscellaneous There is a Linux project called OpenPrinting (https://openprinting. github.io) to support IPP (Internet Printing Protocol) for printing to local network or internet-connected printers. It also supports legacy printers with appropriate drivers. Engineering & mathematical software Engineering & maths software are heavily dominated by paid and closed-source software such as Altium Designer, AutoCAD and MATLAB. However, there are a surprising number of good alternatives. Computer-aided design software FreeCAD (www.freecad.org & Fig.18) is a free and open-source CAD program mainly for mechanical engineering design, although it can be used in other areas, such as architecture Graphing, visualisation & analysis Gephi (https://gephi.org) is a network analysis and visualisation or electrical engineering. It runs on Windows, macOS and Linux. See the video titled “Learning FreeCAD with These Basic Steps” at https://youtu.be/ rglvJH9z5ng KiCad (www.kicad.org & Fig.19) is a free and open-source electronic design automation (EDA) suite for Windows, macOS and Linux. It can create circuit diagrams and comes with a large library of symbols. It can then perform checks to ensure they follow basic electrical rules such as check for output pin conflicts, missing drivers and unconnected pins and create a “netlist”, which defines the connectivity of the circuit. Once a circuit has been drawn, you can then use it to lay out a PCB, using a built-in library of component footprints (it also has matching 3D models). It can import, export and migrate to and from other CAD (computer-­ aided design) tools. Its PCB editor includes an interactive layout router, side-by-side visualisation of the circuit and layout, design rules checks, trace length tuning for high-frequency designs and a footprint editor. It also has a 3D viewer to examine the proposed PCB design with components in place. LibrePCB (https://librepcb.org) is another open-source ECAD program similar to KiCad. Some people say it has a better library manager than KiCad and is easier to use in other ways. CircuitMaker (www.altium.com/ circuitmaker) and EasyEDA (https:// easyeda.com) are two examples of free PCB design software that we have mentioned in the magazine in the past, but they are not open-source. We reviewed CircuitMaker in the 1 0.8 0.6 0.4 0.2 0 -0.2 -0.4 10 5 10 5 0 0 -5 -10 -5 -10 Fig.16: a sample screen from GNU Octave, an opensource alternative to Matlab. 18 Silicon Chip Fig.17: a sample plot from gnuplot. Source: https://gnuplot. sourceforge.net/demo_5.4/pm3d_lighting.html Australia's electronics magazine siliconchip.com.au Fig.18 (left): a sample screen from FreeCAD v1.0. Source: https://wiki. freecad.org/screenshots Fig.19 (below): a PCB design and 3D rendering underway in KiCad. Source: https://docs.kicad.org/master/ en/pcbnew/pcbnew.html January 2019 issue (siliconchip.au/ Article/11378). 3D modelling software OpenSCAD (https://openscad.org) is a solid 3D CAD modelling program that will run on Windows, macOS and Linux. It is not an interactive modelling program, but rather, the user describes an object using a scripting language (see Fig.20) and renders the 3D model from that. There is a video showing creation of a simple object titled “3D Modeling with Code! The best demo (OpenSCAD)” at https:// youtu.be/KrFttd5D1cw RepRap or replicating rapid prototyper (https://reprap.org) is a project to develop low-cost 3D printers that can print their own components; the Skeinforge ‘slicing’ program was developed as part of this. While Skeinforge now appears to be obsolete, several open-source slicing programs exist. Two we have used are Slic3r (https://slic3r.org) and PrusaSlicer (https://github.com/prusa3d/ PrusaSlicer). Open-source 3D printer operating firmware is another important facet of the RepRap project. The Marlin firmware (https://marlinfw.org) is designed to run on an Arduino Mega board and is compiled using the open-source Arduino IDE. siliconchip.com.au SPICE (Simulation Program with Integrated Circuit Emphasis) is opensource software for circuit simulation, developed at the University of California, Berkeley. It is very powerful but a little difficult to set up and use. Linear Technology’s GUI version, LTspice (siliconchip.au/link/ac2p), is not open source but it is free and it is popular because it is so easy to use, and comes with lots of builtin component models. Since LTspice only runs on Windows, Ngspice (https:// ngspice.sourceforge.io) is a free and open-source alternative that also runs on Linux, macOS and other operating systems. Amateur radio FreeDV (https://freedv.org & Fig.21) is a free and open-source digital voice app for SSB amateur radio. It can run on Windows, Linux and macOS. It is helping the transition from analog to digital voice modes of HF amateur SSB, the previous major Fig.20: a sample screen of OpenSCAD showing its scripting language; insert is a 3D printer modelled using OpenSCAD. Source: https://i.materialise.com/en/3ddesign-tools/openscad & https://github.com/martinbudden/BabyCube Australia's electronics magazine February 2025  19 Fig.21: a screenshot of the FreeDV digital voice app for SSB amateur radio. Source: https://freedv.org transition being from AM to SSB in the 1950s and 1960s. Unlike many other digital modes, the voice codec used by FreeDV is not proprietary and is also open source. It uses neural net speech coding (LPCNet) and provides 8kHz of audio bandwidth while using only 1.6kHz of RF bandwidth. It is thought to be the first use of such neural net speech encoding for real-world applications. MMANA-GAL (http://gal-ana.de/ basicmm/en) is an antenna design program that is free for non-commercial use but is copyrighted by the author, although the source code is available. WSTJ-X (https://wsjt.sourceforge. io/wsjtx.html & Fig.22) implements Fig.22: a sample screen from the WSTJ-X digital radio software. Source: https://wsjt.sourceforge.io/wsjtx.html several popular amateur radio digital modes such as FST4, FST4W, FT4, FT8, JT4, JT9, JT65, Q65, MSK144 and WSPR. It also has one called ECHO for detecting your own radio signals reflected from the moon when you try to ‘moon bounce’. It runs on Windows, macOS and Linux. Data Acquisition LDAQ (Lightweight Data Acquisition, https://github.com/ladisk/LDAQ & Fig.23), is a Python-based toolkit for data acquisition that is said to be powerful and user-friendly. It is intended for use by researchers, engineers or hobbyists. It works in all Python environments. Fig.23: a sample screenshot of the LDAQ data acquisition software. Source: https://github.com/ladisk/LDAQ/blob/master/docs/source/images/FRF_ visualization.gif 20 Silicon Chip Australia's electronics magazine OpenDAX (https://opendax.org) is an open-source framework to build parts of data acquisition systems such as distributed control systems (DCS), programmable logic controllers (PLCs) and supervisory control and data acquisition (SCADA) systems. The authors describe the software as not yet ready for mission-critical applications. Operating Systems Android is based on Linux (see below) and its basic implementation is open-source. We discussed some open-source versions of Android in our article on privacy phones (June 2024; siliconchip.au/Article/16280), GrapheneOS (https://grapheneos.org). FreeDOS (https://freedos.org) is an open source DOS-compatible operating system for IBM-PC compatible computers. It is intended for running legacy software and embedded systems. Microsoft has released the MS-DOS v1.25, v2.0 and v4.0 source code under an MIT license for others to view and experiment with. See https://github. com/microsoft/MS-DOS Open Network Linux and SONiC are open-source network operating systems (www.opennetlinux.org & https://sonicfoundation.dev). A commercial example (not open source) of a network operating system is Microsoft Windows Server. GNU (www.gnu.org) is a collection of hundreds of items of free software siliconchip.com.au that can be used as a Unix-like operating system or as parts of an operating system. It includes applications, libraries, developer tools and games. GNU is the original free software concept project by Richard Stallman, started in 1983, with software development starting in 1984 and the free software philosophy published as the GNU Manifesto in 1985 (www.gnu. org/gnu/manifesto.html). The release of the GNU suite was the first time an operating system could be run using free software. The completed GNU components (except for the kernel, the core part of an operating system) led to the independently created Linux operating system, developed by Linus Torvalds from 1991, which is now the main use for these GNU components. Linux is released under a GNU license, and the Linux kernel is what is most used with the GNU software components. According to GNU, Linux should be called GNU/Linux because it wouldn’t work without both sets of components (see www.gnu.org/gnu/linux-and-gnu. html). Linux is actually the name of the kernel of the Unix-like GNU operating system (created by Linus Torvalds), not the entire operating system itself. While it is certainly true that the Linux kernel would not exist without GNU’s tools, and that it relies on many of their libraries to be useful, we’re referring to it as Linux for brevity (it’s a somewhat controversial topic). Linux is an enormously popular alternative to commercial operating systems like Windows. It is available in around 1000 distributions (‘distros’), each tailored to particular uses or tastes, with different applications included. Some Linux distributions may contain commercial software, as Linux has commercial and industrial applications as well, but most distros contain free and open-source software. If you want to migrate to Linux, as I might do in the near future, you can try ‘live’ distros that you can install on a USB stick or other removable media without altering the data on your computer. Distros that can be used live include Debian, SUSE, Ubuntu (Fig.24), Linux Mint, MEPIS and Fedora Linux. Some distributions specifically for live use include Knoppix, Puppy Linux, Devil-Linux, SuperGamer, SliTaz Linux and dyne:bolic. As for which distribution to use, that would siliconchip.com.au Fig.24: an example of the Ubuntu Linux desktop. Source: www.dreamhost.com/ blog/linux-distros Fig.25: most of the world’s top supercomputers run some version of Linux, including the Summit supercomputer at Oak Ridge National Laboratory in the United States, which runs RHEL. Source: https://w.wiki/BsLo need a whole article in itself, however Ubuntu (https://ubuntu.com) is considered a good choice for beginners; it does come with some proprietary device drivers, although it is still free. Another distribution cited as being suitable for beginners, which is based on Ubuntu, is Linux Mint. Ubuntu, in turn, is based on Debian. Apart from home users, many scientific, commercial and industrial users employ Linux, including on supercomputers, the International Space Station and SpaceX vehicles (Dragon, Falcon 9 and Starship). Australia's electronics magazine You can watch a video explaining why one Windows user switched to Linux and the basics of Linux Mint at https://youtu.be/fDDtBKOqTKI Traditionally, supercomputers (Fig.25) ran proprietary operating systems. Today, most run some variant of Linux, such as Red Hat Enterprise Linux (RHEL). RHEL is a commercialised version of Linux but it is based on the free and open-source Fedora Linux and CentOS Stream versions of Linux. AlmaLinux (https://almalinux.org) is a FOSS substitute for RHEL. Some February 2025  21 supercomputers use other versions of Linux, such as Ubuntu. Development and back-end software Compared to the old days of paid compilers and software demos distributed on physical media, there is a lot of choice for people who want free software. Compilers & development software Git (https://git-scm.com) is a free & open-source distributed version control system that can be used for software development or any other time a set of text files will undergo many revisions, possibly by a team of people. Subversion or svn (https://subversion. apache.org) is another similar free & open-source tool that we use (because we find it easier than git). The GNU Compiler Collection (GCC, https://gcc.gnu.org) is a collection of free compilers for Ada, C, C++, D, FORTRAN, Go, Objective-C, Objective-C++ and Rust for various operating systems and computer architectures. GCC compilers are used for most GNU projects and for the Linux kernel, along with many other opensource projects. LLVM (www.llvm.org) and its frontend Clang is a compiler for C languagues (C, C++, CUDA etc). It also is the default compiler for macOS. Processing (https://processing.org) is a combined graphics library and integrated development environment intended for graphical programming. We used it for our LED Christmas Tree project in the December 2018 issue, but you will probably be more familiar with it as the basis for the Arduino IDE. Python (www.python.org) is a dynamically typed, high-level programming language that many people like because it is easy to learn and use but much more powerful than languages like BASIC. Like many modern programming languages, it is also an open-source project. MicroPython is a variant of it that runs on microcontroller boards like the Raspberry Pi Pico. Visual Studio Code (https://code. visualstudio.com) or VS Code is an open-source integrated development environment (IDE) released by Microsoft. It is based on their earlier proprietary Visual Studio program but can run in Windows, Linux, macOS or even a web browser. 22 Silicon Chip Databases MySQL (www.mysql.com) is a free and open-source relational database management system. It is available under either a free and open-source licence or a proprietary licence. PostgreSQL (www.postgresql. org) is a free and open-source relational database management system, which claims to be the world’s most advanced. We think PostgreSQL is very well designed and well worth looking into if you need a relational database. One of its most impressive features is that it supports most concurrency features without any locking, meaning it is almost immune to deadlocks, something that can be a real problem in other database systems. Instead, it uses a versioning system. This allows you to do things like take a ‘snapshot’ to back up the entire database while it is in active use! Web Content Management & Servers Apache (https://httpd.apache.org) is the “number one HTTP [web] server on the internet”, although it was recently overtaken in popularity by NGINX. Together, the two packages power over 60% of all web servers. Apache is one of the earlier opensource projects and, as such, even created its own class of open-source licence that is now used by other projects (the Apache license mentioned earlier). There is also Apache Tomcat (https://tomcat.apache.org), which is an ‘evolution’ of the Java EE (enterprise applications) platform. Drupal (www.drupal.org) is web content management software that is used by the US White House and 14% of the top 10,000 websites worldwide (see siliconchip.au/link/ac2q). WordPress (https://wordpress.org & Fig.26) is a very popular web content management system and blog software that is free and open-source. It is supported by about 60,000 (or more) plugins from other developers. Artificial intelligence There are quite a few open-source AI models and tools, although some AI models claimed to be open source do not meet accepted standards of opensource software. Open source models: ● Stable Diffusion (https://stability. ai/stable-image & Fig.27), which is free for non-commercial or limited commercial use. It generates images from a text description. ● The source code for GPT-2 is publicly available but the trained model and data is not (https://github.com/ openai/gpt-2). ● GPT-NeoX and GPT-J are pretrained language models. ● Llama by Meta (Facebook, www. llama.com) is listed as open-source and includes pre-trained models. It can be used for commercial applications but has restrictions around licensees with “greater than 700 million monthly active users in the preceding calendar month”. Open source libraries/frameworks: ● TensorFlow (www.tensorflow.org & Fig.28), a software library for deep learning and artificial intelligence. ● PyTorch (https://pytorch.org), a library for machine learning and Fig.26: WordPress has a large amount of pre-made themes that can be used to quickly create a website. Source: https://wordpress.com/themes Australia's electronics magazine siliconchip.com.au Fig.27: some example images generated by Stable Diffusion 3.5. Source: https://stability.ai/news/introducing-stablediffusion-3-5 deep learning for applications such as vision and natural language processing. ● Scikit-learn (https://scikit-learn. org/stable/), a machine-learning library for predictive data analysis. ● Hugging Face Transformers (https://github.com/huggingface/ transformers), a collection of models for text-based tasks such as answering questions, summarisation, image classification, object detection, speech recognition and audio classification. Open-source datasets: ● ImageNet (https://image-net.org), an image library for object recognition research. ● Common Crawl ‘crawls’ the web and stores the data in its archives (https://commoncrawl.org), which are made freely available to researchers and developers. It has stored 250 billion web pages over the last 17 years, with 3-5 billion pages added every month. This data can be used to train artificial intelligence models. Cluster & grid computing A computer cluster is a potentially very large collection of computers that are managed to act as a single large computer. Computer clusters are used for calculation-­intensive tasks such as scientific computing (eg, weather prediction, protein folding or fluid dynamics) rather than tasks with high input/ output requirements like databases. Most supercomputers these days use computer clusters. Grid computing utilises the capacity of numerous individual computers to perform individual parts of various computational tasks. The capacity utilised might be otherwise unused; ‘spare’ CPU cycles are ‘donated’ to a distributed computing project such as BOINC or SETI<at>home. Apache Mesos (https://mesos. apache.org) is software to manage computer clusters. Twitter used to use Apache Mesos, but now uses Kubernetes. A Beowulf Cluster is a supercomputer made from many inexpensive computers, generally running Linux and other free and open-source software such as Open MPI, a message passing interface, and Open Source Cluster Application Resources (OSCAR) high-performance computing management software. BOINC (Berkeley Open Infrastructure for Network Computing, https:// github.com/BOINC/boinc) is an opensource project to facilitate distributed grid computing projects. It was originally developed to manage SETI<at> home, which analyses radio telescope data via millions of PCs worldwide. It has now been expanded to other distributed computing projects in the areas of astrophysics, biology, environment, linguistics, mathematics, medicine and others. Kubernetes (https://kubernetes.io) is a containerised application management system that was originally SC authored by Google. Fig.28: an image (left) after applying 10 iterations of DeepDream (right) that was trained on dogs. DeepDream is implemented using TensorFlow. Source: user MartinThoma – https://w.wiki/5fek siliconchip.com.au Australia's electronics magazine February 2025  23 Using Electronic Modules with Jim Rowe Mini Uninterruptible Power Supply (UPS) If there’s a blackout when using your computer, it might keep running (eg, off its internal battery or a UPS) but what about your WiFi router? It will likely drop out and not come back until power is restored. This low-cost UPS module can keep it going as well. M ost consumer-grade uninterruptible power supplies (UPSs) have similar configurations, with a storage battery that’s charged when mains power is available and switched to running an inverter to replace mains power when it fails. Many use a sealed lead-acid (SLA) battery to store the energy. In most cases, the switchover takes only 10-25ms, which usually doesn’t cause problems with loads like PCs or LCD monitors. When delivering power from the battery via the inverter, most UPSs can do so for at least 20 minutes, even when the load requires its full rated output power. That is generally enough to allow you to save your work and shut down the computer safely. The mini UPS module we’re looking at here is a bit different from that. It is intended to provide continuous 9V or 12V DC power to small electronic devices like WiFi routers while being powered from 5-12V DC. It can supply up to 12W of output power continuously, making it suitable for powering most WiFi routers and many other small devices. Instead of a sealed lead-acid (SLA) battery, it uses a small lithium-ion battery like a single 18650 cell, which is much smaller than just about any leadacid battery. All of the mini UPS module’s circuitry is on a PCB measuring 50 × 20mm. It doesn’t have an onboard battery holder; the Li-ion battery (which is not supplied) is intended to be connected alongside it. We obtained the module pictured from an AliExpress supplier called ACELEX, which had it available for only $2.01 plus shipping. Another supplier on AliExpress called MOKCUM seemed to have an identical module for $4.02 plus shipping – twice the price, but still surprisingly low. From the supplier’s photos, the MOKCUM module is set to produce a Fig.1: the block diagram for the mini uninterruptible power supply (UPS) module. It is a straightforward design with only two main sections. 24 Silicon Chip Australia's electronics magazine 9V DC output, whereas the ACELEX module produced an output of 12V DC as received. However, as we’ll explain shortly, the modules can be easily changed to produce either output voltage. How it works After examining the module’s PCB, I was able to glean enough information to produce the basic block diagram shown in Fig.1. There are two main circuit sections; on the left is the lithium-ion charging circuit, while on the right, there is a DC/DC step-up (‘boost’) converter. The offboard Li-ion cell connects to the lines between the two sections. The charging circuit accepts the incoming 5-12V DC input power and produces a regulated 4.2V DC output to charge the Li-ion cell while also driving the step-up converter to provide either 12V or 9V to the load on the right. Link JP1 lets you switch the step-up converter’s output between 12V and 9V. When a solder bridge links its pads, the module delivers 12V to the load; when they are not linked, it delivers 9V instead. Link JP2 changes the maximum charging current for the Li-Ion battery. If the pads are not joined by a solder bridge, the maximum charging current is limited to 500mA (0.5A); if they are linked, the maximum charging current is 1A. Most 18650 cells can happily charge at 1A (well under 1C for their typical capacity), but if you are unsure, you can leave it at the safer 500mA setting. For small LiPo cells like those used siliconchip.com.au Fig.2: the wiring diagram for the mini UPS module. Multiple cells can be wired in parallel if required. in mobile phones, it’s best to leave the JP2 pads open. If you want to use a large cell or several cells in parallel, you will probably want to go for the higher charging current. The LEDs shown at upper left in Fig.1 are not supplied with the module, but are regarded as an ‘optional extra’. The sketchy data provided with the modules suggests that you should fit a common-anode dual red/blue LED (even though the legends on the PCB show R−, + and G−), but of course, you can use a red/green LED or even two separate 3mm LEDs. The blue (or green) LED indicates whether a load is connected to the output of the module, while the red LED indicates the charging state of the Li-ion battery. If the red LED is flashing, no battery is connected; if it is on continuously, the battery is being charged; if it is off, it is fully charged. Fig.1 shows no circuitry to perform the switchover to battery power when the mains-derived input power fails. That’s because there is no switchover as such. The Li-ion battery is already connected to the input of the step-up converter, so it will provide current and power when needed. No switch­ over time at all! battery and a low-voltage load like a WiFi router is quite straightforward, as shown in Fig.2. The incoming DC supply connects to the IN+ and IN− pads on the left, the output load to the OUT+ and OUT- pads on the right, and the Li-ion battery to the B+ and B− pads at bottom middle and bottom right. If you want to add a couple of LEDs (or a dual LED), these can be added at centre left, as shown. Just make sure you use high-efficiency LEDs because the driving currents are low. Link JP1 is just to the left of the output pads, as indicated by the red circle. It’s shown linked by a solder bridge, so the boost converter provides a 12V DC output. If you want 9V instead, simply remove the solder bridge with a soldering iron and some solder-­wicking braid. However, note that diode D1 connects the input to the output, so if you set the unit up for a 9V output, you can’t use a 12V supply. Link JP2 at lower left is indicated by the second red circle. As shown in Fig.2, it usually comes without a solder bridge, limiting the battery charging current to 500mA. It’s best to leave it this way unless you know your battery can handle charging at 1A. By the way, the B−, OUT− & IN− terminals are not all connected together, so make sure your supply, load and battery have independent grounds or else the circuit will not work. Trying it out To check out the module, I powered it from a standard 5V DC, 1A plugpack and connected its output to a programmable DC load. I then fired up my bench DMMs and connected one to measure the module’s output and the other to measure the Li-ion battery voltage. Silicon Chip kcaBBack Issues $10.00 + post $11.50 + post $12.50 + post $13.00 + post January 1997 to October 2021 November 2021 to September 2023 October 2023 to September 2024 October 2024 onwards Hooking up the module to a low-­ voltage power source, a lithium-ion All back issues after February 2015 are in stock, while most from January 1997 to December 2014 are available. For a full list of all available issues, visit: siliconchip.com.au/Shop/2 PDF versions are available for all issues at siliconchip.com.au/Shop/12 siliconchip.com.au Australia's electronics magazine Setting it up February 2025  25 The mini UPS module is compact, measuring 50 × 20mm; the photos above are enlarged for clarity. The module is typically supplied as shown with JP1 bridged, JP2 unbridged and no LED(s). After making sure the sole Li-ion 18650 cell was fully charged, I switched off the input voltage and tested its performance at both output voltages, with load currents of 100mA, 200mA and 300mA. These tests took a few hours, and the results are summarised in Fig.3. The red/mauve and cyan/blue lines show the module’s output voltage at either voltage setting and for the tested load current levels for up to three hours from the removal of input power. For the lightest loads, 100mA in both cases, the output voltage at either setting remained essentially constant for more than two hours after input power removal. That corresponds to a load power of 1.2W at the 12V voltage setting and 0.9W at the 9V setting. There was no significant voltage droop over this time. In fact, the voltage on both settings remained within ±2mV for the duration of the tests. However, it did not last quite as long with a load drawing more current. On the 9V setting, with the load drawing 200mA, the cell voltage fell to 3.2V and I terminated the test after around 2.5 hours. I repeated the test at 300mA, which naturally gave a shorter runtime, and also with the output set to 12V, which also reduced the runtime. With the UPS module fed with 12V from a big bench supply (rated at 5A), and two charged 18650 cells in parallel, the module delivered 600mA to the load at 12V for about 10 minutes before the battery voltage dropped to 3.095V. With a third 18650 cell in parallel and the load current increased to 800mA, even with fully charged cells, the unit could only supply 12V to the load for about 5 minutes before the cell voltage dropped to 2.97V and I turned it off. The small inductor in the output boost converter became very hot in that short time. So the Mini UPS module is really only really suitable for loads up to 600mA, even with three 18650 cells in parallel. It may be rated to supply 1A, but it wouldn’t be able to do so for a useful time. That’s probably enough to power the average WiFi router; many are supplied with a 1A plugpack, although I doubt they draw anywhere near that upper limit unless they are going ‘flat out’. This UPS should be able to power your WiFi router in a blackout for long enough to make it worthwhile with sufficient battery capacity, although that is the kind of thing you should test if you are going to rely on it. Conclusion This module is nicely made, low in cost, has no switchover time and performs reasonably well, with the ability to power low power (<12W) DC loads like WiFi routers for about 10-60 minutes, depending on how much current SC they draw. Fig.3: test runs to see how long it would take the module to discharge at 100mA, 200mA & 300mA loads. The unit can deliver up to about 600mA (a little short of the 1A advertised) with reduced runtime unless larger/more cells are used. 26 Silicon Chip Australia's electronics magazine siliconchip.com.au February altronics.com.au GADGET POWER UPS Great deals on tools, power, AV and more - only until Feb 28th. 279 $ C 5162 189 $ Why pay $300 or more? C 5161 4K video or 20MP still shot ! SAVE $50 NEW! 209 $ 99 $ T 5096 S 9446E DC Control Box & Power Meter A complete pre-wired DC connection solution for your auxiliary battery system. Great for caravans and 4WDs. 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B 0002 Subscribe to JANUARY 2025 ISSN 1030-2662 01 9 771030 266001 $13 00* NZ $13 90 INC GST INC GST CompaCt HiFi part 2 hea dpHone ampliFie �� r 1W into 16Ω Australia’s top electronics magazine 3.5mm & 6.5mm headphone jack �� Class-AB operating mode �� 9-12V AC plugpack Current Probe 5MHz 40A Data Centres, Servers and Cloud Computing Monarch AA5 Radio Model Train Silicon Chip is one of the best DIY electronics magazines in the world. Each month is filled with a variety of projects that you can build yourself, along with features on a wide range of topics from in-depth electronics articles to general tech overviews. 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To start your subscription go to siliconchip.com.au/Shop/Subscribe High-Bandwidth Differential Probe This high-bandwidth, high-voltage differential probe is ideal for use with oscilloscopes, although it could have other uses. It has an internal rechargeable battery and fits in the same case as the Isolated Current Probe we published last month. It will be an invaluable addition to your test equipment arsenal! By Andrew Levido I f you ever work with high-voltage circuits, a differential probe is an indispensable piece of test equipment. In fact, they’re also useful with many low-voltage circuits; any time you want to monitor a differential voltage between two points in a circuit. This one can be built for a fraction the cost of a commercial device with similar performance and functions. The ground sides of most oscilloscope inputs are connected directly to mains Earth. This means you can only measure Earth-referenced signals – either those already referenced to Earth, or those that you can safely connect to Earth on one side for the purposes of the measurement. That generally includes truly floating circuits, such as battery-powered devices. Unfortunately, many signals in circuits such as switch-mode power supplies or motor controllers are referenced to voltages well above Earth potential. Connecting a scope to these using a standard probe would create a short from the circuit reference to mains Earth, via the probe ground lead and the ‘scope itself. This will potentially be catastrophic for your scope, the probe and your circuit. Even if your circuit is floating and you can safely Earth one point for testing, if you want to measure another voltage at the same time that’s referenced to a different point, you’re out of luck. That’s because if you Earth two different points in your circuit, you are adding a short circuit; usually not a great idea! A differential probe (or multiple probes) totally solves that problem. As an interesting and slightly terrifying aside, my very first oscilloscope, an Australian made BWD830 purchased in the early 1980s, actually has a “ground isolate” switch on the back panel that allows the user to open the mains Earth connection, allowing the scope common to float. Fig.1: a high-voltage differential probe is essential if you want to see signals that cannot be Earth referenced on your oscilloscope. In this example, three probes help to measure the phase-to-phase voltages of a variable speed drive. The scope display is a real capture made with the prototypes. 32 Silicon Chip Australia's electronics magazine siliconchip.com.au ● Maximum common-mode voltage: ±400V DC (280V RMS) ● Maximum differential-mode voltage: ±400V DC (280V RMS) ● Common-mode input impedance: 2MΩ || 2.5pF ● Differential-mode input impedance: 4MΩ || 2.5pF ● Attenuation ranges: 100:1, 10:1 ● Basic DC accuracy: better than 1% ● Bandwidth: >30MHz (x100), >25MHz (x10) ● CMRR: >100dB (DC-100Hz) ● Battery Life: >4 hours ● Charging time: <3 hours ● Charging socket: USB-C ● Input sockets: 4mm banana sockets, 20mm spacing ● Output socket: BNC This avoids the risk of blowing up the scope, but can allow the scope case and front panel terminals to rise to lethal voltage levels! Thankfully, this dangerous practice is a thing of the past. (And it still doesn’t help for monitoring multiple points referenced to different voltages anyway...) Fig.1 shows an example of where a differential probe is indispensable. Here, the three phase-to-phase PWM output waveforms from a variable speed drive (suspiciously similar to the one we published in the November & December 2024 issues) are displayed on three channels of an oscilloscope. None of the U, V or W phases can be safely Earthed, and the voltages involved are in the order of 400V peakto-peak. The differential probes provide 100:1 attenuation of the differential voltage and over 10,000:1 (100dB) attenuation of the common-mode voltage, allowing the phase-to-phase voltages to be measured safely. The waveforms shown on the scope are from a real screen capture made using three of these devices. A high-voltage differential probe translates the difference in voltage between its two high-impedance inputs into a voltage that you can safely connect to your oscilloscope’s Earthed input. The output is proportional to the difference in voltage between the positive and negative inputs. Any common-mode signal present on both inputs is almost entirely rejected. The differential probe is housed in a small plastic case measuring 82 × 65 × 28mm. The inputs are two shrouded 4mm banana jacks at one end, with a 20mm spacing. That is close enough to the 3/4-inch (19.05mm) standard for siliconchip.com.au dual-banana-plug accessories to fit. The BNC output, range switch and USB-C connector are at the other end of the case. The power and charge LEDs are visible through the top of the case via two light pipes. Design goals When I set out on this project, I set myself a few design goals. I wanted a probe that could safely be used in mains-voltage projects like that described above. This means the device should be able to measure differential-mode signals of ±400V magnitude and withstand a similar level of common-mode voltage. This corresponds to an AC voltage of 280Vrms. We want to show these on a standard scope, so an attenuation of 100:1 (-40dB) would be appropriate to give a ±4V full-scale output signal. Sometimes, we will want to measure a low-voltage signal riding on a high common-mode voltage; for example, to examine the gate signals of the IGBTs in Fig.1. These signals would normally be within a ±40V range, so a 10:1 (-20dB) attenuation range was also one of my requirements. The common-mode rejection ratio (CMRR) at DC to mains frequency should be at least 100dB. This means a 400V common mode voltage would contribute less than 4mV at the output. The input impedance should be in the megohm range with fairly low parallel capacitance (say <10pF). I wanted an upper bandwidth limit as high as I could reasonably get, at least 25MHz, to get good representation of high-speed switching signals with fast rise-times. Bandwidth and rise time are related according to the Australia's electronics magazine approximation trise ≈ 0.35/BW, so a 25MHz bandwidth means the fastest rise time we will see is about 14ns, which should be short enough. I also wanted the unit to have the smallest form factor possible and include an internal rechargeable battery. My bench gets cluttered enough as it is without having bulky probes and their power cables added to the mix. More than three hours’ battery life and USB-C recharging was mandatory. Operating principles In principle, the concept of a differential probe is pretty straightforward: a matched pair of input attenuators followed by a classic three op-amp differential instrumentation amplifier will do the job. Fig.2 shows the bare bones of the circuit, along with differential and common-mode voltage sources we will discuss later. You can think of this circuit has having three sections: a dual input attenuator, a buffer stage and a difference amplifier stage. The overall differential-­mode gain of the circuit is given by multiplying the gains of each of these stages, which are given in the figure. We have to set the gains of each stage such that we respect the input common-mode voltage range of each op amp (voltages A+/A- and C+/C- in the figure) and their maximum output swings (voltages B+/B- and Vout). With ±5V power rails, it seemed fairly safe February 2025  33 Fig.2: the differential probe consists of two matched attenuators followed by a classic three-op-amp instrumentation amplifier. The latter has a buffer stage with a gain programmable via a single resistor (RG) and a difference amp stage with a fixed gain. to assume an input common-mode voltage of ±2V and an output swing of ±4V as a starting point. A division ration of 200:1 would give 2V at point A with a 400V input, leaving the rest of the circuit to provide a gain of 2 or 20 to achieve the overall target of 100:1 or 10:1 attenuation. As you can see from Fig.2, the voltage at any one of the inputs will actually be a combination of some common-mode voltage, Vcm, plus one half of the differential-mode voltage, Vdm. The maximum voltage of 400V at the inputs will therefore be made up of a combination of common-mode and differential-mode voltages. We can construct a graph (the yellow area in Fig.3) showing the allowable ranges of input voltage that keep the op amp voltage within the ±2V band. This input range is more than enough to measure signals likely to be encountered in a circuit powered by 230-240V AC. The area shown in pink is the combination of inputs that can be measured on the 10:1 attenuation range. In this case, the range is limited by the ±4V output swing of the op amps, rather than the input common-mode voltage. It is important to keep in mind that all of these are limits relate to the faithful reproduction of the input signal. The maximum voltage that the inputs can safely withstand is considerably higher, as we shall see. With the attenuator gain determined to be 1/200th, we can consider the gains for the other two stages. The buffer stage gain can be set by selecting a single resistor RG, so this is the obvious candidate for switchable part of the gain. We can’t put all the remaining gain in this stage, or we run the risk of exceeding the difference amplifier’s 34 Silicon Chip common-mode input voltage range. Thus, I chose to make the buffer stage gain switchable between 1 and 10 and set the difference amp stage gain to a fixed value of two times. That’s about it for the high-level design – a pair of matched 200:1 attenuators, a ×1/×10 switchable buffer stage and a ×2 difference amplifier. Now we just have to make it all work – and the devil is in the details, as they say. The attenuator The circuit diagram (Fig.4) shows the complete design. The attenuators have to withstand high voltages, have reasonably high input impedance and be very closely matched to maximise CMRR. The attenuators are identical, so I will focus this description on the positive side for simplicity. The resistors I have chosen for the Fig.3: the range of common-mode voltage and differential-mode voltages the probe can faithfully reproduce at the output. The yellow area is for the ×100 range and the pink area is for the ×10 range. It can safely tolerate much higher voltages without damage. Australia's electronics magazine upper leg of this divider are 1MW ±0.1% ¼W devices with a voltage rating of 700V. The maximum continuous voltage we can apply across each of these resistors is limited to 500V by power dissipation. With two resistors in series, the inputs can withstand a sustained voltage of 1kV (DC or AC RMS), giving a comfortable safety margin. The resistance value required in the bottom leg of the divider for 200:1 attenuation is 10,050W. In each half, this is made up of a 10kW resistor, a 10W resistor and half of 100W trimpot VR1. This trimpot is shared with the negative attenuator, allowing us to tweak the divisor ratios so that they are precisely equal, as necessary for maximum rejection of common-mode signals. With VR1 centred, the total resistance of each resistor string is 10,060W, not 10,050W as calculated. The extra 10W is necessary to compensate for the 10MW resistors, which are effectively in parallel with the lower leg of each divider. You can ignore trimpot VR2 in this calculation, since its value is much smaller than the error due to the 1% tolerance in the value of the 10MW resistors. The overall resistance of the lower leg of each divider is therefore 10,060W || 10MW = 10,050W. The purpose of VR2 is to allow us to inject up to ±5mV into one input to compensate for any op amp offset errors. We will discuss this further below. Diode pairs D1 & D2 protect the op amp inputs from overvoltage by limiting the voltage swing at the divider output to ±5.6V or thereabouts. That covers the DC performance of the attenuator, but we want the divider to operate properly up to 25MHz or more. We know that there will inevitably be some capacitance at the output of the divider. The protection diodes, for example, will contribute about 1.5pF each; the op amp input capacitance will be about the same. There will also be 3pF or 4pF of stray capacitance inherent in the layout. At 25MHz, this ~10pF of total capacitance will have an impedance of around 630W, reducing the divider ratio to something in the order of 1/3500. The incidental capacitance is moreor-less unavoidable, so we potentially have a real problem. The solution is to deliberately add some capacitance across the upper leg siliconchip.com.au of each divider to reduce its impedance by the same ratio and maintain the attenuation. With a 200:1 divider, we would need an upper leg capacitance 199 times lower than the ~10pF in the lower leg. Clearly, this is impractical. Instead, we put a small known value of capacitance across the upper leg and add more capacitance across the lower leg to compensate for it. I selected series pairs of 4.7±0.1pF 1kV NP0 capacitors for the upper legs, to match the high-voltage tolerance of the input resistors. Together, they amount to 2.35pF of capacitance in the upper leg of each divider, requiring 467pF of capacitance in the lower leg to compensate. This latter capacitance is made up of the ~10pF of incidental and stray capacitance we have already mentioned, plus the parallel combination of 390pF and 27pF fixed capacitors, plus VC1 (12-60pF). This combination gives us a range of capacitance adjustable from nominally 440pF to 490pF. It is useful to have a range to account for capacitor tolerances and other uncertainties. Moreover, the overall bandwidth we can ultimately achieve will be quite sensitive to perfect frequency compensation. The buffer stage The op amp we use for the buffer stage is critical. It must have high input impedances so as not to load the attenuator, and low bias currents since the input impedance is ~10kW. Thus, a FET input op amp is required. It must also have a high large-­signal bandwidth, and a common-mode input range of ±2V with ±5V supplies. I chose the ADA4817, which is expensive at around $15 each, but it fits the bill nicely. It has an input impedance of 500GW in parallel with 1.3pF and the input bias current is ±20pA. The large signal bandwidth extends to 200MHz, with 0.1dB gain flatness to 60MHz. The worst-case offset voltage is ±4mV (which is good for a FET input op amp), and the input common mode voltage range is -4.2V to +2.2V with ±5V rails. If I only required a gain of one for this stage, I could have simply wired IC1 and IC2 as non-inverting buffers. But since we need the option of a gain of 10, I had to close the feedback loop around each op amp with resistors. It is a good idea to choose a fairly low value for this resistor as it will form an RC low-pass filter with the op-amp’s input capacitance, the effect of which will be to increase the gain of the buffer as the frequency rises, causing unwanted ‘peaking’ in the frequency response. When the ×10 range is selected via S1, the parallel combination of the 110W and 220W resistors is switched in between the two buffer amplifiers’ inverting inputs. The resistance values were chosen to give this stage a gain of 10 in this configuration. Consistent with the attenuator, I used 0.1% tolerance resistors for gain-setting. Fig.4: the complete probe circuit. Power is provided by an 800mA Li-ion cell via a dual-rail DC-to-DC converter (REG5). The battery is charged via a USB Type-C connector (CON4) and IC4. siliconchip.com.au Australia's electronics magazine February 2025  35 You can see the input attenuator components arranged vertically outside the banana sockets near the top. The 510W resistors in series with the non-inverting inputs of IC1 and IC2 are critical to the stability of the circuit. High-speed op amps like the ADA4817 love to oscillate. One of the (many) things that can bring this on is extraneous capacitance on the inputs, and we have plenty given the compensation network we just discussed. The 510W resistors are ‘stopper’ resistors that isolate the op amp inputs from this capacitance. The 500GW input impedance and 1.5pF input capacitance mean that these resistors don’t otherwise affect the operation of the probe. Just as for the input divider, we add 10pF & 47pF frequency compensation capacitors to this gain stage. I did not bother with a variable capacitor here because the low impedance of the surrounding circuit makes it less sensitive to an error of a few picofarads one way or the other. Difference amplifier The requirements for the difference amplifier (IC3) are not quite as stringent as for the buffers, but we do need a high large-signal bandwidth and good output characteristics. The LMH6611 fits the bill. It has a large signal bandwidth of 85MHz and a gain-bandwidth product of 115MHz. The output swing with ±5V rails is ±4.5V into a 150W load and the output drive current is ±120mA. 36 Silicon Chip The LMH6611’s input common-­ mode voltage range is -5.2V to +3.8V, giving plenty of headroom. As a bonus, it is considerably cheaper than the ADA4817s. IC3 is set up as a difference amplifier with a fixed gain of two using low-value 0.1% tolerance resistors. The 10W resistor helps overall stability by providing a little bit of isolation between the LMH6611’s output and any load capacitance. This stage does not need frequency compensation due to the low gain and low impedances involved. My design calculations indicate that the end-to-end gain error of this circuit should be comfortably under 1% over the temperature range of 0-40°C, and nearer to half this at 25°C. However, the untrimmed offset error could be in the order of ±5mV on the ×100 range and ±45mV on the ×10 range. The big difference is due to the buffer stage amplifying the ADA4817’s offset when on the ×10 range. This is why it is necessary to add the offset trim. If we added the offset to the difference amplifier (where we would in ideal world), we would need a different offset trimpot for each range and an extra gang on the range switch to select the right one. The compromise I selected was to add the offset before the gain stage, meaning we can trim out most of IC1’s and IC2’s offset but may not be able to fully eliminate that from IC3. Since this is ±4mV at the output (0.1% error) in the worst case, I decided I could live with it. Power supply The ±5V power supply is derived from a single Li-ion 14500 (AA-sized) 800mAh cell via a TPS65133 dual-rail switching power supply (REG5). This chip accepts a 2.9-5V input and can source up to 250mA on each rail. It is Fig.5: use this overlay diagram to place the components. We recommend mounting the LCC-packaged DC-DC converter (REG5) and supporting components first. Once you have confirmed they are working, you can move on to the rest of the parts. Australia's electronics magazine 92% efficient at 100mA and requires only a couple of inductors and three capacitors to operate. The chip has an undervoltage lockout to protect the Li-ion cell from over-discharge. The TPS65133 generates very little noise as far as switching converters go, but to be safe, I added an LC filter (10μH/220μF) between each output of the switcher and the analog circuitry. A green LED (LED2) across the power rails provides user indication that the power is on. The cell is charged from a USB-C power-only connector via a MAX1555 charger chip (IC4). This charges the cell at around 280mA, which is enough to charge an empty cell in under three hours. A yellow LED (LED1) lights when the MAX1555 is charging the Li-ion cell and extinguishes when it is fully charged. The charging voltage comes from USB-C connector CON4. Resettable PTC fuse PTC1 and transient voltage suppressor diode TVS3 protect against reverse-polarity circuits and overvoltage conditions. The two 5.1kW resistors pull the USB power delivery control channel lines down to passively signal to the source to supply 5V. The power is switched via a second set of contacts on range switch S1. In the Off/Charge position, the cell is connected to the charger and isolated from the rest of the circuit. In either the 100:1 or 10:1 position, the battery is connected to the switcher and isolated from the charger. PCB design High voltages can be present at the banana sockets’ exposed conductors and either end of the first resistor and capacitor of each attenuator. Those components are on the PCB outside the banana sockets. The track clearances PCB follow IPC2221-B standard B4 for boards with solder masks below 3050m altitude. So you must build this on a commercially-­ made PCB with a solder mask. During assembly, you should apply a conformal coating over the top half of the board once all components other than trimpots/trimcaps have been fitted. That will allow it to resist arcing even under extreme conditions (eg, very high humidity). These coatings are available in spray cans (see the parts list), are easy to apply and can be soldered through, although they should be reapplied later if you do that. Construction All the components mount on a small double-sided PCB coded 9015-D that measures 56.5 × 82.5mm. Most are through-hole or hand-solder-friendly surface-mount types. The only really tricky device is REG5, the TPS65133 switch-mode regulator. Unfortunately, all the useful power chips like it seem to only be available in tiny ‘leadless’ packages. During construction, refer to the PCB overlay diagram (Fig.5) to see which components mount where and with what orientations. Because of REG5’s package, we recommend assembling and testing the power supply first. REG5 has a thermal pad underneath the chip, so reflow (either hot air or IR) is the only realistic option to mount it. The best way I have found to do this is to use solder paste. Apply a small smear of it to all the pads. Don’t worry if a little gets between pads as it will ball up under surface tension when reflowed. Place the chip carefully, using the screen-printed lines as a guide. Make sure the orientation is correct. Heat the chip and the surrounding board with hot air until the solder melts, including that on the thermal pad. I use tweezers to hold the chip in place until I feel the surface tension of the solder ‘pull’ it into place. You can tell the thermal pad solder has melted if the chip re-aligns itself if you nudge it very slightly out of siliconchip.com.au Parts List – High-Bandwidth Differential Probe 1 double-sided PCB coded 9015-D, with solder mask, 56.5 × 82.5mm 1 Hammond 1593LBK 92 × 66mm case [element14 4437858] 1 adhesive panel label, 55 × 80mm 1 14500-size 800mAh Li-ion cell with PCB pins (BAT1) [Altronics S4981] 1 red PCB-mount banana socket (CON1) [Cal Test CT3151SP-2] 1 black PCB-mount banana socket (CON2) [Cal Test CT3151SP-0] 1 PCB-mount BNC socket (CON3) [Molex 73100-0105] 1 USB-C power only socket (CON4) [Molex 217175-0001] 2 4.7μH 1.1A M2520/1008 shielded ferrite inductors (L1, L2) [Würth 74404024047] 2 10μH 350mA M2012/0805 shielded ferrite inductors (L3, L4) [TDK MLZ2012M100WT000] 1 0.75A 24V M3226/1210 PTC polyfuse (PTC1) [Littelfuse 1210L075/24PR] 1 right-angle DP3T PCB-mount slide switch (S1) [E-Switch EG2310] 1 top-adjust 100W 3296-style multi-turn trimpot (VR1) [Altronics R2370A] 1 top-adjust 10kW 3296-style multi-turn trimpot (VR2) [Altronics R2382A] 2 0.6in (15.24mm) convex light pipes [Dialight 51513020600F] 2 No.4 × 6mm self-tapping screws 4 small self-adhesive rubber feet 1 can of conformal coating [Jaycar NA1610, Altronics T3175] Semiconductors 2 ADA4817-1ARDZ-R7 410MHz precision op amp, SOIC-8-EP (IC1, IC2) 1 LMH6611MK/NOPB 135MHz precision op amp, TSOT-23-6 (IC3) 1 MAX1555EZK-T Li-ion battery charger, TSOT-23-5 (IC4) 1 TPS65133DPDR dual DC-DC converter, WSON-12 (REG5) 1 SMBJ5.0C 5V transient voltage suppressor, DO-214AA (TVS3) 1 SMD M2012/0805 yellow LED (LED1) 1 SMD M2012/0805 green LED (LED2) 2 BAV99 dual series signal diodes, SOT-23 (D1, D2) Capacitors (all SMD M2012/0805 size 50V NP0/C0G ceramic unless noted) 2 220μF 10V solid tantalum, SMC case 5 10μF 16V X7R 8 100nF X7R 2 390pF 1 47pF 2 27pF 2 10pF 4 4.7pF 1kV 2 6mm diameter 12-60pF variable capacitors (VC1, VC2) [EW GKG60015] Resistors (all SMD M2012/0805 size ±1% ⅛W unless noted) 2 10MW 4 1MW ±0.1% M3216/1206 size ¼W 700V [Vishay TNPV12061M00BEEN] 2 10kW ±0.1% 10ppm [element14 1140912] 2 5.1kW 1 1.8kW 3 510W 2 360W ±0.1% 25ppm [Panasonic ERA-6AEB361V] 2 330W ±0.1% 25ppm [Panasonic ERA-6AEB331V] 1 220W ±0.1% 25ppm [Panasonic ERA-6AEB221V] 2 180W ±0.1% 25ppm [Panasonic ERA-6AEB181V] 1 110W ±0.1% 25ppm [Panasonic ERA-6AEB111V] 3 10W position. Once it cools down, you can remove any excess solder or obvious shorts with solder wick around the edges (adding a bit of flux paste [not solder paste] makes the wick work better). Then clean up the flux residue with isopropyl alcohol. Australia's electronics magazine Next, fit the four inductors, L1– L4, the four capacitors around REG5 and the two large tantalum capacitors in the upper-left corner of the PCB according to the overlay. You are then ready to test the power supply. Solder a couple of lengths of fine February 2025  37 Fig.6: drill the enclosure end panels and top according to this diagram. The slots can be formed by drilling a pair of holes inside the perimeter and using a craft knife and files to open them up to the required dimensions. hookup wire to the board to power it externally. The easiest place to connect the negative supply is the through-hole for the battery negative terminal (the single hole on the righthand side of the board). The best place to connect the positive supply is the bottom right-hand through-hole in the group of six where the switch will later be mounted. These locations are marked by small triangles on the PCB silk screen overlay. Connect an external power supply set to deliver 4V with a current limit of 100mA and switch it on. The current draw should be negligible, and you should be able to measure 5V across both of the large tantalum capacitors. If there is a problem, switch off, check your work and, if necessary, reflow REG5 again. If all is well, you can remove the wires and proceed with mounting all the other parts, leaving the battery till the very last. IC1 and IC2 also have thermal pads on the bottom, so these will have to be reflowed too. However, they are SOIC-8 packages so are much easier to solder than REG5. Remember to apply the conformal coating we mentioned earlier on both sides of the board above the battery location before soldering the trimpots and trimcaps. Reapply it on the underside after soldering those components so their joints are covered. Immediately after you mount the battery, screw the board into the case bottom. This will help prevent accidental shorts under the board. The 38 Silicon Chip energy density of the Li-ion cell is such that accidental shorts can easily burn out tracks or cause other damage. Case preparation Drill the enclosure end plates and top case according to Fig.6. The slots can be most easily made by drilling a couple of holes inside the outline and finishing with a craft knife and small files. You will need to remove the two plastic bosses on the inside of the top case where the banana jacks are located – you can just snip them out with a pair of side cutters. The label (Fig.7) is simply glued to the front panel with some adhesive. I printed mine on glossy photo paper and covered it in transparent adhesive film for protection. Consistent with oscilloscope probes and commercial probes of this kind, the label describes the 100:1 and 10:1 attenuation ranges as ×100 and ×10, respectively. This refers to the multiplication factor you need to apply to the ‘scope’s vertical scale. For example, a 1V/division on the scope represents 100V/division on the ×100 range and 10V/ division on the ×10 range. Once it has been applied, punch out the holes for the light pipes and push them in from the front. They can be secured with a drop or two of The assembled PCB before it was installed in the case. Australia's electronics magazine cyanoacrylate adhesive (superglue) on the back side. Assemble everything except the top case and you are ready for calibration. Testing and calibration Start the calibration process by fully charging the battery. Connect a USB-C power supply to the probe and make sure the switch is in the off/charge position. The yellow LED should light, indicating the battery is charging. When full charge is reached, the LED will go out. This may take two or three hours if the battery is nearly discharged. Once charged, remove the USB cable and power the unit on by selecting either the ×100 or ×10 range and recheck that the power supplies are at ±5V as before. The green LED should be lit. The first step in calibration is to zero out the offset correction. We need to do this to make sure it does not impact the setting of the CMRR trim in the next step. Switch the probe to the 100:1 range and adjust VR2 until the voltage at its wiper is as close to zero as you can get it. You can clip your voltmeter’s negative lead to the GND test point and read the wiper voltage on the bottom end of the vertically orientated capacitor immediately below VR1, marked by a small square on the PCB overlay. You should be able to adjust the voltage to within a few millivolts either side of zero. Anything under ±10mV is fine. Now we need to adjust the CMRR trim. Set your bench power supply to the highest (safe) voltage you can get. For example, connect two channels of a dual 30V supply in series for 60V. Connect the positive lead of the power supply to both of the probe inputs (shorted together) and the negative lead to the GND test point. Switch the probe to the 100:1 range. Use your meter to measure the voltage between the mid-points of the voltage dividers while you adjust VR1. The suggested probe points are marked by small circles on the PCB overlay, immediately to the left of D1 and the right of D2. Adjust VR1 for a reading as close to zero as you can get at these points. You should be able to get a reading below ±20µV. With a 60V input, a reading below ±20µV implies a CMRR of 130dB. But you can probably do better than that with a good meter and some patience. Now you can set the offset voltage trim. Remove the power supply but keep the two inputs shorted. Measure the output voltage at the BNC connector with respect to the ground test point. Trim VR2 to get the output close to zero on both ranges. This may require a little backwards and forwards between ranges and the acceptance of some compromise (for reasons described above). For example, the best I could do was -1.1mV on the 100:1 range and +1.5mV on the 10:1 range. You should be able to get to within ±10mV of zero on both ranges simultaneously. The final step is to trim the frequency compensation. You will need siliconchip.com.au a function generator and an oscilloscope. The function generator should be set to deliver a 1kHz square wave at the highest amplitude you can manage. Connect the differential probe to the scope using a BNC-to-BNC cable and make sure the scope’s bandwidth limit is disabled. To set up the positive divider, connect the function generator’s output to the positive input of the probe and its common to the ground test point. Also connect the probe’s negative terminal to the ground test point. Switch the probe to the 100:1 range. Set up your scope to get a stable display of the square wave output of the differential probe and adjust compensation trimmer VC1 for optimum compensation, just as you would for an oscilloscope probe. The correct compensation is achieved when the rising edge of the square wave shows no overshoot or undershoot, as shown in Fig.8. Use a non-metallic tool to make this adjustment. It’s better to err very slightly on the side of over-compensation (a small amount of overshoot) if you are unsure, as this will maximise the probe’s bandwidth. Repeat the whole process for the negative divider, connecting the function generator output to the negative input of the differential probe and the probe’s positive terminal to the ground test point. This time, tweak VC2 for optimum compensation. Using it Screw the lid on and your probe is ready to use. I added four small self-­ adhesive rubber feet to the bottom of the case to prevent it from sliding around too much on the bench. Always take special care when you are using the probe with high voltage circuits. Make all connections – including that from the probe to the scope – before powering up any circuit under test. Never disconnect any high-voltage differential probe from the scope while the test circuit is powered on. If you do, the BNC connector on the probe can float to high voltages. There is no isolation barrier in these devices. Not much current can flow due to the high impedance of the probe, but you can still get a shock. Always use quality test leads with shrouded banana plugs for high-­ voltage connections, and check everything twice before powering it up. SC Australia's electronics magazine Fig.7: this label artwork can be downloaded from the Silicon Chip website as a PDF. For details on how we make front panels see siliconchip. com.au/Help/FrontPanels Undercompensated Correct Compensation Overcompensated Fig.8: correct compensation is achieved when the square wave’s leading edge shows no undercompensation droop or overcompensation overshoot. February 2025  39 Antenna Analysis and Optimisation This series is about understanding how antennas work and designing matching circuits for them. This first article will cover antenna fundamentals, reactance, Smith charts and some related topics. Next month, a follow-up article will go into using antenna analysis software. Part 1 by Roderick Wall, VK3YC R adio Amateurs (hams) frequently build and install antennas. We know that the Voltage Standing Wave Ratio (VSWR) needs to be as close as possible to 1:1 for good performance (to achieve efficient power transfer to the antenna). To help us achieve this, we can use an antenna analyser hardware device. Software is also available to aid in this endeavour. Traditional Smith charts can help us understand how to adjust antennas or design matching circuits. We will also demonstrate how some common antenna types work. Amateur radio clubs often have antenna analysers for members to use. They can usually measure complex impedance and indicate the sign of the antenna image. Those that only measure |Z| magnitude and do not show the image are not as useful. When an antenna analyser is connected to a set of antenna terminals, it ‘sees’ your antenna as being made up of what looks like three components: • An inductor (L) with inductive reactance gives a positive imaginary +jW impedance component. • A capacitor (C) with capacitive reactance gives a negative imaginary -jW impedance component. • A resistor (R) that dissipates some of the power as heat and radiates the rest, with a real resistance (W) value. Editor’s note: j is being used as the engineering substitute for the complex value i, where i = √-1. Like resistance, the SI unit for impedance is ohms (W). An antenna can be considered a complex resistive-­ inductive-capacitive (RLC) network – see Fig.1. Antennas can have impedances like: • 50W of real resistance with a capacitive reactance of 25W, written as (50 – j25)W – see Fig.2. • 25W of real resistance with 50W of inductive reactance, written as (25 + j50)W – see Fig.3. • 50W of real resistance with no reactance; the antenna is resonant. Written as (50 + j0)W – see Fig.4. Some antenna analysers use X rather than j to represent reactance. The three antenna states The above are three possible antenna conditions that an antenna analyser will display. Real resistance will always be there, while reactance can either be inductive (+j), capacitive (-j) or absent (j0). At some frequencies, it may have inductive (+j) reactance; at other frequencies, it could have capacitive (-j) reactance. At a specific frequency, both reactances will be equal in magnitude, but opposite in influence and cancel each other out (j0). The antenna is said to be resonant at the specific frequency that the impedance is purely resistive. Real resistance The real resistance is where power is dissipated. The power dissipated in radiation resistance (Rr) is radiated as electromagnetic waves, while the power dissipated in loss resistance (Rl) is lost as heat. For an antenna to be efficient, the radiation resistance should be as high as possible compared to the loss resistance. However, an antenna analyser is only able to measure the total resistance. It is not easy to measure each resistance separately and indicate antenna efficiency, where efficiency = Rr / (Rr + Rl). However, refer to the later section on an experimental method to derive the loss resistance, Rl. Reactance (1) Modelling the complex impedance of an antenna as three passive components in series. (2) An antenna with a -jW complex impedance component has capacitive reactance. (3) An antenna with a +jW complex impedance component has inductive reactance. (4) An antenna with a j0W complex impedance component is purely resistive. Reactance is the imaginary part of electrical impedance. Antennas can have both inductive and capacitive reactance. These reactances are opposing, so the presence of both will mean that they partially (or possibly wholly) cancel. The antenna analyser will only display the net resultant reactance as inductive, capacitive or no reactance. Australia's electronics magazine siliconchip.com.au 40 Silicon Chip When the reactance is j0W, the antenna is said to be resonant. Often, it is not practical for the reactance to reach j0W at the desired frequency; with a low reactance value, close to zero, we may still say that the antenna is resonant. Ideal inductors and capacitors do not dissipate power; they store energy and then return it. However, real inductors and capacitors are not ideal components and will have some resistive loss, even if it is small. The antenna reactance stores power in the antenna’s near field and gives it back. Inductance is measured in henries (H), while capacitance is measured in farads (F). The amount of reactance a capacitor or inductor has depends on the value (in henries or farads) and the frequency. The formulas are Xc = 1 ÷ (2πfC) and Xl = 2πfL. When the frequency increases, a capacitor’s reactance decreases while an inductor’s reactance increases. If frequency decreases, the opposite happens. An antenna analyser can determine the resonant frequency of tuned circuits and antennas. Simulating an antenna Discrete components can be used to make the equivalent circuits shown in Figs.2-4 for a given fixed frequency. However, the radiation resistance (Rr) will be low compared to the loss resistance (Rl), and the circuit’s efficiency as an antenna will be very low. Electromagnetic waves will not travel far; most of the power will be dissipated as heat. These equivalent circuits can be helpful as calibration standards to check the accuracy of antenna analysers at specific frequencies. For example, a 1% 50W non-inductive resistor is equivalent to the Fig.4 circuit. An antenna analyser should give a reading of (50 + j0)W and a VSWR of 1:1 if the resistor leads are short, with no inductive reactance, and the antenna analyser impedance is 50W. How complex impedance determines VSWR Let’s start by using Cartesian coordinates to draw a real resistance line, as shown in Fig.5, with 0W at one end and infinite ohms at the other. The 50W system impedance is in the middle. We can place +j inductive reactance above the real resistance line and -j capacitive reactance below it. siliconchip.com.au Fig.5: by plotting the complex impedance on a Cartesian plane with an X-axis that ranges from zero to infinite ohms, we obtain circles of constant VSWR, with the ideal 1:1 VSWR in the centre. Next, we can draw constant VSWR circles to indicate various VSWR values. Larger circles indicate a higher VSWR than smaller circles. A VSWR of 1:1 is a dot at the 50W point in the middle of the real resistance line, where the VSWR circle has collapsed into a dot. A 100W resistor would have a VSWR of 2:1, as would a 25W resistor. To achieve a VSWR of 1:1, the resistance has to match the system impedance, which is 50W in this case (it may be 75W in some situations). For a VSWR of 1:1, the antenna must also be resonant, ie, having no reactance (j0W). If your antenna has a worse VSWR, it may be possible to adjust it to get closer to 1:1 or use LC matching circuits, which use an inductor (L) and a capacitor (C) to improve the VSWR. T and Pi matching circuits can also be used; how to design matching circuits will be discussed later. The Smith chart The Smith chart was invented by Philip H. Smith (1905-1987). It is a graphical aid or nomogram designed Australia's electronics magazine for engineers specialising in radio-­ frequency engineering to assist in solving problems with transmission lines and matching circuits. The Smith chart shows complex impedance, real resistance and imaginary reactance for a single frequency or a range of frequencies. Fig.6 shows a modern version of the original Smith chart, published in Electronics magazine, January 1939, under the title “Transmission Line Calculator By P. H. Smith, Radio Development Department Bell Telephone Laboratories”. It has been rotated on its side, as that is how we usually see Smith charts these days. The beauty of Smith charts is that they make it easy to plot impedance changes and impedance matching on paper. Software for plotting Smith charts is also available, which can be more accurate than drawing on paper and can usually perform component calculations – so no maths is required! Smith charts are often displayed on modern RF test instruments, including antenna analysers. February 2025  41 Fig.6: a blank Smith chart, which is similar to Fig.5 except that lines of constant reactive impedance are curved rather than straight. The Smith chart is similar to Fig.5, except instead of having straight vertical lines for real resistance and straight reactance lines, the Smith chart has constant circles and constant curves. Smith charts also have constant VSWR circles. The Smith chart shown in Fig.6 is a normalised version, with 1.0 at the centre. That means it can be used with any system impedance (50W, 75W etc). To convert those values to ohms, you multiply by the system impedance. To convert back to a normalised chart, you divide by the system impedance. 42 Silicon Chip Fig.7 is a simplified version of the Smith chart with some highlighted lines and points. The impedance values on it are shown for a 50W system; the red dot in the middle represents (50 + j0)W. Several of the constant-­ resistance circles are highlighted in green and labelled with their values. For example, any point on the constant resistance circle that goes through 50W has a real resistance component of 50W. A mauve dot has been placed on the constant real-­ resistance 25W line (it is at [25 + j50] W). The left-most point on the Smith Australia's electronics magazine chart represents 0W, while the rightmost point represents infinite ohms. The 50W circle is called the unity resistance circle or Z-matching circle. It is the road home to where VSWR is 1:1, in the middle of the Smith chart. The blue lines and values in Fig.7 show the inductive imaginary +j portion of the complex impedance. The mauve dot mentioned earlier is on the +j50W line, hence its value of (25 + j50)W. The equivalent circuit of an antenna that falls at this point was shown in Fig.3. It comprises a series resistor and inductor. siliconchip.com.au Fig.7: this simplified Smith chart show lines of equal inductive reactance (blue), capacitive reactance (red) and resistance (green). The red lines are the imaginary -j (capacitive) part of the complex impedance. The yellow dot is at (50 – j25)W, and its equivalent circuit, with a series capacitor and resistor, was shown in Fig.2. If you analyse an antenna and find it is above or below the horizontal line at the centre, you generally want to try to get it onto that horizontal line, ie, make it resonant. But remember that the VSWR will only be 1:1 at the system impedance, 50W in the example shown in Fig.7. If the real resistance is higher or lower siliconchip.com.au than that, you ideally want to make changes to move it to the 1:1 VSWR point in the middle for maximum efficiency. Wavelength vs frequency In a vacuum, electromagnetic waves travel at the speed of light, c ≈ 3 × 108m/s (light is a type of electromagnetic wave). For most practical purposes, air is sufficiently close to a pure vacuum that you can use the same figure. A signal’s frequency and wavelength can therefore be determined using the following formulas: Australia's electronics magazine ƒ = c ÷ λ or λ = c ÷ ƒ The Greek letter lambda (λ) is the wavelength in metres, while c is the speed of light (in m/s). The wavelength is the distance travelled by one cycle of an electromagnetic wave, while the frequency ( ƒ) is in cycles per second (Hz). Antenna impedance vs wavelength How you adjust an antenna to obtain a VSWR of 1:1 depends on the type of antenna. The following may give some ideas. February 2025  43 ground systems and objects around them will have different complex impedance curves than the one shown. Dipole antennas Fig.8: a plot of resistance (cyan) and reactance (red) versus length as a fraction of the wavelength for a lossless Marconi vertical antenna with a perfect ground plane. Fig.9: how a monopolar (“Marconi”) antenna with a ground plane (left) can be reconfigured into a dipole (right). The Fig.8 plot is for a Marconi vertical antenna with a perfect ground that has no losses. It shows the antenna drive point real resistance (Rd, cyan) and the imaginary reactance (Xd, red) as functions of the length of the driven vertical element. Positive values above the X-axis are for the real resistance (Rd) and inductive reactance (+j), while values beneath it indicate capacitive reactance (-j). The antenna’s resonant points are when reactance is j0, ie, where the red Xd curve crosses the X-axis. The horizontal scale shows the length of the driven vertical element as a multiple of the wavelength (λ). This graph was made using data from the EZNEC antenna simulator. The antenna is resonant at points (b) 1/4 wavelength, (d) 1/2 wavelength, (f) 3/4 wavelength and (g) one wavelength. Points (c) and (e) are if the driven element is cut in length so that the real resistance is 50W, to match a 50W system impedance. Point (b) indicates 44 Silicon Chip that if you were cutting the 1/4-wave driven element for resonance at a fixed frequency and the reactance is capacitive, you need to make the driven element longer to make it resonant. Likewise, if the reactance is inductive, the driven element must be shorter. For a 1/2-wave driven element, if it is inductive, make it longer, or shorter if it is capacitive. Point (d) shows that the real resistance for a 1/2-wavelength resonant antenna is 1889W. You may find that 1889W is too high for an antenna tuner to cope with. In that case, you may want to reduce or increase the driven element length to reduce the real resistance, to allow the tuner to work. If it is a multi-band antenna, you may need to select a length that is not a 1/2-wavelength or multiple of it on the other bands. The Marconi vertical antenna with a perfect ground and no losses used in Fig.8 is a reasonable reference, but practical antennas with different Australia's electronics magazine So, how does the plot in Fig.8 relate to a 1/2-wave dipole antenna? A dipole antenna can be built by combining two Marconi antennas, as shown in Fig.9. Section (a) on the left is a Marconi 1/4wave vertical antenna. Replacing the no-loss ground with a conductive (eg, tin) sheet gives us (b). Adding another 1/4-wave Marconi vertical antenna on the other side of the tin sheet results in the configuration shown in (c). Because the field lines between the top and bottom elements line up and match each other, the tin sheet can be removed, giving (d). The complex impedance for each 1/4wave resonant vertical antenna is (36 + j0)W. Connect them in series doubles the antenna impedance to (72 + j0)W, as in (e). To make the vertical polarised 1/2-wave dipole a horizontal polarised dipole, we just need to lay it horizontally. A 1/2-wave dipole can be broken into two 1/4-wave lengths called elements. The elements are set at 180° from each other and fed in the middle. This type of antenna is called a centre-feed 1/2wave dipole. Its impedance is (72 + j0) W in free space. When placed near the ground, the complex impedance will be different. In Fig.9, we showed how two 1/4wave Marconi antennas can be made into a 1/2-wave centre-feed dipole. The same can be done with two 5/8-wavelength vertical antennas, converting them into a centre-feed Extended Double Zepp antenna. Reflected power & transmission line losses Fig.10 shows an antenna matching circuit at the transmitter end of the transmission line and not at the antenna end (as would be the case when the antenna tuner is part of the transceiver). Because the VSWR at the antenna is not 1:1, power is reflected back from the antenna towards the matching circuit. The matching circuit reflects and adds the reflected power to the forward power from the transmitter. Thus, the forward power supplied to the antenna is now higher than the power supplied to it just from the transmitter. This must happen if the matching siliconchip.com.au Fig.10: when the antenna matching circuit is at the transmitter end, some power is reflected back at the antenna end and circulates to ensure the antenna receives the full transmitter power. circuit at the transmitter end is doing its job, delivering the full transmitter power to the antenna when the antenna VSWR is not 1:1. The reflected power circulates from end to end of the transmission line. Essentially, the matching circuit boosts the power level on the transmission line until all the power from the transmitter reaches the antenna. The ‘extra’ power on the transmission line does not come out of thin air, it is simply recirculating power from the transmitter that has not yet reached the antenna. The level of the reflected circulating power depends on the antenna VSWR. If antenna VSWR is 1:1, there is no circulating power and you do not need a matching circuit. In this example, an SWR meter inserted at either end of the transmission line will indicate a standing wave, while an SWR meter between the transmitter and matching circuit will indicate a VSWR of 1:1. Losses will increase because of the extra distance the reflected power travels. Because the forward power from the matching circuit to the antenna is now higher than the power from the transmitter, forward transmission line losses will also increase. The ARRL Antenna Book presents detailed graphs of increased line losses as a function of VSWR for a variety of real lines. Some transceivers have an Antenna Tuning Unit (ATU) built inside them. This allows the ATU at the transmitter output to be tuned at any frequency within the band. And make the transmitter output VSWR to be close to 1:1 across the band. It also allows the antenna VSWR to be higher than 1:1 as in Fig.10. If the VSWR at the antenna is not 1:1 and there is no matching circuit at the transmitter end, the reflected energy is dissipated in the transmitter’s output resistance. Some transceivers measure siliconchip.com.au VSWR to determine the reflected power and reduce the transmitted power to protect the transceiver if VSWR is high. A low-loss balanced transmission line can reduce losses under dry conditions, as shown in Fig.11; losses can increase in wet conditions with such a configuration. An added advantage is that the antenna tuning unit (ATU) does not need to be mounted in the air at the antenna connection terminals. An experimental method to derive Rl Point (b) in Fig.8 for a 1/4-wavelength resonant vertical antenna shows that the feed point impedance is (36 + j0)W when the ground is perfect and has no loss resistance (Rl). In chapter 7 of the ARRL book “Antenna Physics: An Introduction” by Robert J. Zavrel, Jr W7SX, he describes a non-­ambiguous method to determine Rl for a given location for a 1/4-wavelength vertical antenna. To do this, install a 1/4-wavelength resonant vertical over the ground plane. The base feed point impedance should show as little reactance as possible (ideally, it is a pure real resistance, but some small reactance value is acceptable). We are only interested in the real resistance of the impedance that dissipates power. In this case, the value for radiation resistance (Rr) will be very close to 36W. Therefore, the loss resistance (Rl) will simply be Rl = Rfeed – 36W As you add radials, change their lengths and so on, the feed point complex impedance should change accordingly. These can form the basis of an approximation for general use. However, the multiple differentials involved will vary by antenna location. As you experiment, simply use the following equation to approximately determine efficiency: Efficiency = Rr ÷ Rr + Rl Ground properties can also affect radiation resistance (Rr). For example, if the ground under the vertical/ radial system has very low conductivity and dielectric constant and is gradually made more lossy, it will begin to approach the characteristics of free space. In this case, the antenna radiates electromagnetic waves into the ground, which are lost as heat. Consequently, an accurate differentiation between radiated and absorbed power is nearly impossible. A radiated resistance (Rr) calculation accounts for all radiated power, even that which goes under the surface and can never reach the receiver. Next month That covers all the basic theory we need to analyse and tune antennas. Next month, we are using software to SC make antenna analysis easier. Fig.11: this configuration can reduce losses in the line between the transmitter and antenna when the matching network is at the transmitter end. Australia's electronics magazine February 2025  45 CIRCUIT NOTEBOOK Interesting circuit ideas which we have checked but not built and tested. Contributions will be paid for at standard rates. All submissions should include full name, address & phone number. IR Repeater, based on the IR Helper I was so impressed by Tim Blythman’s IR Helper in the September 2024 issue (siliconchip.au/Article/16577) that I bought an XC4431 Tiny Leonardo from Jaycar the next day and tried the sketch out. On receipt of specific commands from a TV remote, it sends new commands to control an AV amplifier. I modified the IR_Helper.ino sketch to change some of the commands sent. As the audio from our TV is fed through an optical cable to my home theatre AV receiver, the TV does not use its remote MUTE, VOL- and VOL+ IR signals. This unit converts those unused buttons to control the amplifier, so that when watching TV, only one remote is needed. I made a circuit board to hold the Arduino module, an IR receiver and a 3.5mm socket. I shifted the visible LED to the Arduino output pin A0 so it confirms that a code is being transmitted. The PCB accepts either through-hole or surface-mount components. The IR LED(s) are in a multi-emitter cable that plugs into a 3.5mm socket on the PCB. You can find this cable on AliExpress (search for “IR repeater extender”). Choose 2-5 emitters per cable and select the 3.5mm plug cable only. The matching IR receiver cable is not needed. Put heatshrink tubing over any emitter that you will not be using to prevent stray IR signals being picked up again by the receiver on the PCB. The unit is powered by the USB socket on the TV. When the TV powers up, it signals the amplifier to also switch on and, after a delay, selects the TV Audio input. When the TV is switched off, there is enough time for the power-off signal to be sent to the amplifier before the TV switches off the USB 5V powering the Arduino. The PCB is mounted under the TV next to its IR receiver. Arduino Sketches I modified Tim’s IR_Helper.ino sketch by adding seven extra lines. The first six were inserted at the top of the loop() function and serve to print the received commands to the serial console so that I can record them for later modifications. The last of these three lines is needed to echo all received commands to the transmitters: IrReceiver.printIRResultShort( &Serial); IrReceiver.printIRSendUsage( &Serial); IrSender.write( IrReceiver.read(), 1); Run the Arduino IDE before plugging in the USB cable to the board, to avoid a message “downloading index: package_index.tar.bz2” (stuck at 50%) at the bottom of the IDE. Use the sketch to obtain the TV and amplifier codes of your remotes. At this stage, the cable with the IR LEDs (emitters) does not need to be plugged into the 3.5mm socket. You can then modify your version of IR_Convertor-Repeater.ino sketch, replacing my lines with your own. For the <number of repeats> I found that one was enough for on/off functions like MUTE or POWER. As volume changes need more time for you to hear the change, the code is written so that one press on the remote results in 10 repeats being sent out. When installing the device, I held one of the IR emitters over the front plastic panel of the amplifier where I thought its IR receiver might be. I kept testing by using the MUTE button of the TV remote until I got the best response from the amplifier. This is the position to mount the emitter using double-sided tape clear pads. I use double-sided clear tape made by Sellotape as I did not need to make a hole in the pad, the IR signals will pass through. As Teletext is obsolete worldwide, there are four unused coloured buttons on most TV remotes. I intend to add to my IR_Convertor-Repeater.ino sketch, and use those buttons to replace another IR remote which controls a dimmable LED light strip around the ceiling of our home theatre room. The software for this project can be downloaded from siliconchip.au/ Shop/6/834 Robbie Adams, Tauranga, New Zealand. ($80) The Tiny Leonardo board on a custom-made 33 × 35mm PCB. We recommend putting heatshrink tubing over any unused emitters to reduce interference. The IR Repeater is shown attached underneath a TV in the photo at the top of the page. The circuit draws around 35mA. 46 Silicon Chip Australia's electronics magazine siliconchip.com.au Vehicle accessory power control This circuit may offer a simplified solution to that presented in the recent Dashcam Power Control Circuit Notebook (September 2024; siliconchip. au/Article/16572). Some years ago, I designed a digital speedometer for a 4WD that had a speedo which could not be seen in bright sunlight(!). My digital speedo was connected via the car’s OBD-II interface connector. Initially powered via a small toggle switch, I came up with this replacement for that switch. Based on a Design Idea published in EDN (September 18th, 2008) for a single LED car voltage monitor, it is simple, cheap, requires no microprocessor and can be built on a scrap of prototyping board or a tiny PCB. Q2 switches on and powers the output when the battery voltage exceeds 2.5V × (1 + [R1 ÷ VR1]) and turns off when the battery voltage falls below 2.5V × (1 + R1 × R2 ÷ R3 × [R1 + R2]). With 14V on the input, I adjusted the 4.7kW or 5kW preset (VR1) for 2.5V on TP1. This meant my ODB-II speedo would switch on at 14V and off at 13V, a setting suited to my car. VR1 can be adjusted to switch on Q2 at input voltages from 9V to 15V. The component values shown result in a switch-off voltage about 1V below this set point. My digital speedo’s current consumption was around 30mA. If your load exceeds this, you will need a higher-­current transistor for Q2 or possibly even a high-power Darlington. Andrew Woodfield, Christchurch, New Zealand. ($70) Power supply transformer tap switching I successfully built two of your 45V 8A Linear Bench Supplies (October-December 2019 issues; siliconchip.au/Series/339) with a few changes. The lack of transformer secondary switching results in high heat output at lower voltages. The circuit shown here is my implementation of secondary switching. It requires the use of a good-quality, high-current relay. To show the output voltage, I simply rescaled a 50V analog meter movement I had lying around. The analog ammeter was a bit more involved. I had to remove the coil assembly of my original ammeter, siliconchip.com.au as it was a low-resistance type (20W), and replace it with a higher resistance type. I then rescaled the ‘ammeter’ to read FSD with 328mV at TP4 (which on my supply equates to 500mA output). I then calculated resistor values for my ‘ammeter’ based on TP4 providing 6.2V at 9A. The section at lower right shows my method of current range switching. I then used the Tonne meter movement design software to print new fascias for both the ammeter and voltmeter. On the left of the power supply face is a switch above the current set pot, to select the 500mA or 10A Australia's electronics magazine current range (S2). The switch to the left of the current pot shorts the output terminals, which is helpful when setting the current limit. On the far right is an output breaker rated at 15A (a Carling type), in case when charging a battery the PSU develops a shorted output. Then the breaker will simply trip. Brett Neale, Bertram, WA. ($80) February 2025  47 Mini Projects #021 – by Tim Blythman SILICON CHIP Wireless Flashing LEDs Wireless power transmission has been researched for over 100 years ago and is currently used for charging things like toothbrushes, smartphones and even electric cars. Here we show you how to build wireless LEDs that can be programmed to flash in sequence. I t’s difficult to transmit a lot of power wirelessly, so we decided to see what was possible on a smaller scale. Most of these technologies depend on the transformer principle: an alternating current in a coil will induce a current in a second nearby coil. Since the inverse square law applies, the closer the coils, the much more effective the energy transmission. We first saw the concept of wireless LEDs on YouTube (Big Clive – youtu. be/UQ3K0suY1Dc). In the video, he demonstrates a kit purchased online. It consists of a small circuit board attached to a coil about 8cm across and some small LED modules. When the circuit is powered, it drives the coil at around 200kHz and LED modules that are nearby light up. He goes on to explain the circuit and show some waveforms. We wanted to see if this was something we could replicate, or possibly improve on. By adding some smarts in the form of an Arduino Uno, we’ve made it possible to control the LEDs better and make them flash in a pattern. You might have seen the Circuit Notebook entry on Wireless power transfer from our December 2023 issue (siliconchip.au/Article/16048). The principle here is similar, although our circuitry is simpler and much more compact. However, it does require a microcontroller. Circuit details Fig.1 shows the fairly simple circuit. It depends on using the values shown, particularly the capacitors and inductors, for proper operation. In the Transmitter on the right, the Uno generates a PWM waveform at its D3 pin, which drives the coil/capacitor pair via a transistor. The 100μF capacitor provides a steady power supply for this part of the circuit. When pin D3 goes high and the transistor switches on, the current builds in the inductor until D3 goes low, and Parts List – Wireless LEDs (JMP021) Transmitter (one required) 1 Arduino Uno compatible main board [Jaycar XC4410] 1 TIP31 3A NPN transistor, TO-220 [Jaycar ZT2285] 1 100μF electrolytic capacitor [Jaycar RE6130] 1 100nF MKT capacitor [Jaycar RM7125] 1 1kW resistor [Jaycar RR0572] 1 2m length of 0.5mm solid-core insulated wire (enamelled or with plastic insulation) 1 2-pin header [cut from Jaycar HM3211] 1 length of electrical tape to secure coil Receiver (per unit, multiple can be used) 1 100μH unshielded SMD inductor [Jaycar LF1400, pack of 10] 1 6.8–10nF ceramic capacitor [Jaycar RC5346, RC5347 or RC5348; see text] 1 high-brightness 5mm LED [Jaycar ZD0290, ZD0291, ZD0292, ZD0293 or ZD0295] 48 Silicon Chip Australia's electronics magazine A screenshot from the YouTube video by Big Clive (https://youtu. be/UQ3K0suY1Dc). It shows the inspiration for this project. siliconchip.com.au Fig.1: a parallel tuned LC network is driven by a PWM signal from a microcontroller. Energy is radiated from the coil that’s part of that LC network; the Receiver picks up the energy and uses it to power the LED. the transistor switches off. The current continues to flow through the inductor to charge up the capacitor. The energy in the capacitor is released on the next cycle. By using a capacitor here instead of a diode to catch the inductive spike, the energy in the inductor is saved instead of being dissipated. The well-known formula for the resonant frequency of an LC (inductor-capacitor) circuit is: f = 1÷(2π√LC) For the components we have chosen and the coil’s dimensions, this works out to around 250kHz. As we will see later, the circuit will operate mostly below that frequency, from about 160kHz to 200kHz. The components in the Receiver on the left have a resonant frequency of 193kHz for a 6.8nF capacitor, down to 159kHz for a 10nF capacitor. An 8.2nF capacitor would be resonant at 176kHz. The presence of a so-called non-linear device (the LED) will change this somewhat. It is not a sharp resonance (like a radio tuner), so the circuit will also respond to close frequencies as well. In this case, resonance means that the circuit will tend to reinforce signals that occur at a specific frequency. An analogy would be pushing a Similarly, our coil has a diameter of 8cm, with five windings. We used a bottle as a former, then taped up the wires to help it keep its shape. Twisting the two wires together also helps to keep the coil from unravelling. siliconchip.com.au Australia's electronics magazine playground swing; when the pushes occur at the correct frequency, it will swing higher than if they are not. The operation of the two parts of the circuit depends on the two inductors being ‘coupled’; their magnetic fields must interact. This will also change the behaviour of the two circuits. They can be considered to form an air-cored transformer. In practice, the frequency of the Transmitter coil is determined by the PWM frequency. As it is operating near resonance, the waveform is amplified somewhat. The Receiver will also resonate around its characteristic frequency, allowing it to develop enough voltage to light the LED. We wrote an Arduino sketch that allows the frequency to be manually adjusted. Broadly speaking, the closer the Receiver is to resonance, the brighter the LED will light. You might also see a neat trick in our design. By having Receivers with different resonant frequencies, we can change the Transmitter frequency and have different Receivers light up at different times. By cycling through different frequencies, the different LEDs will flash in a sequence. Construction We wound a five-turn coil on a bottle; you can use a similar cylindrical object, making sure to leave 10cm or so of wire to connect at each end. You could use enamelled wire, but we had some solid-core cable (from an old network cable). You might prefer this as it is easier to strip than enamelled wire. Use tape to secure the windings neatly and twist the ends to keep them Each Receiver consists of an inductor, capacitor and LED soldered in parallel. Using different capacitor values allows the Receiver to be tuned to react to different frequencies. The Receivers are small and cheap enough that you can easily build a dozen using different coloured LEDs. February 2025  49 Just a handful of parts are needed to experiment with wireless power. We built our Transmitter circuit on a small pair of header pins, and it too only needs a handful of components. Fig.2: this is how we laid out our prototype; only three connections are needed to the Uno. Check with the circuit diagram as you go to ensure you wire things up correctly. together, as you can see in our photos. Place the two-pin header in the 5V-GND position on the Uno and solder the 100μF capacitor across it, making sure that the negative stripe on the capacitor goes to ground. Follow with the transistor; its pin 3 emitter should go to ground, too. Note from the photos how it is mounted upside-down. Solder the 1kW resistor to the base pin (pin 1) of the transistor and push the other lead into the socket for pin D3. The coil and 100nF capacitor are both connected between the collector (pin 2) of the transistor and 5V. Look closely at our photos to check that this is all correct. Receiver Each Receiver simply has the LED, an inductor and a capacitor wired in parallel. We soldered the capacitor to the inductor first after trimming its leads down to a few millimetres. The LEDs leads are similarly trimmed to sit over the capacitor and also soldered in place. Check out our photos; you can see that the solder pads on the inductor are quite large. You should be able to straighten the leads so that the Receiver sits flat on the top of the inductor. This will help with testing. Place the Receivers inside the Transmitter coil. 50 Silicon Chip Software The software uses direct register writes to achieve a higher PWM frequency than plain Arduino code would allow. Thus, it only works on the Uno or ATmega328-based boards, such as the Nano. The code in setup and the setF() function could be used in your own sketches. You can download the Arduino sketches from siliconchip.au/Shop/6/583 Load the sketch file WIRELESS_ LED_FREQUENCY_TEST onto the Uno and open the Serial Monitor at 115,200 baud. You should see a report that the Uno is delivering a 222kHz waveform. Change the frequency by entering a value in kHz (eg, 220<Enter>). The Uno will find the nearest achievable match (between 100kHz and 250kHz) and display it. We found that the Receivers with 6.8nF capacitors were brightest at around 195kHz, 8.2nF worked best at 185kHz and 10nF at 165kHz, although the 10nF Receivers were much less bright than the others. This is due to the Transmitter coil being further from its 250kHz resonance. The red and yellow LEDs also tended to be less sensitive to frequency; their lower forward voltage allows operation over a wider range. You might find that the transistor is getting warm at this stage. The Australia's electronics magazine WIRELESS_LED_FLASHING sketch cycles through three different frequencies in turn and flashes the Receivers briefly, so the transistor is not working all the time. You can tweak the frequencies and delays with the arrays and defines near the start of the sketch. Using frequencies between the peaks noted above will allow multiple LEDs to light at the same time. The Receivers work best in the plane of the coil (and parallel to it), but will still work if a thin piece of paper or plastic is between. Thus, you can hide the Transmitter if you want to. Of course, the Receivers are very simple and well-suited to experimentation. It appears that around 200kHz is the sweet spot and you could try different value capacitors. You can also try different inductors, but we found that this style and value worked best. You can also use the Receivers to test other wireless power devices. A mobile phone charging pad causes the Receivers to flash briefly; many of these use a coded protocol to avoid running continuously and wasting power. Wireless power transmission is still only practical over short distances, but this project shows how to easily experiment with the concept and create an entertaining display. SC siliconchip.com.au Need to make amazing projects with our MICROCONTROLLERS & MINI COMPUTERS? 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It can check the type and pinout of bipolar transistors and measure their gain, as well as examining Mosfets, diodes and LEDs to provide information like the pinout and forward voltage. T he best feature of this Tester is that it can help you work out the pinouts of unknown devices. However, it also gives you important parameters like the DC gain (β or hfe) of bipolar transistors or the threshold voltage (Vgs(th)) of Mosfets. The Transistor Tester can detect and measure: • Bipolar transistors: pinout, polarity and β (gain). • Logic-level Mosfets: pinout, polarity and gate-source threshold voltage. • Single/dual LEDs and diodes: pinout/polarity and forward voltage. For LEDs, it will light them up so you can see what colour they are and how bright they are. It’s great for dual LEDs too, allowing you to test each element separately and see how it is connected. The Tester has a 16×2 character LCD screen on a ‘shield’ that also includes six tactile pushbuttons, making it easy for us to display information and accept user input. Fig.1 shows the circuit; note the connections around the six resistors and the test header. The test header is a three-way socket or similar so it can be used to connect a three-lead device such as a transistor. Two-lead devices can plug into any two of the three locations. The Arduino Leonardo microcontroller module connects to the LCD shield through fixed headers; the LCD shield’s pinout dictated most of the remaining pin choices we have made. Fig.2 shows a possible configuration when testing of a typical NPN transistor such as a BC548. Pin D3 is taken high, effectively connecting it to 5V and supplying the base of the transistor via a 10kW resistor. Next, pin D0 is taken low, connecting the emitter to circuit ground via 1kW. Finally, pin A2 is taken high, directly connecting the collector to 5V. We can then measure the voltage at the A3 and A4 analog inputs using the Leonardo’s 10-bit ADC (analogto-­digital converter) peripheral. With those voltages, we can establish that the base sits around 0.7V above the emitter and that the current through the 1kW resistor (and thus emitter) is much greater than the current through the 10kW resistor and transistor base. From that information, we can determine that the connected device is an NPN transistor with the pinout as noted. We can also calculate its DC gain from the ratio of the emitter and The Transistor Tester is primarily built from these three modules: an Arduino Leonardo, prototyping shield and LCD shield; the finished project is shown in the lead photo. The LCD shows information about the device connected to the Transistor Tester, while the buttons run detailed tests for further information. The three-way header on the LCD shield is where a component can be connected for testing. siliconchip.com.au Australia's electronics magazine February 2025  55 Table 1 – test information Component Initial test Button Specific test procedure Bipolar transistor Check for two PN junctions (baseemitter and basecollector) LEFT Check polarity and β in different configurations and confirm pinout based on higher β value. Mosfet Check for one PN junction (body diode) RIGHT Check which polarity switches on Mosfet and confirm threshold voltage and polarity. Diode Check for one PN junction UP Measure forward voltage, display test current and confirm pinout. Single LED Check for junction with Vf higher than a silicon diode UP Light up LED, measure forward voltage, display test current and confirm pinout. Dual LED Check for two junctions with Vf higher than a silicon diode UP and DOWN Light up LED, measure forward voltage, display test current and confirm pinout. UP measures one junction, DOWN measures the other. Fig.1: it’s a simple circuit, but quite powerful when combined with the digital and analog peripherals of a microcontroller. Fig.2: when testing an NPN transistor, this circuit is formed by setting various pins to a high or low level, or high-impedance (those pins are not shown). It can measure the voltages to determine the current through the resistors and thus different component leads. 56 Silicon Chip Australia's electronics magazine base currents. This is just one set of connections that the Tester can make. The Arduino can set any pin to be an input, meaning it is in a high-­ impedance state; that means that it is effectively disconnected from the circuit. So we can probe individual pairs of pins in isolation, which we do to work out the potential location of PN junctions, as found in diodes or transistors. We don’t have space to describe all the internal operations in detail, but the Tester starts by probing pairs of pins to suggest what devices might be connected based on the PN junctions present. The initial tests use only the 10kW resistors, so minimal currents are applied to connected devices. The user can then press one of the buttons to run a specific test to further characterise a connected device such as a transistor, diode or LED. Table 1 has some more details on the initial and detailed tests and the buttons used to perform them. For example, potential LEDs and diodes are checked by measuring the voltage between two pins while ignoring the third. If the voltage is between 200mV and 750mV, it could be a silicon or schottky diode. If the voltage is higher (but less than 5V), it’s likely a light-emitting diode (LED). Multiple LEDs are found by scanning the various pin combinations. Thus, common-anode, common-cathode and dual (back-to-back) LEDs can be identified and their pinouts confirmed. A Mosfet is initially detected as a single diode, which is the body diode between the source and drain. Assuming that the other pin is the gate, it is then just a matter of checking whether it is a P-channel or N-channel Mosfet by driving the gate high or low to see if the Mosfet switches on. Note that this only works for Mosfets with threshold voltages comfortably under 5V. Non-logic-level Mosfets usually have a threshold around 4V and are switched on fairly hard by 4.5V, so while it’s possible or even likely they would be detected correctly, it isn’t guaranteed. A resistor might be identified as a back-to-back LED, since it will conduct in both directions. A resistance reading is also provided for this reason. Construction In addition to the LCD shield, we used a prototyping shield to simplify siliconchip.com.au Parts List – Transistor Tester (JMP020) 1 Arduino Leonardo [Jaycar XC4430] 1 Arduino prototyping shield [Jaycar XC4482] 1 alphanumeric character shield [Jaycar XC4454] 3 10kW axial ¼W or ½W resistors [Jaycar RR0596] 3 1kW axial ¼W or ½W resistors [Jaycar RR0572] 1 3-way female header [cut from Jaycar HM3230] 1 micro-USB cable to suit Leonardo [Jaycar WC7724] construction, as it looks much neater and is easier to follow. Check Fig.3 before soldering the resistors in place to match. We used a short section of socket header soldered to the LCD shield to allow components to be plugged in for testing. You might consider clip leads or some flying leads to a breadboard as an alternative. Plug the three shields together once all the components are fitted. You must use a Leonardo for this; an Arduino Uno or Mega won’t work because both those boards use the D0 and D1 pins for serial communications. Software You’ll need the Arduino IDE to load the software. The LiquidCrystal library might need to be installed; this can be done from the Library Manager. We used version 1.0.7. The sketch folder includes the lcdkeys.h file for interacting with the buttons on the LCD shield. Choose the Leonardo board and its serial port and upload the sketch. You should see a splash screen like Screen 1, after which the display should indicate that nothing is detected (Screen 2). If something is detected, you might have a wiring error! Operation The Tester tries to be as smart as possible while still allowing the user to select what tests to run by pressing suggested buttons. The pinout is displayed from left-to-right, matching the order on the test socket. Try not to touch the leads while the test is happening. Fingers can pass enough voltage to switch on the gate of a Mosfet, which would alter the results. We tried out the Tester on numerous common devices, but we can’t predict what it might display for unusual ones. Screens 3–8 show the results of connecting different devices to the test header, followed by pressing the suggested button. Screens 3 and 4 show a 2N7000 N-channel Mosfet being tested. The RIGHT button gives the report shown in Screen 4. Screens 5 and 6 are the readings for a BC558 PNP transistor, with a press of the LEFT button resulting in Screen 6. Such a transistor can still work (albeit poorly) even if the collector and emitter are reversed, so we pick the arrangement with the highest β value; the reverse value is shown on the bottom line for comparison. A yellow LED shows Screen 7, then Screen 8 when UP is pressed. You’ll also see the LED light up while the UP button is held, with brief flashes off as other scans run. The current display on the bottom line alternates with a calculated resistance value, which will be useful if a resistor is connected. If you have a dual LED of any sort, the DOWN button can be used to scan the second LED in the package. Conclusion The Tester is easy to build and easy to use. It can help identify parts and determine their pinouts. You could even use it to sort and match transisSC tors and LEDs for projects. Screens 1 & 2: the splash screen and idle screen are seen here. If you don’t see the idle screen (Screen 2) when nothing is connected, there may be a wiring problem. Screens 3 & 4: a Mosfet’s body diode is detected although the Tester cannot immediately determine which type it is. Running different tests with the UP or RIGHT buttons can narrow down the choices. Screens 5 & 6: the text at lower right in Screen 5 indicates the relative location of the PN junctions in the part. In this case, they happen to match the connected PNP transistor. Screen 6 shows the device’s pinout at lower right. Fig.3: this shows how we soldered the resistors to the prototyping shield. The circuit we have used lends itself to a tidy protoboard layout. siliconchip.com.au Screens 7 & 8: if a dual LED is connected, Screen 7 will show the type (common cathode, common anode, etc) and the respective pins. Use UP and DOWN to probe the individual LED devices. Australia's electronics magazine February 2025  57 picomite for the words and mmbasic : geoff graham firmware : peter mather Raspberry Pi Pico 2 This new MMBasic interpreter for the Raspberry Pi Pico 2 and Pico 2 W takes advantage of the new features of the Raspberry Pi Pico 2. It is a comprehensive programming environment that converts the Pico 2 into an easy-to-use and powerful platform for beginners and experts alike. T he Raspberry Pi Pico is a complete package with its own power supply, USB interface and more. It is sold at an extremely good price, making it the perfect drop-in microcontroller for many applications. As described in the December 2024 issue, the new Raspberry Pi Pico 2 has more memory, better performance and more features. If you wish to delve into the details, see the article (siliconchip. au/Article/17316). The highlights of the Pico 2 are: • A faster base clock speed, up from 133MHz to 150MHz. • More efficient CPU cores, up to 50% faster. • More on-chip RAM, up from 264kiB to 520kiB. • More flash memory for program storage, up from 2MiB to 4MiB. • New features, such as the HSTX peripheral for HDMI output and support for external PSRAM. We published the original PicoMite in the January 2022 issue (siliconchip. au/Article/15177). It is essentially a port of MMBasic from the Micromite/ Maximite to the RP2040 chip used on the Pico modules. Yo u c a n n o t simply use that firmware on this new processor. The Pico 2 uses a different type of CPU core (the ARM M33 rather than M0), so the firmware must be The top side of the Raspberry Pi Pico 2. rebuilt to suit the new instruction set. If you try to load the old firmware, the new processor will simply ignore it. With our new release (version 6.00.01) of the PicoMite firmware, we now support both the Raspberry Pi Pico 2 and the original Pico. However, this firmware is much more than just a recompiled version of the original. We have changed it substantially to make the most of the speed and additional features of the new processor. Headline features include HDMI video output in various resolutions up to a wide screen resolution of 1280 × 720, and up to 32,768 colours in other resolutions. The video interface has extensive support for sprites, multiple layers, BLIT and other features used in creating detailed graphics for applications such as games. A new feature in v6.00.01 is a USB interface for connecting a USB keyboard, USB mouse and game controllers. This interface includes support for a USB hub, so you can connect up four devices simultaneously. The new firmware also supports the extra memory available on the Raspberry Pi Pico 2, which provides BASIC programs with up to 256kiB of program space and 228kiB of general-purpose RAM. We exploit the extra speed of the processor with a default CPU clock rate of 150MHz. It can also be overclocked to nearly 400MHz. At the core of the PicoMite firmware is the MMBasic interpreter. This is a fully featured BASIC interpreter that is mostly compatible with Microsoft BASIC. It includes features such as long variable names, multiple data types (float, integer and string) and modern structures, such as multi-line IF and CASE statements. New Raspberry Pi microcontrollers The Raspberry Pi Pico 2 uses a new Australia's electronics magazine microcontroller called the RP2350A, developed by the Raspberry Pi Foundation. There are three other variations of this chip. The first, the RP2350B, is the same as the RP2350A except that it has 20 more pins. This allows for a total of 48 GPIO pins, with eight capable of operating as analog inputs. The PicoMite firmware for the RP2350 will work with either the A or B variants and will automatically recognise the extra I/O pins when it is running on the RP2350B. They will be available to the BASIC program as GP30 to GP47. Currently, only a few modules use the RP2350B, primarily supplied by Pimoroni (https://pimoroni.com). However, it is likely that other suppliers will soon follow with their own versions. The RP2354 A and B chips are the same as the RP2350 versions, but they have 2MiB of flash memory integrated in the package. The PicoMite firmware may support these in the future, but currently they are not readily available for purchase. The RP2350 includes some security features intended to prevent third-­ parties from accessing the program and interfering with its operation. These features are not supported in the PicoMite firmware, as we doubt users will be that concerned with security. Upgrades for the original Picos This release still supports the RP2040 microcontroller used in the original Raspberry Pi Pico and many other third party modules. If you use the new firmware on a board with an RP2040 chip, you will gain many of the extra features listed here, such as USB keyboard support. While the original Raspberry Pi Pico is a little slower than the Pico 2 and has a less memory, it is still more than enough for most projects, so you don’t need to throw away your old modules. siliconchip.com.au ∎ More flash and RAM for user programs ∎ HDMI video output, up to 1280 × 720 pixels and up to 32,768 colours ∎ VGA video output, up to 640 × 480 ∎ USB keyboard, mouse, Wii controller and hub support ∎ Improved TCP/IP stack for WiFi boards ∎ High-speed frequency counter input ∎ Overclocking up to 400MHz Load them with this firmware and they will still be very useful. The Raspberry Pi Pico 2 W The Pico 2 W has also been recently released, with the same RP2350A processor and the addition of a WiFi interface. This, and the RP2040 version of the MMBasic firmware that we called the WebMite (August 2023 issue; siliconchip.au/Article/15897), are also supported by this new firmware. The WebMite firmware running on the RP2040 suffered from a problem that caused the processor to reboot intermittently for no reason. The cause of this was buried deep in the TCP/ IP protocol stack that is used to communicate with the wireless interface and, despite a lot of effort, it proved impossible to eliminate. In this new firmware version, we have completely rebuilt the networking features using a different protocol stack, eliminating the annoying reboots. Thus, we strongly recommend that any designs based on the previous WebMite be upgraded to this version. The Raspberry Pi Pico 2 W using the RP2350A processor (also called the WebMite) also uses this new protocol stack, so we are confident it will not suffer from the same problem. The E9 erratum As explained in our December 2024 article, the Raspberry Pi Foundation issued an erratum called E9 for the RP2350. This describes a hardware fault that affects the GPIO and PIO pins that interferes with the use of internal pulldown currents when they are used as digital inputs. We have implemented some workarounds in the firmware, so you can continue to use the pulldown option for pins configured as digital inputs. However, ideally, an external resistor of 8.2kW or less should be used instead. siliconchip.com.au Beta testing of the PicoMite firmware revealed that this error also affected the ability of MMBasic to communicate using the 1-wire protocol that’s used to measure temperature and humidity with DHT22 sensors. However, workarounds added to the firmware for the RP2350 mean these functions are now unaffected. HDMI support The RP2350 includes an internal peripheral called HSTX. This is a high-speed serial transmission circuit that streams data to up to eight output pins in parallel. It balances the delay between these outputs to within 0.3ns, making it perfect for generating the signals required for DVI/HDMI video. To produce such a signal, the Pico­ Mite firmware builds the video image in a reserved portion of RAM (the video buffer) and then configures a DMA (direct memory access) channel using the second CPU core to rapidly push that data to the HSTX peripheral. The firmware supports three screen resolutions: 640 × 480, 1280 × 720 (wide-screen/720p) and 1024 × 768 pixels. Within each resolution, there are several modes (set by the MODE command) that can trade resolution for more colours and features. The MODE command can save memory by doubling or quadrupling the size of each pixel, both horizontally and vertically. The monitor will still see the same resolution (ie, the same pixel rate). However, since there will be fewer pixels in the video buffer, the memory saved can be used for more colours. For example, the resolution can be set to 1280 × 720 using the RESOLUTION command. Following this, the MODE 1 command can be used to generate an image of 1280 × 720 pixels in monochrome, or MODE 3 can be used for a 640 × 360 image in 16 colours, while MODE 4 will provide a 320 × 180 pixel image in 256 colours. The memory saved by doubling and quadrupling each pixel can also be put to other uses. For example, MODE 4 also releases enough memory for two optional video layers that can be used for an independent overlay. A typical use of this would be to create an image of a moving vehicle overlaid on a background image of a stationary road. The TILE command Another handy feature is the TILE Australia's electronics magazine command, which allows you to colour individual characters in otherwise monochrome text. So, using the 1280 × 720 HDMI resolution in MODE 1, you can colour each character in one of 16 colours. This is used by the built-in editor in MMBasic, which uses cyan for keywords, yellow for comments, green for constant numbers and so on. Screen 1 shows a screen grab of the editor running in the 1280 × 720 resolution with colour coding turned on. This gives you a productive development environment with a colourful wide-screen program editor. In Screen 1 (shown on page 60), the editor was using the default font 3, which gives 80 characters by 30 lines. If you want more, you can switch to font 1 and have an expansive editing window of 160 characters by 60 lines, still with colour coding. HDMI overclocking To generate the DVI/HDMI signal, the firmware needs to overclock the RP2350 to as high as 372MHz. Overclocking means running the CPU clock at a higher frequency than the maximum stated in the data sheet. The firmware automatically does this to accommodate the requirements of the video output. All the Raspberry Pi Pico 2 modules that we tested work perfectly at these speeds. However, overclocking also depends on other components that accompany the processor, and manufacturers might decide to use components that are less tolerant. For this reason, the HDMI capability won’t necessarily work across all thirdparty modules using the RP2350 processor. Connecting an HDMI monitor Fig.1 illustrates how to connect the Raspberry Pi Pico 2 to an HDMI socket. At the high frequencies used by DVI/HDMI, it is important The underside of the Raspberry Pi Pico 2. Screen 1: using the HDMI output in wide-screen format, you have an excellent editing experience with the built in MMBasic editor. The text is clear and the colour coding identifies elements in the program with cyan for keywords, yellow for comments, green for constant numbers etc. to keep the signal lines short and of the same length. To minimise reflections in the signal path, it is also recommended that surface-­mount resistors be used. We have seen poor quality cables that exhibited significant crosstalk, ruining the signal. So, if you get a poor image on your monitor, check your HDMI cable as well. The signal generated by the Pico­ Mite firmware is actually a DVI signal, but HDMI transparently supports DVI and, because HDMI monitors are more common, we recommend using an HDMI connector. However, keep in mind that the PicoMite 2 does not support the transmission of audio in the HDMI signal. The tile feature described above also works with the built-in editor using the VGA video output. As a result, you can still edit your program in a reasonable resolution (640 × 480 pixels) while enjoying a colourful editing experience, even though the output is nominally monochrome. USB keyboard support The USB connector on the Raspberry Pi Pico (both the original and Pico 2) is normally used to load the firmware and to access the MMBasic command prompt as a virtual serial interface over USB. However, the Pico’s USB connector and electronics are USB OTG (On The Go) compliant, similar to the connector on many mobile phones. This means that it is possible to switch the connector from a USB client (when loading firmware) to a USB host, which is required for communicating with a USB keyboard, mouse or similar. When you load a PicoMite firmware image with USB capability (available for both the Pico and Pico 2), this switch will be made automatically when the upload is complete and MMBasic starts running. Using an adaptor cable, you can then plug in a USB keyboard and it will be immediately recognised and start operating normally with auto key repeat, function keys, etc. The Raspberry Pi Pico even supplies the 5V necessary to power the keyboard. You need the adaptor cable because keyboards usually have a Type-A host plug, while the Pico has a micro Type-B USB socket. These adaptors are common (see Photo 1 for an example) and you can find them online or in stores such as Jaycar (Cat WC7725). Because the USB interface on the Pico is now used for a keyboard/ VGA video output If you do not want to use HDMI, VGA video output is another option. VGA-capable versions of the firmware are available for both the Raspberry Pi Pico 2 (RP2350 processor) and the original Raspberry Pi Pico (RP2040). It is simpler to connect it to a VGA connector compared to HDMI, and that the processor does not need to be overclocked (although it still can be). VGA works the same as it did in the previous versions of the Pico­Mite firmware. It provides two video resolutions: 640 × 480 in monochrome and 320 × 240 with 16 colours. The RP2350 version supports a third mode, with a resolution of 640 × 480 in 16 colours. Refer to the VGA PicoMite article to see how the VGA socket is wired up (July 2022; siliconchip.au/Article/15382). Fig 1: this is how you connect a Pico 2 to an HDMI monitor. It is important to keep the PCB traces short and the same length. The adjacent table shows the function of each pin on the HDMI socket and how they are connected. 60 Australia's electronics magazine Silicon Chip siliconchip.com.au mouse, you will not be able to power the Pico via this connector, so the Pico must be powered via 5V applied to the VSYS pin. Another consequence is that you won’t be able to use the serial console over USB. In a self-contained computer with a keyboard and HDMI or VGA video, this is not normally a problem, as the MMBasic console output will be available on your monitor. For users who wish to retain access to the serial console, MMBasic automatically switches the console to pin 11 (GP8) for the serial transmit signal and pin 12 (GP9) for receive. It will also set the baud rate to 115,200. To access this console, you will need a USB to serial bridge that provides a TTL serial interface on one side and a USB interface on the other. These are cheap and commonly available on eBay and similar sites (search for modules using the keyword CP2102). You can also get them from the Silicon Chip Online Shop (Cat SC3437). Using a USB hub The PicoMite USB capability supports a USB hub and, by using one of these, you can connect up to four USB devices, including keyboards, mouse and Wii game controllers. You can even plug in multiple keyboards if you wish, and they will all operate in parallel, although why you would want to do that will remain a mystery! It is better to use an unpowered hub (ie, one that is powered by the Pico’s USB connector). This is because the USB protocol stack running on the Pico cannot reset the hub so, if the power on the Pico is cycled without powering down the hub, the hub will keep its previous connections and be confused when the Pico tries to reconnect. This phenomenon can also cause the hub to be confused if devices are swapped while the hub is powered. If this causes trouble, the simple solution is to cycle the power on the Pico followed by the hub, then plug in the USB devices one by one. USB mouse support The USB interface also supports a computer mouse. The main use for this is within the MMBasic program editor, but you can also use it within a program. If you use the editor with VGA/ HDMI video, colour coding turned on and a mouse connected, the mouse position will be indicated by a character in red on a white background. When you move the mouse, this highlight will move accordingly. Photo 1: you need this kind of converter to connect a standard USB keyboard, mouse, game controller or hub to the Pico or Pico 2. This example is Jaycar Cat WC7725. Clicking on the left mouse button will move the edit cursor to that position (like if you had used the arrow keys on the keyboard), while clicking the right button is the same as pressing F4 on the keyboard (ie, select and cut to the clipboard). Finally, clicking the scroll wheel is the same as using F5 (copy and paste). This means that, within the editor, you can use the mouse to position the edit cursor, cut or copy text to the clipboard, then paste it in a different location, all without touching the keyboard. This is similar to using an editor in a desktop computer’s graphical interface (such as Windows) and makes for a very productive environment. The mouse position and button states can also be read from within a program by using the DEVICE(MOUSE) function. Similarly, one or more USB Wii Classic game controllers can be used within a program using the DEVICE(WII) function to determine the position of the joysticks and buttons (you may need a USB adaptor to connect them). HDMI Pin Function To Pico Pin 1 TMDS Data 2+ (Red) GP16 (pin 21) via 220Ω resistor 2 Shield Ground 3 TMDS Data 2− (Red) GP17 (pin 22) via 220Ω resistor 4 TMDS Data 1+ (Green) GP18 (pin 24) via 220Ω resistor 5 Shield Ground 6 TMDS Data 1− (Green) GP19 (pin 25) via 220Ω resistor 7 TMDS Data 0+ (Blue + Sync) GP12 (pin 16) via 220Ω resistor 8 Shield Ground Support for external PSRAM 9 TMDS Data 0− (Blue + Sync) GP13 (pin 17) via 220Ω resistor 10 TMDS Clock+ GP14 (pin 19) via 220Ω resistor 11 Shield Ground 12 TMDS Clock− GP15 (pin 20) via 220Ω resistor 13 CEC (Consumer Electronics Control) NC (no connection) 14 ARC (Audio Return Channel) NC 15 DDS Clock (I2C Clock) NC 16 DDC Data (I2C Data) NC 17 Ground Ground 18 +5V +5V via schottky barrier diode 19 HPD (Hot Plug Detect) NC New in the RP2350 is support for PSRAM (pseudo-static RAM). This is a type of RAM chip that sits on a quad SPI bus (similar to flash memory) that can be used to increase the amount of RAM accessible by the RP2350. For MMBasic, this feature has limited application, as the RP2350 already has plenty of internal RAM (520kiB) for BASIC programs. Because it is accessed via a serial bus, PSRAM is slower than the internal RAM. However, there are programs that might need to create very large arrays and would not mind the slower access. siliconchip.com.au Australia's electronics magazine February 2025  61 An example of a module that includes PSRAM is the Pimoroni Pico Plus 2, which comes with 8MiB of PSRAM, a dramatic increase on the internal RAM of the RP2350. The PicoMite firmware supports PSRAM with the OPTION PSRAM command. When this is enabled, MMBasic will simply add this extra RAM to the general memory used for I/O buffers, strings and arrays. This is transparent to the BASIC programmer, who can then define truly enormous arrays. Clock speed We have mentioned before that the Pico’s processor can run at various clock speeds. It turns out that the RP2040 and RP2350 processors are quite tolerant of overclocking to above the stock frequency listed in the data sheet. The standard clock speed for the RP2040 is 133MHz, while for the RP2350, it is 150MHz. These are the defaults used by MMBasic. Most chips will run fine at speeds up to 400MHz, and will only experience a temperature rise of 5-6°C, which is hardly significant, so additional heatsinking is not required. When the clock speed is increased, it is also necessary for the CPU core voltage (supplied by an internal voltage regulator) to be increased in a balanced manner. The PicoMite firmware does this automatically; the programmer only needs to use the command CPU SPEED to set the clock rate. For example, with the RP2350 at its base clock speed (150MHz), the CPU core voltage is set to 1.1V, but above 300MHz, it is automatically increased to 1.4V. At these higher clock speeds, programs run proportionally faster. However, it might not be as easy as that. The main limitation on overclocking is not so much the RP2040 and RP2350 CPUs, but the layout of the Pico’s board and the memory (flash and PSRAM) attached to the quad SPI bus. All the official Raspberry Pi Pico and Pico 2 modules we have tested ran at high speeds without a problem (some as high as 400MHz), but other manufacturers might decide to use components that are less tolerant. For this reason, the degree of overclocking cannot be guaranteed and there is no way of knowing beforehand how a module may perform. The only 62 Silicon Chip certain way of discovering this is to test it yourself. Versions of the PicoMite firmware that support video output (VGA or HDMI) need to run at a specific clock frequency to generate the correct video timing and this is enforced by the firmware. For example, the HDMI firmware is fixed at 315MHz for a 640 × 480 pixel resolution and 372MHz for 1280 × 720. VGA defaults to a clock speed of 126MHz and you can only select integer multiples of this, such as 252MHz or 378MHz, if you wish to run it faster. RP2350 or RP2040 that have 16MiB of flash memory, the size of drive A: is almost 10MiB. That is a lot of storage (relatively speaking) and allows you to store many images, music tracks, configuration files, log files and more on the Pico without needing to connect an SD card. You can even store multiple versions of your program as you edit and experiment with it. Once you get used to it, you will find this feature invaluable. The amazing thing is that it is all internal to the Pico – nothing extra is required. The internal drive A: Another feature that was available in the previous firmware version but is worth mentioning is the library facility. This allows you to add your own commands, functions and features to MMBasic so that they are a permanent part of the BASIC language. To install components in the library, you write them as normal MMBasic subroutines and functions and use the LIBRARY SAVE command to transfer them to the library. They are then permanently added to MMBasic and will be available to any BASIC program running on the Pico. For example, you might have written a series of subroutines and functions to retrieve data from a specialised sensor. You could also add them to the library to perform similar functions to those that are already part of the language. This feature is very handy as, from time to time, you can find yourself thinking that it would be nice if MMBasic implemented some feature that you often need. Now you can easily add that feature yourself. One very useful feature in the firmware is drive “A:”, an internal filesystem created when the firmware is loaded. This feature was released in the previous version of the Pico­Mite firmware, but it is so handy that it deserves to be mentioned here. Drive A: is a portion of the flash memory on the Pico that is reserved to create a pseudo drive that looks like an SD card or hard disk. It has a normal file system with subdirectories and long filenames, and acts much the same as an SD card, except that you cannot remove it. Within MMBasic, you can open files on drive A: for reading/writing, rename files, create subdirectories, search for a file, list files and so on. By opening a file for random access, you can even create and operate a miniature database, all within the Pico. On the Raspberry Pi Pico 2 (with 4MiB flash), its size is just over 2MiB and this will increase if more flash memory is available. For example, with third-party boards using the Library support We recommend using a controller like this clone of a SNES controller which has a USB Type-A connector, so you don’t need to worry about adaptors. siliconchip.com.au More RP2350 features Some additional features available on the RP2350 versions include the ability to play MP3 audio files, so you can create your own MP3 player or employ high-quality music as a background to your games. Other audio formats that are supported are WAV, FLAC and MOD. For high-quality audio, you can use a VS1053 CODEC module or a MCP4822 DAC, so building your own music player is a possibility. MMBasic also includes support for the extra features in the PIO (Programmable Input/Output) peripherals provided by the RP2350. There are three in the RP2350, and each acts like a miniature CPU controlling a set of I/O pins. Within MMBasic, you can load program routines into the PIOs, set them running, and pass data to/ from them. The VGA output in the PicoMite firmware uses one PIO to generate the video. This is a good example of what you can achieve using this feature. The ability to convert GP1 into a high-speed frequency counter input is another new feature introduced with the RP2350 that MMBasic supports. This allows you to accurately measure frequencies up to half the CPU clock frequency. Firmware files When you unpack the Ver 6.00.01 firmware zip, file you might be surprised to find there are 12 firmware images in it. These are needed to cover the variations between the CPU (RP2040 or RP2350), the keyboard support (PS/2 or USB), the video output (none, VGA or HDMI) and whether it has WiFi capability or not. The features provided by the various firmware images fall into one of three categories. The first is a general embedded controller. This is where the Pico might be the brains inside a heating controller, burglar alarm etc. For this application, you might, for example, select a firmware image that supports an attached LCD panel. The second category is the self-­ contained, boot-to-BASIC computer reminiscent of the home computers in the 1970s and 80s, such as the Apple II, Tandy TRS-80, Commodore 64 and so on. This is where you turn the computer on and it boots straight to the MMBasic prompt, at which point you can enter a program, edit it and run it (no operating system is required). For this, you would select a firmware image that supports a PS/2 or USB keyboard and VGA or HDMI video output. The third category is the web/internet capable controller (ie, the WebMite) and you have two choices, using either the Raspberry Pi Pico W or the Pico 2 W. These can have an attached LCD panel for displaying data, but their best feature is that they can connect to your WiFi network to serve web pages, access the internet, send emails etc. This does not mean that you cannot use firmware optimised for one job in another role. The above categories are simply to help make sense of the available options and ultimately the choice will depend on what works best. A typical filename for a firmware image is “PicoMiteRP2350­ VGAUSBV6.00.01.uf2”, where: • RP2350 is the processor that the firmware is compiled for. • VGAUSB is the feature set supported (VGA and USB). • V6.00.01 is the version number. This will be incremented in future releases. • .uf2 is the extension, indicating a loadable Raspberry Pi Pico firmware image. Table 2 makes it easy to identify the feature set you need and the corresponding firmware image file. Conclusion The PicoMite firmware is a comprehensive BASIC programming environment for the Raspberry Pi Pico and Pico 2 that converts the Pico hardware into an easy-to-use platform for beginners and experts alike. It is completely free to download and use. In this introduction, we have covered many features of the firmware but, in reality, we have just skimmed the surface. There are many more features that are both useful and amazing. For the full story, download the Pico­ Mite User Manual and work your way through that. This manual runs to over 200 pages and covers all the features of the PicoMite firmware in detail. It even includes a beginner’s tutorial in programming in BASIC, so it is easy to get started. Both the firmware and user manual are available for download from: • https://geoffg.net/picomite.html SC • siliconchip.au/Shop/6/833 Table 2 – firmware variations Filename prefix CPU LCDs Keyboard Video WiFi Flash Default clock Max. clock PicoMiteRP2040 RP2040 Yes PS/2 None None 128kiB 133MHz 420MHz PicoMiteRP2350 RP2350 Yes PS/2 None None 256kiB 150MHz 396MHz PicoMiteRP2040USB RP2040 Yes USB None No 128kiB 133MHz 420MHz PicoMiteRP2350USB RP2350 Yes USB None No 256kiB 150MHz 396MHz PicoMiteRP2040VGA RP2040 No PS/2 VGA No 100kiB 126MHz 378MHz PicoMiteRP2350VGA RP2350 No PS/2 VGA No 180kiB 126MHz 378MHz PicoMiteRP2040VGAUSB RP2040 No USB VGA No 100kiB 126MHz 378MHz PicoMiteRP2350VGAUSB RP2350 No USB VGA No 180kiB 126MHz 378MHz PicoMiteRP2350HDMI RP2350 No PS/2 HDMI No 180kiB 315MHz 372MHz PicoMiteRP2350HDMIUSB RP2350 No USB HDMI No 180kiB 315MHz 372MHz WebMiteRP2040 RP2040 Yes PS/2 None Yes 88kiB 133MHz 252MHz WebMiteRP2350 RP2350 Yes PS/2 None Yes 208kiB 150MHz 252MHz siliconchip.com.au Australia's electronics magazine February 2025  63 ~ Tim Blythman’s NFC Programmable ~ IR Remote Control Keyfob Sometimes you need a small infrared (IR) remote control for just a handful of functions. This remote is about the smallest we’ve seen, it can hang on your keychain and you can make it yourself. It has three buttons that can trigger separate functions that are programmable wirelessly via NFC. W e’ve used the Jaycar XC3718 IR Remote Control for Arduino in several projects, most recently in the Multi-Channel Volume Control from December 2023 and January 2024 (siliconchip.au/Series/409). Its small size is a perfect match for the handful of functions that are needed in that project. Another project that supported that remote is the Eight Channel Learning IR Remote Receiver from October 2024 (siliconchip.au/Article/16669). Unfortunately, the XC3718 remote has been discontinued, so we were keen to find a replacement. Rather than having buttons that send fixed IR codes, and rely on the receiver to be able to adapt to that, we felt we could improve it. It would be handy for such a device to be programmable. The difficulty lies in adding a way to allow codes to be added or changed easily. We don’t want to massively complicate the device with a screen, more buttons etc! Nor would it be ideal to build external hardware to plug into a socket on the keyfob. Luckily, there is a neat solution. You might recall our Dynamic NFC/ RFID Tag from July 2023 (siliconchip. au/Article/15860). It combined a small chip with a PCB trace antenna to create a programmable NFC/RFID tag that can be used to hold and transfer small amounts of information. NFC (near-field communications) is a protocol based on RFID (radio frequency identification) technology. It allows communication with devices over short distances, typically up to 5cm. It’s the technology that’s used in things like contactless credit cards and transit passes. Here, we have used NFC to add 64 Silicon Chip a programming interface to the IR Remote Control. An external device such as a mobile phone becomes the programmer, and the setting can be transferred wirelessly to the Remote Control, without needing a socket or opening the case! The Remote Control is simply placed against the NFC reader on a device (eg, on the back of a mobile phone) and an app is used to control the transfer of data. The NFC chip we are using doesn’t even need external power, so the data can be transferred without a battery fitted to the Remote Control. Other features Since we are using the same chip as the earlier Dynamic NFC/RFID Tag, you can use this device similarly if you wish. The ST25DV04 has 512 bytes of EEPROM that can be used to store all manner of information, as well as the configuration for the Remote Control. Compatible chips with more storage are also available (up to at least 8kiB). The NFC protocol allows up to four different NDEF (NFC Data Exchange Format) records to be stored. Programming the Remote Control only requires a single text format NDEF record to hold the programming data; the remaining space can be used to store any other information you want. For example, an NDEF record can contain a URI to link to a webpage, or a WiFi record that contains the information needed to connect to a WiFi network. It could even contain a virtual business card, embedding data relating to contact details and phone numbers. The MIME record type could contain a complete file, such as an image, Australia's electronics magazine although its utility is somewhat limited by the small amount of memory on the chip. So you could also use the RFID Programmable IR Fob Remote as a portable NFC tag which can be used to pass around information such as webpage links or virtual business cards. The data is transferred by simply tapping the fob against an NFC reader. Circuit details Fig.1 is the complete circuit diagram. Power comes from coin cell BAT1 in a holder, which has a 22μF capacitor across it. This relatively large capacitance helps to even out the demands on the coin cell. Its life­ span can be adversely affected by high loads. A further 100nF capacitor provides local bypassing for IC2, a PIC16F15224 microcontroller. This is a fairly basic 8-bit 14-pin part, but it has PWM and timer features to allow the modulation and timing needed to implement an IR transmitter. This chip also has a very low-power sleep mode, which is handy for a device powered by a small cell. IC2 connects to the CON1 header for in-­ circuit serial programming (ICSP) at its pins 1, 4, 12, 13 and 14. A 10kW pullup on pin 4 sets the microcontroller to run normally unless a programmer overrides this signal. IC1 is the ST25DV04 dynamic NFC tag chip. Pins 2 and 3 connect to a PCB trace inductor which has a nominal inductance around 4.7μH. When combined with the chip’s internal 28.5pF capacitance, it is resonant at NFC’s 13.56MHz frequency. The trace inductor consists of eight loops on the back of the PCB. siliconchip.com.au ● Compact keyfob case: 61 × 36.5 × 15.8mm ● Can attach to a keyring ● Three buttons to trigger the IR emitter ● Power supply: CR2032 lithium coin cell ● Standby current: <1μA ● Active current: 3mA ● Status indicator: red/green LED ● IR protocols supported: NEC, Sony SIRC and Philips RC5 & RC6 ● Low battery indicator ● Integrated NFC tag ● Programmable with ST25 NFC Tap mobile app ● Can work with our Multi-channel Volume Control, Eight Channel Learning IR Remote Receiver and other projects IC1’s pin 4 is ground, while power on pin 8 is supplied by IC2’s pin 8, along with a 100nF bypass capacitor. This means that IC2 can completely power off IC1 by setting that pin low (although IC1 can still get power from its antenna in that case). Pins 6 and 7 of IC2 connect to pins 5 and 6 of IC1 for the I2C interface; each also has the requisite 4.7kW pullup to the switched power line. Tactile pushbuttons S1, S2 and S3 connect to pins 5, 3 and 2 of IC2 respectively, with the other sides connected to ground. The microcontroller applies an internal pullup to those pins so that the switch state can be detected; these pins are in a high state until the button is pressed, then it goes low. The remaining circuitry drives the infrared (IR) transmitter LED2, and a bicolour indicator LED1. LED1’s red and green junctions are in inverse parallel with other, with both in series with a 2.2kW resistor connected between pins 9 and 10 of IC2. By driving one pin high and the other low, either the red or green LED can be lit. The section around IR LED LED2 has been designed to provide high bursts of current to drive the transmitter, while at the same time enforcing a low average current draw on the coin cell. The 470W resistor and 22μF capacitor provide a local buffer, while the 100W resistor limits the peak current. With the values used, the average current draw of LED2 during transmission is 2mA, while the IR LED sees peaks of 15mA, which gives a good compromise between transmission power and the draw on the coin cell. LED1 and IC2 will also draw current while the transmitter is active, adding to the load on the cell. siliconchip.com.au This simple circuitry only switches the IR LED to be on when IC2’s pin 11 takes Mosfet Q1’s gate high, and off when pin 11 is low. The microcontroller must modulate the signal to suit the receiver detection frequency and the expected protocol. Firmware The microcontroller runs with a 2MHz instruction clock, much lower than its 8MHz maximum. That reduces its current draw when it is active by about 2mA. If the clock was much slower, the micro would struggle to generate the necessary waveforms for IR transmission. The micro is normally in sleep mode and it draws less than 1μA. Our Coin Cell Emulator from December 2023 (siliconchip.au/Article/16046) gives a reading of 0.0μA in this state! Thus, the cell life will be dominated by how much the Remote Control is used and the cell’s shelf life. When a button is pressed, the micro ‘wakes up’ from sleep mode and acts upon the button presses. When the buttons are released and transmission has ceased, the micro checks the supply voltage. If it is 2.6V or higher, the green LED in LED1 is flashed briefly; otherwise, it flashes red. This is a simple but effective battery status indicator. The IR transmitter combines a timer and PWM peripheral to generate the IR modulation, which can vary between 36kHz and 40kHz, depending on the Fig.1: the Remote Control circuit is straightforward. IC1 is powered from one of IC2’s I/O pins, allowing it to be fully powered off to minimise battery drain. The circuit around Q1 and LED2 allows LED2 to be driven at 15mA peak while limiting the draw on the coin cell to only 3mA. Australia's electronics magazine February 2025  65 NFC programming That IC1 can be programmed via its RF interface is completely transparent to the rest of the circuit. Its electrical interface is much the same as many I2C EEPROM devices, although its contents also include a header identifying the size and nature of the data, which needs to be read and validated before the data is processed. In the event that all three buttons are pressed at the same time, the micro quickly alternates the green and red LEDs to alert the user. When the buttons are released, it powers on IC1 and attempts to read an NDEF text record from its internal memory, then powers down IC1 immediately. If the read is successful and correctly formatted data is found, the codes are loaded into memory and are available for use the next time any of the buttons are pressed. During this sequence, LED1 blinks in various patterns to report on the status of the programming. We’ll discuss the text format, programming and LED colour codes in more detail later, as well as the use of the ST25 NFC Tap mobile app. Construction The PP43 fob enclosure that we are using for this project comes equipped with buttons (to actuate the switches) and a 3mm hole that suits LED1. However, since these cases are designed to house RF transmitters, they lack a hole for the IR LED. We recommend adding this hole as the first step, since it will be easier and neater to tweak the location of the IR LED than to modify the hole in the case. You can use the PCB to mark it out, or use the measurements in the Fig.2 drilling diagram. Firmly tape the two case halves together. The hole is centred on the division between the two halves. We found it worked well enough to drill both 2.2kW LED1 K Figs.3 & 4: most of the components on the front of the PCB are M3216/1206 passives or SOIC chips, so they should be easy enough to solder, even for those inexperienced with surfacemount work. Don’t forget to fit the two components on the back of the PCB. The 22μF capacitor helps protect the coin cell from brief high-current demands that could shorten its life. This diagram is shown at 175% scale. 66 Silicon Chip 22m F S1 IC1 K 100W S2 100nF PCB assembly The remaining parts can now be fitted to the PCB. They are mostly SOIC or M3216/1206-sized SMD devices, along with some through-hole parts. So it is easy enough to construct even if you have had minimal experience working with SMD parts. At a minimum, you will need a syringe of flux paste and tweezers if you are accustomed to working with through-hole parts. Your flux will likely recommend a cleaning solvent; if not, isopropyl alcohol works well for most fluxes (you can use methylated spirits in a pinch). We also recommend you keep on hand some solder-wicking braid, a magnifier and a fume extraction fan. If you don’t have such a fan, work near an open window or outdoors. Working on a uniform light-coloured background will help you find any parts that you drop, and a magnifying lamp can also be helpful. S3 100nF Q1 4.7kW BAT1 4.7kW 10kW IC2 CON1 CR–2032 10kW 22mF + Fig.2: you can use the dimensions here to locate the hole for the 5mm IR LED, or simply slot the PCB into place and use it to place marks on the case. There is a locating pin on the case to ensure that the PCB is aligned. 470W halves at the same time as long as they were securely held together. Check our photos to confirm the placement of the hole and start with a smaller pilot hole to locate it accurately. You can see that the hole sits underneath the black button. It is a 5mm diameter hole and, of course, it goes in the opposite end of the case from where a keyring would attach. The metal battery tabs included with the case are not needed since we are using a cell holder fitted to the PCB. LED2 protocol. The processor encodes the IR signal as a series of active and inactive phases at the carrier frequency. The Remote Control supports the NEC, Sony (SIRC) and Philips (RC5 and RC6) protocols. John Clarke explained all of these in detail in the article on the Eight Channel Learning IR Remote Receiver. An interrupt is triggered on each PWM cycle, providing the timing to step through the active and inactive phases of the encoded signal. LED1 is driven in time with the active phases of the IR signal, to give confirmation that transmission is occurring. The colour reflects the battery state; it flashes green if the battery is fine or red if its voltage is low. When the transmission cycle ends, the buttons are checked and the IR transmission continues if the button is held down. For most protocols, that means simply repeating the previously sent sequence, but the NEC protocol uses a special repeat packet instead. Australia's electronics magazine siliconchip.com.au Proceed to fit the parts in the locations shown in the Fig.3 and Fig.4 overlay diagrams. Start with Q1, the only SOT-23 device. Smear a tiny bit of flux paste on its pads (that will make soldering much easier) and rest it in place according to the silkscreen marking, with its leads flat against the PCB. Tack one lead, then check the positioning of the remaining pins over their pads and adjust as needed by remelting the solder. With it correctly located, solder the remaining leads and refresh the first lead by adding a tiny amount of extra flux paste before touching the iron to it. The same process can be used to solder IC1 & IC2. Before soldering any pins, it’s most important that you identify pin 1 on the IC, which is usually indicated by a dot or divot in one corner. Failing that, look for a chamfered edge along one side. With that side on the left and the writing facing you, pin 1 will be at upper left. Match each chip’s pin 1 with the markings on the PCB and Figs.3 & 4. Solder these parts in place, just like Q1. If you get a solder bridge between two pins, leave it in until all the leads are soldered. To remove a bridge, add more flux, then gently push the braid against the solder with the iron. When it draws in the excess solder, gently slide both away from the part. Now you can fit the four capacitors using the same technique. There are two 100nF parts, one adjacent to each IC. These will be thinner than the 22μF parts. One 22μF part is near the top of the PCB, while the other is on the back. The resistors will be marked with value codes (see the parts list). Make sure that the values match the silkscreen and overlay in Fig.3. There are seven resistors to be fitted. Next, solder the cell holder in place. Align it with the markings on the PCB, being sure that the cell entry faces the corner of the PCB near the BAT1 marker on the silkscreen. You can also compare its orientation to the photos. Take care to line it up correctly, since it may prevent the screw being fitted if it is too close to the edge of the board. Apply a little solder to one pad, then check its position. If you’re happy with that, apply a generous amount of solder to both pads to give mechanical strength. That completes the surface-mounting parts, so clean off any excess flux using your solvent and allow the board Parts List – IR Remote Control Keyfob siliconchip.com.au Australia's electronics magazine 1 double-sided PCB coded 15109231, 30.5 × 52mm 1 Supertronic PP43 keyfob enclosure 1 2032-size SMD coin cell holder (BAT1) [eg, Linx BAT-HLD-001] 1 CR2032 or CR2025 3V lithium coin cell (BAT1) ♦ 1 5-way pin header, 2.54mm pitch (CON1; optional, for ICSP) ♦ 3 4.3mm-high 6×6mm through-hole tactile switches (S1-S3) 1 M2×6mm Nylon machine screw and hex nut 1 lid label sticker Semiconductors 1 ST25DV04K-IER6S3 (or equivalent) dynamic RFID tag chip, SOIC-8 (IC1) 1 PIC16F15224-I/SL 8-bit microcontroller programmed with 1510923A.HEX, SOIC-14 (IC2) 1 3mm bicolour (red/green) LED (LED1) 1 5mm IR emitter LED (LED2) [TSAL6200 recommended] 1 2N7002 N-channel Mosfet, SOT-23 (Q1) Capacitors (all SMD M3216/1206, X7R ceramic) 2 22μF 10V 2 100nF 50V Resistors (all SMD M3216/1206, 1%, ⅛W) 2 10kW (code 1002 or 103) 2 4.7kW (code 4701 or 472) 1 2.2kW (code 2201 or 222) 1 470W (code 470R or 471) 1 100W (code 100R or 101) This Remote Control will easily fit in your pocket and can trigger up to three different functions. It makes the perfect compact companion for devices like the Eight Channel Learning IR Remote Receiver. The PCB slots into the case and aligns with a pin moulded into its base. The ICSP (in-circuit serial programming) header can be left in place. Note the screw to prevent the coin cell being removed by children. Both images are shown enlarged for clarity. SC7421 Kit ($25 + P&P): includes all parts listed except the two marked with ♦. The microcontroller is pre-programmed, but the NFC chip will be blank. February 2025  67 to dry thoroughly. Take the time to inspect it under magnification for bridges or bad solder joints, since they will be easier to correct now than later. Through-hole parts Bend IR LED2’s leads at right angles directly behind the body. Make sure they are bent in the right direction, such that the shorter cathode lead will go into the hole marked K on the PCB. Push it into the holes and solder one of the leads so that the lens points out parallel to the PCB, then trim both leads (leave the unsoldered one long enough to solder later). You can now place the PCB in the bottom of the case and confirm that LED2 lines up with the hole. Having only one lead soldered will make it easier to adjust the position. Once it is aligned with the hole, solder the other lead. Next, fit the three tactile switches. These must be no more than 4.3mm high; any taller and they would be permanently depressed by the case buttons. They must also be mounted flat against the PCB for the same reason. You can now use the PCB along with the top half of the case to check the position of the bicolour LED, LED1. Its top lip should sit no more than 7mm above the PCB. The K cathode marking on the PCB corresponds to the cathode of the red LED in the package. Test this with a multimeter set to diode mode. When the LED lights up red, the pin connected to the multimeter’s black lead is the one that should be placed in the hole marked K. If you have a pre-programmed chip for IC2, you can fit a coin cell and test the LED’s operation. Pressing and holding one of the buttons should cause the LED to flicker and flash green, assuming a fresh cell has been fitted. If it’s red instead, you should swap its leads. Programming IC2 The hole shown in Fig.2 allows the IR emitter LED to poke out through the end of the case, as seen here. Note the location relative to the buttons. 68 Silicon Chip The ICSP header is only needed if you have to program a blank chip for IC2. A standard height (11mm total) header strip will not foul the case when fitted, so we recommend that you solder this in place and leave it; it will not affect the operation at all. The software we use for programming PICs is Microchip’s IPE (integrated programming interface), which can be downloaded as part of the MPLAB X IDE from www.microchip. com/en-us/tools-resources/develop/ mplab-x-ide A Snap, PICkit 4 or PICkit 5 programmer can be used for programming. A coin cell should be fitted to provide power if needed; for example, the Snap cannot provide power. Connect the programmer to the CON1 header, aligning the pin 1 arrows on the programmer and PCB. Choose the PIC16F15224 as the part, click connect and confirm that communication is established. If powering IC2 from the programmer (PICkit), you will need to enable that before clicking the connect button. Load the HEX file, program it into the chip and check that Australia's electronics magazine it verifies correctly. The LED should briefly flash green as programming finishes or when you disconnect the programmer. Final assembly Fit the cell (+ side up) and secure it with the machine screw and nut through the adjacent hole, feeding the screw from the bottom of the PCB. The case should neatly snap together around the PCB. Check that the buttons actuate correctly and the LED lights up as described earlier. If the LED lights when no button is pressed, one of the switches may be stuck. Check the solder joints on the back of the PCB and trim down any that are too tall. The PCB should slot neatly into the case and sit flat. The default programming is to suit the Multi-channel Volume Control, with the red and blue buttons increasing and decreasing the volume, respectively (think hotter and colder!). The black button controls the mute function. The default codes are for an NEC device at address 0, with command codes 21 (red), 7 (blue) and 67 (black). You might see these values reported differently on some systems. An example is the Micromite or PicoMite IR decoder, which will report codes 168, 224 and 194 respectively because it uses a reversed bit order. The device code is still reported as 0 as the bits are the same when reversed. Simple hardware, such as the IR Keyboard we created in August 2018 (Turn any PC into a media centre; siliconchip.au/Article/11195), can also be used to interact with this and other IR transmitters. The excellent irremote Arduino library makes it easy to receive all sorts of IR signals. Programming the NFC chip To use the Remote Control with other hardware, you will need to program it to use new codes. First, you need to determine the protocols and codes to use. If you do not have a manual or other reference for these, hardware similar to the IR Keyboard can be used to read codes from an existing remote. The Arduino irremote library comes with a sample sketch called “ReceiveDump”, which reports the protocol and details of received IR signals. We used this extensively during our testing of the Remote Control to check that it was delivering the correct codes. siliconchip.com.au The NDEF text record required to program the Remote is much the same as a CSV (comma separated variable) file. The first field in each row is a code that identifies the protocol; the codes and protocols are listed in Table 1. The next field is the address code in decimal, followed by the command or data field, also in decimal. You will need the ST25 NFC Tap mobile app and a device that has NFC capabilities. We used an Android phone and downloaded the app from the Play Store (siliconchip.au/link/ ac38). We haven’t tested it, but the app also appears on the Apple App Store (siliconchip.au/link/ac39). There may be other apps that will work; we previously tried the NXP TagInfo and TagWriter apps. Any app that can read and write NFC NDEF records should work. Screen 1 shows the welcome screen Table 1 – protocol codes for RFID Programmable IR Fob Remote for the ST25 NFC Tap. Hold the back of the Remote Control against the back of the phone (or other device). Screen 2 shows what you will see when the NFC tag in the Remote Control is read. Tap the NDEF tab to see Screen 3, then the blue button at bottom right and select the option to add a plain NDEF text record to the tag. Screen 4 shows the text field; you simply enter the codes and values as shown, pressing the Enter key between each line. The red button is on the first line, blue on the second and black on the third. When you have finished making changes, save the new text to the tag using the save button at top tight. The ST25 app uses a line feed (LF, ASCII 0xA) as the line separator, so if you use a different app, make sure that this is the same. The values shown in Screen 4 are equivalent to the default settings provided by the Remote Control. You can also add an extra column with notes or comments about each line. Just be sure to separate it from the other values with a comma and be aware of the limited memory available. There are many other things that you can do with the app. For example, Screen 3 has a copy button at top right that can be used to clone tags. Screen 1: the ST25 NFC Tap comes from STMicroelectronics, who produce the ST25 range of chips. It’s a good idea to open the app before scanning a tag. Otherwise, your device might open a different app when the tag is brought near. Screen 2: when a tag is first scanned, some basic information is provided, including the serial number. The tabs along the top provide more options. Screen 3: a blank tag will have no NDEF records yet. The blue button at the bottom right of this page allows records to be added. siliconchip.com.au Addr. Protocol Code bits Cmnd. bits NEC N 8 8 Philips RC5 5 5 6 Philips RC6 6 8 8 Sony 12-bit S 5 7 Sony 15-bit T 8 7 Sony 20-bit U 13 7 Australia's electronics magazine February 2025  69 The Memory tab can be used to read, write or erase the tag’s EEPROM. If a tag is not working correctly, you can try erasing the EEPROM and rewriting the settings. The Memory tab also allows the tag contents to be read from or written to a file. The DEFAULT.BIN file in the downloads for this project can be written to the tag to similarly reset it to containing the default IR codes. You can add other NDEF records to the tag. In our experience, a device will typically act on the first valid record that it recognises. So if you wish to add an alternative record for people to scan (such as a WiFi handover record or URI record), we suggest adding it before the text record for the Remote Control. Non-text records are simply ignored by the microcontroller. Remote Control use While the above process writes a set of codes to IC1, these are not automatically loaded. Instead, the buttons are used to do this under user control. Pressing all three buttons at the same time will trigger the read sequence. While the buttons are held down, the LED will alternate red and green. Releasing the buttons starts the reading process. Firstly, IC2 checks if IC1 is present and if it is not, the LED flashes red once for about a second. If IC1 is present but no NDEF text record is found, then nothing is shown on the LED. This can be expected with a blank NFC chip, such as if construction has just been completed. If a valid NDEF text record is found, the LED will flash once for each of the three button codes, in order from left to right, green if it is valid or red if it is not. If it is valid, the code will be updated; otherwise, the current code is kept. After this, normal operation resumes and you should see a brief flash indicating the battery status. In general, the code requires a valid protocol code as per Table 1. The address and command values provided must fit within the number of bits prescribed. For example, a value of 256 for either the address or command of an NEC code would be invalid, since these are eight-bit values and 256 requires nine bits to encode. After programming, the Remote Control operates with the new codes. Simply push each button and the corresponding IR code will be sent for as long as the button is held down. If a second button is pressed, while the first is still down, the first code will continue until the first button is released, then the second code will start. If you start to see the LED flashing red instead of green during operation, then the battery is getting low; down around 2.6V. The circuit itself will function down to near 2.0V, but IR range will suffer due to the lower SC current provided to LED2. Completion We’ve created a label for the keyfob shown below. There’s space for functions to be added in permanent marker below each button on the sticker. The kit for this project will include a sticker with this artwork – attach it to the front of the fob case. As we noted, the files in downloads include a DEFAULT. BIN file (containing the values seen in Screen 4) that can be written directly to the EEPROM, if you wish to experiment with it. The downloads also include the HEX file for programming the microcontroller and the MPLAB X project files. Screen 4: the text shown here matches the default settings of the remote control. The text can be stored to the tag with the SAVE button (floppy disk icon) at upper right. 70 Silicon Chip Screen 5: the Memory tab provides access to low-level read and write functions. You can also store the tag contents to a file. Australia's electronics magazine There are spaces on this label to add a legend for each button so you know what it does. This will be provided as a sticker in kits purchased from the Silicon Chip Shop, and will also be available to download. siliconchip.com.au SOnline ilicon Chip Shop Kits, parts and much more www.siliconchip.com.au/Shop/ Compact OLED Clock & Timer September 2024 Short-Form Kit SC6979: $45 siliconchip.au/Article/16570 This kit includes everything needed to build the OLED clock, except the UB5 Jiffy box and Li-ion cell. Dual Mini LED Dice August 2024 Micromite-Explore 40 October 2024 Complete Kit SC6991: $35 SMD LED Complete Kit SC6961: $17.50 TH LED Complete Kit SC6849: $17.50 siliconchip.au/Article/16418 siliconchip.au/Article/16677 Includes either 3mm through-hole or 1206sized SMD LEDs. Choice of either white or black PCB. CR2032 coin cell not included. Includes the PCB and all onboard parts. Audio Breakout board and Pico BackPack are sold separately. ESR Test Tweezers Mains Power-Up Sequencer Complete Kit SC6952: $50 February-March 2024 June 2024 siliconchip.au/Article/16289 This kit includes everything needed to build the ESR Test Tweezers. Does not include the CR2032 (or CR2025) coin cell or optional 5-pin header CON1. USB-C Serial Adaptor Complete Kit SC6652: $20.00 June 2024 siliconchip.au/Article/16291 Includes the PCB, programmed microcontroller and all other parts required to build the Adaptor. Hard-To-Get Parts SC6871: $95 siliconchip.au/Series/412 The critical components required to build the Sequencer such as the PCB, micro etc. Other components need to be sourced separately. → Subscribers receive a 10% discount on all purchases, except for subscriptions (postage is not discounted). → Prices listed do not include postage. Postage rates within Australia start at $12, rates are calculated at the checkout. Precision Electronics Part 4: Signal Switching In this fourth article in this series, we will look at how to extend the current measurement range of the circuit we’ve been working on so far. To achieve that, we’ll have to switch between two or more shunt resistors. By Andrew Levido I n the previous article in this series, we developed our current-sense circuit (Fig.1) to the point where we could measure a 0–1A current in the highside of a hypothetical power supply with a worst-case 25°C precision of around ±0.03%. Over the temperature range of 0–50°C, this error rose to just under ±0.2%. That was the analog error only; it did not include any errors introduced by the analog-to-digital converter (ADC), which we will go into in a future article. To achieve this level of precision, we were planning to apply a fixed gain calibration and a dynamic zero offset calibration in software, using the two switches shown in Fig.1. This level of precision would allow us to meaningfully measure current from 1A down to a few tens of milliamps, since our resolution is limited to ±2mA. To achieve the microamp or better current sensitivity that we desire, we determined that we needed to switch in different shunt resistors to provide a series of current ranges. So far, we have been using a 0.1W shunt resistor for the 1A range, which develops 100mV across it at full scale. This requires a differential-mode gain of about 25 to get our signal to a nominal 2.5V level for the ADC. Assuming our power supply has some voltage headroom, there is nothing stopping us from increasing the shunt resistance by an order of magnitude, so it drops 1V at full scale. We can then decrease the gain to a nominal value of 2.5. The power dissipation in the shunt resistor will increase accordingly, but any offset errors we see on the input side, including those that change with temperature, will be smaller in relation to the full-scale signal. That will improve the overall precision of the circuit. This will be important as the complexity – and therefore sources of uncertainty – of the circuit increases. Table 1 (below) shows the ranges we could potentially implement, the current resolution we could expect, and the shunt resistors we would need for each one. This table assumes we can maintain the ±0.2% error we have achieved so far. It suggests we should be able to realise our sub-microamp resolution ambitions if we can maintain a similar level of precision as we did with our previous efforts. Before we get into the details of how we will switch the shunt resistors in and out, and the impact that will have on precision, we should look at the options available for signal switching. There are basically only two options: 1. We can use a mechanical switch such as a signal relay if we want to control it with a microcontroller. 2. Alternatively, we can use some form of electronic analog switch, which will most likely be based on field-effect transistors (FETs). Signal relays Signal relays are similar to power relays, but their design is optimised for low on-resistance and high linearity instead of power handling. They are usually rated for currents of 2A or less and for switching voltages under 50V. These aren’t hard and fast definitions; there is plenty of grey area between the top end of signal relays and the bottom end of power relays. Relays have the advantage of excellent on-resistance linearity with applied voltage and temperature. They have a very high off-resistance (essentially infinite) and virtually zero leakage since the switching path is electrically isolated. Typical initial on-resistances for signal relays range from about 10mW to 200mW. The word “initial” is important here – the on-resistance of signal relays generally increases with the number of operations, as shown on the right side of Fig.2. This is an extract from a data sheet for Panasonic TQ-­series relays, although all brands behave in more or less the same way. It’s also worth noting that the operating and release voltages, shown on the left, also worsen slightly with time Table 1 – current ranges using a fixed 0.2% error Fig.1: this is the circuit we designed last time. It is capable of a measurement resolution of a couple of milliamps; to measure lower currents, we need to switch ranges somehow. 72 Silicon Chip Current range Resolution (±0.2%) RS (gain ≈ 2.5) 1.00A ±2.0mA 1.00W 100mA ±200µA 10.0W 10.0mA ±20µA 100W 1.00mA ±2.0µA 1.00kW 100µA ±200nA 10.0kW 10.0µA ±20nA 100kW 1.00µA ±2nA 1.00MW Australia's electronics magazine siliconchip.com.au Fig.2: relays make great signal switches, but we should be aware that their contact resistance and operating voltages deteriorate with the number of operations. as the relay’s mechanical parts wear. The failure rate data for the TQ relays suggests that 1% will have failed after 3.5 million operations and 10% after about 10 million operations when switching 5V at 1mA into a resistive load. That is a lot of operations, so it probably will not be of concern to the designer, but relays do have a limited life. Panasonic deserves a lot of credit for publishing very comprehensive data for their relays. Not all manufacturers are this up-front in their data sheets. Relays are not always good for very high-frequency applications, since their stray inductance and capacitance can be relatively high. Specialised high-frequency relays are available if you need them. For precision circuits, we often use reed relays, which can have very low stray capacitance (0.5pF) and are available with internal electromagnetic screens which can help minimise induced noise or be used as a “guard” electrode when measuring minuscule currents. A reed relay is essentially a reed switch that’s actuated by an electromagnet. On the downside, relays are usually somewhat bulky and expensive, so designers tend to use them only when their unique characteristics are absolutely necessary. Instead, they generally use more compact and cheaper analog switches where they can (which offer the added benefit of an almost indefinite lifespan). Analog switches Analog switches are typically built from Mosfets since their drain-source resistance is controllable via gate voltage and the channel can conduct current in either direction. Because a Mosfet’s channel resistance is non-­linear with applied voltage, most analog switches use back-to-back N-channel and P-channel Mosfets. The parallel on-resistance of the two devices is more linear than either one alone, as illustrated in Fig.3. The Mosfet substrates are connected to the analog power rails to maximise linearity. By the way, if you are familiar with using discrete Mosfets as high-power switches, you may be puzzled by the comment that they can conduct current in either direction. That’s Fig.3: most analog switches use parallel N-channel and P-channel Mosfets to minimise the effect of the non-linear channel onresistance of Mosfets. because power Mosfets usually have an unavoidable ‘body diode’ in parallel with the channel in one direction, meaning they can only really switch current in one direction by themselves. When fabricating multiple Mosfets on a single substrate as in a CMOS integrated circuit, the body diode is still there, but it is possible to choose where one end of that diode connects. Depending on what potential it is connected to, that body diode may never conduct under normal conditions, so it can effectively be ignored. Thus, Mosfets in ICs (as well as the fairly unusual four-terminal discrete signal types that expose the bulk connection separately) can operate bi-­ directionally, similarly to JFETs. The NMOS+PMOS architecture is used in switches such as those in the industry-standard DG41x series. Fig.4 shows the simplified circuit of one channel, extracted from the data sheet. As well as the back-to-back switching Mosfets, you can see a level shifter, which allows the control voltage (VIN) and logic supply (VL and GND) to be anywhere within the V+ to V– analog supply range. Fig.4: this simplified diagram of one switch from a DG41x series analog switch shows the parallel N-Channel and P-channel Mosfets. The level shifter allows the control signal and logic supply to be anywhere within the analog voltage range. siliconchip.com.au Australia's electronics magazine February 2025  73 Fig.5: the onresistance characteristic of this DG41x series analog switch shows the non-linearity and temperature dependence of the on-resistance. The DG41x series switch on-­ resistance characteristic with ±5V rails is shown in Fig.5. The nominal on-­ resistance is anywhere between 10W and 20W, depending on temperature, and varies about 30% as the signal voltage changes. The imprecision associated with analog switches can best be understood by looking at the on- and offstate equivalent circuits in Fig.6. In the on state (left), the on-resistance Ron appears in series with the source resistance Rsource to produce a voltage divider with the load resistance Rload. As we have seen, Ron is non-linear and temperature-dependant, so the voltage error due to this divider will be uncertain. For this reason, we usually try to keep the load resistance as high as possible with respect to the sum of Rsource + Ron. In the on state, a leakage current Id(on) will produce a DC error voltage proportional to Rload in parallel with Rsource + Ron. This can be minimised by keeping the source impedance as low as possible. The channel capacitance Cd(on) will appear in parallel with Cload and form an RC low-pass filter with Rsource + Ron – another reason to keep Rsource low if you can. In the case of the DG41x family of switches, Ron can be up to 35W, Id(on) can be up to ±15nA and Cd(on) is typically 35pF. In the off state (shown in Fig.6), the leakage current Is(off) will produce a DC voltage across Rsource, and ID(off) will produce a voltage across the load impedance, Rload. The latter can be more difficult to manage, since we generally want to use a high load impedance for reasons described above. The DG41x switches have Fig.6: these equivalent circuits show the leakage currents and internal capacitances present in analog switches in the on and off states. 74 Silicon Chip Australia's electronics magazine off-state leakage currents (ID(off) and Is(off)) of up to ±15nA, and CD(off) can be up to 9pF. Charge injection is another concern with analog switches, especially those with a low Ron value. Achieving low Ron requires physically large Mosfets, which have higher levels of gate capacitance. Whenever the gate of the Mosfet switches, this gate capacitance is charged or discharged via the drain and source. This means a charge is injected into the signal path when the devices switch. The resulting voltage disturbance is a factor of the switch output and load capacitance, as shown in Fig.7. The charge is injected via Cq and appears as a voltage spike or dip at the output, as CD(ON) in parallel with Cload charge or discharge. Each DG41x switch has a charge injection of 5pC. If the external load capacitance were 50pF, this would result in a voltage spike or dip of 59mV every time the switch changes state. This could very well create a significant ‘pop’ when switching audio signals – something to be aware of. Of course, the input signal to this type of analog switch must stay within the power rails. For switches with back-to-back complimentary Mosfets, the signal voltage can extend all the way to both rails. There are some newer analog switches with very good Ron linearity. These appear to use a single N-­ Channel Mosfet with a very flat Ron characteristic. Fig.8 shows the on-­ r esistance characteristic for one channel of the TMUX821x series of analog switches from Texas Instruments (TI). The on-resistance is very flat all the way Fig.7: charge injection can cause voltage transients in the signal path when an analog switch is opened or closed. The effect is usually worse in low-Rds(on) switches, where the gate capacitance (Cq) is higher. siliconchip.com.au Fig.9: this interesting class of optically coupled analog switches may be suitable for some applications. They can switch a few hundred milliamps and provide good isolation between the control signal & switch. Fig.10: this circuit ensures the current-carrying switches (S1a, S2b and S3c) are not in the measurement path. That’s helpful since the voltage drop across them is unpredictable. The shunt voltage sensing switches (S1b, S2b and S3b) carry no appreciable current, so the voltage drop across them will be minimal. from the negative supply up to a few volts short of the positive supply. With the ±15V supplies shown here, the upper limit on signal voltage is around 10V to 12V, depending on how much non-linearity you can put up with. Before we leave this discussion of analog switches altogether, I want to mention one more type that I have found useful in certain applications: optically coupled Mosfet switches, such as that shown in Fig.9. These are a bit of a hybrid between relays, analog switches and opto-­ couplers. They use inverse series Mosfets (for polarity independence), which are switched optically via an internal LED. A typical example, the AQY282GS, is rated for switching up to 60V (AC or DC) at 0.8A. It has a maximum on-resistance of 0.8W at 25°C, rising to twice that at 85°C. The manufacturer does not provide any linearity data, but we can assume it will not be great. They do have good input–output isolation (1000MW and 1.5pF), but up to 1µA of leakage between the output terminals when off. These devices are Fig.11: this circuit configuration was used to obtain the results described. Not shown are the DIP switches used to control the analog switches & relay coils. not super-fast – the switch-on time can be up to 5ms and switch-off up to 0.5ms. They are driven exactly like you would drive an optocoupler. Updating our design So, armed with all this knowledge, how do we go about designing our multi-range current sensing circuit? Whatever type of switch we use to select the shunt resistors, it will add a material and unpredictable voltage drop. We therefore can’t just put the Fig.8: the onresistance characteristic of the TMUX821x is remarkably flat for signal voltages from the negative rail up to a couple of volts short of the positive rail. This suggests a single Mosfet is being used. Note how the Rds(on) is still highly temperaturedependent. siliconchip.com.au Australia's electronics magazine switching element in series with the shunt and measure the voltage across them both. Instead, we need to use the topology shown in Fig.10. One of the “a” switches (S1a, S2a or S3a) is closed to select one of the shunt resistors, depending on the chosen current range. The corresponding “b” switch is also closed, connecting the relevant shunt resistor to the instrumentation amplifier’s inverting input. Since this input has a very high impedance, very little current flows through the “b” switch, so its on-­ resistance and non-linearity are largely irrelevant. The voltage drop across the active “a” switch, where appreciable current does flow, is not in the measurement path, so it does not impact the reading. As a bonus, we get the zero-­ calibration state for free. If we close any “b” switch that does not have its corresponding “a” switch closed, we effectively short the inamp’s inputs together via that shunt resistor, which will have close to zero voltage across it. I decided to build a version of this circuit with 1A, 10mA and 100μA full-scale ranges. In a real application, you would probably implement February 2025  75 Fig.12: this graph, copied from the manufacturer’s data sheet, shows the various leakage currents in the TMUX821x series of analog switches. As you would expect, they increase rapidly with temperature. a range for each decade, but I wanted to keep things manageable for my experiments. I chose to use relays for S1a and S2a (the 1A and 10mA range respectively), although an analog switch could certainly be used for the latter range. The 100µA range (S3a) and the three “b” switches used analog switches. This meant I could get away with just one quad analog switch package. The key parts on the test board are shown in Fig.11. A 3.3V logic power supply and the dip switches driving the relays and analog switch control lines are not shown. I used a 1% tolerance 3W resistor for R1, since high-precision power resistors are super expensive. I did, however, select a resistor with the best tempco (±20ppm/°C) that I could afford, since we can’t trim out the temperature drift as easily as we can trim out the absolute resistance error. It is easier (and cheaper) to get high-precision 100W and 10kW resistors, so I chose devices with 0.1% tolerance and 10ppm/°C tempcos. The relays I used were 3.3V coil 1A relays from Fujitsu’s SY series that I happened to have on hand. The primary concern with selecting the analog switch was to get a unit with a sufficient voltage rating, since the supply voltages would be +24V and -5V, giving a total supply span of 29V. DG41x-series switches are limited to a supply voltage span of 12V. Figs.13 & 14: the voltage error due to analog switch leakage is calculated by substituting the on and off equivalent circuits. As discussed in the text, the 600pA source can be ignored but the other two will cause an error. This diagram shows the 100µA range where the error is worse than the others. The simplified version is shown at right; it summarises the sources of error. 76 Silicon Chip Australia's electronics magazine The TMUX821x range is good to ±50V, which is more than enough. The TMUX8212 includes four independent normally open switches, which is perfect. From Fig.8, we can see that the analog switch on-resistance is under 5W at room temperature, with about ±1W change over the 0°C to 50°C range we are designing for. Fig.12 shows the leakage currents. At 50°C, the worst case for our design, Id(on) is ±10pA or less, while Id(off) and IS(off) are each less than ±300pA. Those figures are for ±36V supplies, so with our lower supply voltages, the values we experience are likely to be lower. However, in the absence of more detailed data, we have little choice other than to use those figures. I used the cheaper of the two instrumentation amplifiers that we tested last time, the INA821, but this time with the gain set to about 2.5. Like last time, the op amp is powered from +24V and -5V rails. Error budget The easiest way to manage the error budget for a circuit with several configurations like this one is to calculate a separate budget for each range. The process is exactly the same as for the examples we created in previous instalments, except for the errors introduced by the analog switches. We can distil the impact of the analog switches down to a single voltage error by substituting them with their equivalent circuits, as shown at the top of Fig.13. Here, the circuit is shown with the 100µA range active (with the two analog switches closed and both relays open). Fig.14 shows the same configuration with the leakage current sources consolidated. The 1W and 100W resistors disappear, since they are in series with current sources, which themselves have very high (theoretically infinite) source resistances. This simplification leaves us with three potential sources of leakage-induced voltage error. The 600pA current feeding into the power rail on the source side of the shunt resistor can be ignored, since this current must flow either back into the regulator (where it does not matter), or through the shunt to the load (where it will be measured as part of the load current). The 10pA source on the load side siliconchip.com.au Table 2: 100μA range At Nominal 25°C Abs. Error Rel. Error 0-50°C (Nominal ±25°C) Error Nominal Value Shunt Resistor: ERA-6ARB103V (±0.1%, 100ppm/˚C) 10kW Abs. Error Input voltage error due to shunt 1V 1mV 0.10% 0.25mV 0.025% Input voltage error due to switch leakage 0V 6.2μV 0% 0mV 0% Input voltage error due to bias (Ios ±0.5nA, ±20pA/˚C) 0V 5μV 0% 5μV 0% InAmp: INA821 (Vos ±35µv, 5µV/˚C) 0V 35μV InAmp Input Voltage error total (Sum of Lines 2-5) 0V 1mV 0.10% 0.380mV 0.038% InAmp Gain Resistor Rg: ERA-6ARB333V (±0.1%, 10ppm/˚C) 33kW 33W 0.10% 8.3W 0.025% 0.10% InAmp Gain Error (0.015% ±35ppm/˚C) Rel. Error 0.025% 125μV 0.02% 0.088% InAmp Gain (Line 7 × Line 8) 2.5 0.0029 0.12% 0.0028 0.113% Vout DM (Line 6 × Line 9) 0V 5.5mV 0.22% 3.8mV 0.151% Vout CM (20V, 100db, ±1.5db over 0-50˚C) 0V 200μV Vout (Line 10 + Line 11) 0V 5.7mV Table 3: 10mA range 37.7μV 0.23% At Nominal 25°C Abs. Error Rel. Error 3.8mV 0.152% 0-50°C (Nominal ±25°C) Error Nominal Value Abs. Error Shunt Resistor: ERA-6ARB101V (±0.1%, 10ppm/˚C) 100W Input voltage error due to shunt 1V 1mV 0.10% 0.25mV 0.025% Input voltage error due to switch leakage 0V 95nV 0% 0nV 0% Input voltage error due to bias (Ios ±0.5nA, ±20pA/˚C) 0V 50nV 0% 50nV 0% InAmp: INA821 (Vos ±35µv, 5µV/˚C) 0V 35μV InAmp Input Voltage error total (Sum of Lines 2-5) 0V 1mV 0.10% 0.3751mV InAmp Gain Resistor Rg: ERA-6ARB333V (±0.1%, 10ppm/˚C) 33kW 33W 0.10% 8.3W 0.10% InAmp Gain Error (0.015% ±35ppm/˚C) Rel. Error 0.025% 125μV 0.02% 0.025% 0.088% InAmp Gain (Line 7 × Line 8) 2.5 0.0029 0.12% 0.0028 0.113% Vout DM (Line 6 × Line 9) 0V 5.5mV 0.22% 3.8mV 0.150% Vout CM (20V, 100db, ±1.5db over 0-50˚C) 0V 200μV 37.7μV 0.038% Vout (Line 10 + Line 11) 0V 5.7mV 3.8mV 0.152% Table 4: 1A range 0.23% At Nominal 25°C Abs. Error Rel. Error 0-50°C (Nominal ±25°C) Error Nominal Value Abs. Error Shunt Resistor: VMP-1R00-1.0-U (±0.1%, 20ppm/˚C) 1W Input voltage error due to shunt 1V 10mV 1% 0.5mV 0.05% Input voltage error due to switch leakage 0V 4.5nV 0% 0nV 0% Input voltage error due to bias (Ios ±0.5nA, ±20pA/˚C) 0V 500nV 0% 500nV 0% InAmp: INA821 (Vos ±35µv, 5µV/˚C) 0V 35μV InAmp Input Voltage error total (Sum of Lines 2-5) 0V 10mV 1% 0.625mV 0.063% InAmp Gain Resistor Rg: ERA-6ARB333V (±0.1%, 10ppm/˚C) 33kW 33W 0.10% 8.3W 0.025% 1% InAmp Gain Error (0.015% ±35ppm/˚C) Rel. Error 0.05% 125μV 0.02% 0.088% InAmp Gain (Line 7 × Line 8) 2.5 0.0029 0.12% 0.0028 0.113% Vout DM (Line 6 × Line 9) 0V 28mV 1.12% 4.4mV 0.175% Vout CM (20V, 100db, ±1.5db over 0-50˚C) 0V 200μV Vout (Line 10 + Line 11) 0V 28.2mV siliconchip.com.au Australia's electronics magazine 37.7μV 1.13% 4.4mV 0.177% February 2025  77 of the shunt will cause an error since this current can flow into the load without being measured. This is the equivalent of under-reading the load current by 10pA, so it will result in a voltage error of up to 100nV (10pA × 10kW) at the op amp input. The 610pA leakage will similarly cause a voltage error, but this time the error will be seen across the series combination of the shunt resistance and the switch on-resistance. This error will be 6.1µV (610pA × [10kW + 6W]). The total voltage error introduced by the switches will therefore be ±6.2µV, which you can see in line 3 of the error budget table for the 100µA range. This is a meaningful amount compared with the instrumentation amplifier’s ±35µV input offset voltage. Given the relatively high shunt resistance, we also have to account for the impact of the instrumentation amp’s input bias currents. The difference between these currents (the input offset current) will cause an additional voltage error across the source resistance. The INA821’s data shows the maximum input offset current is ±0.5nA at 25°C, with a tempco (estimated from the graphs) of ±20pA/°C. This will result in a voltage error of ±5.0uV at 25°C with an additional ±5.0µV over the 0°C to 50°C operating range. This error, shown on line 4 of the error budget, is also similar in magnitude to the input offset voltage. Other ranges As you might expect from the above calculations, the error voltages will be lower for the other ranges where the shunt resistances are lower. I went through the same exercise for these ranges and came up with error voltages due to switch leakage of 4.6nV for the 1A range and 95nV for the 10mA range, plus input offset current errors of 500pV and 50nV, respectively. These are included in the relevant error budget tables (Tables 2-4), but are frankly so small as to be irrelevant given the instrumentation amplifier’s ±35µV offset voltage. The rest of the error budget tables are calculated as we did the last time. The upshot is a worst-case untrimmed 25°C error of ±1.13% for the 1A range and ±0.23% for the 10mA and 100µA ranges. The big difference is due to the 1% tolerance of the 1W shunt compared to the 0.1% tolerance of the other two. Over the operating temperature range, the 1A range has an additional ±0.18% error, with an extra ±0.15% for the 10mA and 100µA ranges. Recall that the circuit in the previous article had a worst-case untrimmed 25°C error of 0.65% with ±0.28% additional error over temperature. This circuit is better (except on the 1A range, where the shunt tolerance range has doubled) because we have used better-tolerance resistors and have reduced the instrumentation amp gain by a factor of 10. Testing As usual, I built the circuit and carefully measured its performance. The results are shown in the tables opposite (Tables 5-7). Again, we achieved much better performance than the worst-case calculations would suggest. The measured untrimmed errors were ±0.5%, ±0.06% and ±0.18% for the 1A, 10mA and 100µA ranges, respectively. To calculate the trimmed error results, I used a gain correction based on the line of best fit, but just used the measured zero-current output value as the offset, mimicking the dynamic offset correction process. The trimmed errors were ±0.036%, ±0.054% and ±0.031% for the three ranges – very similar to the values we achieved previously. The errors over the operating temperature range are around ±0.11%, assuming the offset calibration eliminates the offset component of the input-side temperature drift error. It would be around ±0.18% otherwise. We can probably say that, across all ranges, our circuit achieves better than ±0.06% error at 25°C and ±0.25% over the operating temperature range. This is on par with the performance we saw last time, and means we have more-or-less met the expectations we set in Table 1 for these ranges. As a paper exercise, I calculated the error budget for a possible 1µA full-scale range, assuming a 1MW 0.1% ±10ppm shunt. The worst-case untrimmed error at 25°C is ±0.35%, and the total error over the temperature range would be within ±0.6%, which is pretty good. With trimming, we could probably assume a current resolution in the order of ±5nA. This is about as low as I would go with this circuit. Once we get down to measuring such small currents, things become very challenging. A next obvious step will be to look into the analog-to-digital conversion process, to complete our theoretical PSU current-sensing design. However, in all of our work so far, we have entirely ignored one important source of uncertainty and error: noise. This is an interesting but complex topic that we need to know about before moving on. So we will cover it SC next time. Raspberry Pi Pico W BackPack The new Raspberry Pi Pico W provides WiFi functionality, adding to the long list of features. This easy-to-build device includes a 3.5-inch touchscreen LCD and is programmable in BASIC, C or MicroPython, making it a good general-purpose controller. This kit comes with everything needed to build a Pico W BackPack module, including components for the optional microSD card, IR receiver and stereo audio output. $85 + Postage ∎ Complete Kit (SC6625) siliconchip.com.au/Shop/20/6625 The circuit and assembly instructions were published in the January 2023 issue: siliconchip.au/Article/15616 78 Silicon Chip Australia's electronics magazine siliconchip.com.au Measured Data Untrimmed Error Trimmed Error I (mA) Vout (mV) Absolute (mV) Relative Absolute (mV) Relative 0.0000 -3.480 -3.48 -0.14% 0.00 0.000% 9.0707 223.366 -3.13 -0.12% 0.38 -0.001% 20.1549 500.500 -2.76 -0.11% 0.78 -0.007% 29.1271 723.712 -3.58 -0.14% -0.02 0.006% 38.4297 955.808 -3.77 -0.15% -0.18 0.016% 49.9203 1243.030 -3.46 -0.13% 0.16 0.018% 58.7674 1464.160 -3.24 -0.13% 0.41 -0.021% 72.2879 1801.780 -3.23 -0.13% 0.47 0.031% 80.1932 1998.150 -4.25 -0.17% -0.53 0.000% 86.6674 2160.290 -3.77 -0.15% -0.03 0.000% 95.1638 2373.240 -2.97 -0.12% 0.79 0.000% Silicon Chip Binders REAL VALUE AT $21.50* PLUS P&P Table 5 – 100μA range (Vcm = 20V). Measured Data I (mA) Untrimmed Error Trimmed Error Vout (mV) Absolute (mV) Relative Absolute (mV) Relative 0.00000 -0.773 -0.77 -0.03% 0.00 0.000% 0.98420 244.922 -0.83 -0.03% -0.06 -0.002% 1.98602 495.514 -0.39 -0.02% 0.38 0.015% 2.93840 733.371 -0.34 -0.01% 0.43 0.017% 4.18878 1045.728 -0.20 -0.01% 0.56 0.022% 4.98283 1244.140 -0.06 0.00% 0.70 0.027% 5.85370 1461.660 0.01 0.00% 0.76 0.030% 7.11774 1775.860 -1.42 -0.06% -0.67 -0.026% 7.99387 1996.360 0.31 0.01% 1.06 0.041% 8.68506 2169.050 0.42 0.02% 1.16 0.045% 9.53879 2382.450 0.64 0.03% 1.39 0.054% 10.64341 2658.070 0.44 0.02% 1.18 0.046% Table 6 – 10mA range (Vcm = 20V). Measured Data I (mA) Untrimmed Error Vout (mV) Absolute (mV) Trimmed Error Relative Absolute (mV) Relative 0.000 0.055 0.05 0.00% 0.00 0.000% 100.303 251.400 0.95 0.04% -0.27 -0.011% 199.851 500.786 1.76 0.07% -0.61 -0.024% 300.618 754.046 3.41 0.13% -0.14 -0.005% 400.330 1003.724 4.11 0.16% -0.59 -0.023% 500.944 1255.870 5.03 0.20% -0.85 -0.033% 601.552 1508.490 6.43 0.25% -0.61 -0.024% 701.079 1758.470 7.90 0.31% -0.31 -0.012% 800.656 2008.760 9.55 0.37% 0.18 0.007% 901.122 2261.360 11.29 0.44% 0.75 0.029% 1000.709 2511.350 12.61 0.49% 0.92 0.036% Table 7 – 1A range (Vcm = 20V). siliconchip.com.au Australia's electronics magazine Are your copies of Silicon Chip getting damaged or dog-eared just lying around in a cupboard or on a shelf? Can you quickly find a particular issue that you need to refer to? Keep your copies safe, secure and always available with these handy binders These binders will protect your copies of S ilicon C hip . They feature heavy-board covers, hold 12 issues & will look great on your bookshelf. H 80mm internal width H Silicon Chip logo printed in goldcoloured lettering on spine & cover Silicon Chip Publications PO Box 194 Matraville NSW 2036 Order online from www. siliconchip.com.au/Shop/4 or call (02) 9939 3295 and quote your credit card number. *see website for delivery prices. February 2025  79 Programmable Frequency M DIVIDER COUNTER This small PCB doesn’t cost a lot to build but provides several useful features. It can reduce the frequency of an incoming signal or pulse train by a factor of between 3 and 21,327,000 that’s easily configured over a USB serial port. It can also provide basic frequency measurements. By Nicholas Vinen ● Input frequency range: 300Hz to 77MHz (typical) ● Over 85,000 possible division ratios: — 3–21,327 (with short output pulses) — 30–213,270 in steps of 10 (50% duty cycle output) — 213,300–2,132,700 in steps of 100 (50% duty cycle output) — 2,133,000–21,327,000 in steps of 1000 (50% duty cycle output) ● Configuration: via USB serial interface; default power-up ratio can be selected ● Can measure the input signal frequency to ±0.25% (prototype was within ±0.02% <at> 20MHz) ● Output duty cycle: 10%/50% (if division ratio is multiple of 10; otherwise < 34%) ● Input signal level: 20mV to 3.2V RMS (100mV+ or more at low frequencies) ● Output jitter: estimated at 0.1ns ● Propagation delay: approximately 110ns ● Input/output impedances: 50Ω or 75Ω ● High noise immunity with 23.5mV hysteresis ● Outputs are in phase with inputs ● No output signal toggling in the absence of an input signal ● Power supply: 5-12V DC <at> 20mA ● Power connectors: USB Type-C, 2.1mm/2.5mm ID barrel socket 80 Silicon Chip Australia's electronics magazine y Compact Frequency Divider design, published in the May 2024 issue, was a purposefully simple design (siliconchip.au/ Article/16244). However, the hardware used was capable of doing a lot more than just dividing a signal frequency by a fixed ratio. I could have added jumpers to allow the ratio to be changed, but at least 19 jumpers would be required and figuring out how to set them would be very complicated. I instead decided to add a low-cost microcontroller with a USB interface that would allow you to type any desired division ratio. It could quickly reconfigure the divider chips to achieve the desired ratio. The cost and size of the extra parts are minimal, so the result is still quite compact and affordable. It can easily be configured with a wide range of possible division ratios; the USB cable used to configure it can also power it. For maximum flexibility, you can also save the ratio you set into flash memory and then power it from just about any low-voltage DC source away from a computer. I also realised it was pretty easy to add an onboard frequency counter that would be reasonably accurate, so I did that as well. You just type ‘m’ (for “measure”) and press Enter and within a second or so, it measures and reports the input frequency. So it’s a fairly useful little device that’s small and not too expensive to build. The signal chain is very similar to the May 2024 design mentioned earlier, with four digital logic ICs doing most of the work. There are three 74HC4017s, each dividing their inputs by a factor of 10, plus a 74HC4059 programmable divider chip. Cascading those four stages gives many possible division ratios. Like that earlier design, a highspeed push/pull comparator is used to boost and square up the input signal, while a hex buffer IC provides the drive strength for 50W or 75W termination. Besides the new parts to control the division ratio (a microcontroller and three more digital logic chips), the remainder of the circuit is a linear power supply with reverse polarity protection and a power indicator LED. The unusual 74HC4059 While updating this design, I pondered why the way the 74HC4059 siliconchip.com.au divider works is so strange, and I think I have the answer. The 4059 was designed in a time before microcontrollers. If you wanted to build a digitally controlled, PLLbased (phase-locked loop) radio tuner, you would need to have some sort of ‘register’ chip or chips to store the currently selected frequency. There would be a way to increase or decrease the value stored in that register (using buttons, or a knob, or something like that) and its output would be a binary number that would be fed to the PLL to control the tuning. You would also need a frequency divider as part of that PLL. A PLL essentially works by using negative feedback via a divider to multiply a frequency. So it appears that the 4059 was designed to be that part of the circuit, with its 16-bit digital inputs fed from the register. That explains why it has a programmable prescaler that effectively multiplies the 16-bit (‘J’) value by a figure of 2, 4, 5, 8 or 10. That is the way that you would select the size of the steps caused by each increment or decrement of the J-value. For our purposes, we can use a fixed prescaler value of 8 since we don’t need configurable steps; that value gives us the widest possible range of division ratio (3 to 21,327) and the microcontroller can figure out what values to feed to the 16 J inputs to achieve the desired ratio. To make the circuit more flexible, rather than just controlling the configuration of the 74HC4059, I have also added the ability to bypass one, two, or all three of the 74HC4017 divideby-ten stages. That is done using a single 74HC4052 dual 1-to-4 analog multiplexer chip, as described in more detail later. So we can divide by a factor of 1, 10, 100 or 1000 in addition to the configurable ratio of the ‘4059. Ideally, we want the final divide-by-10 stage in the circuit for two reasons. One is that the output pulses from the 74HC4059 are very narrow (only the length of the period of its input signal) and that last divide-by-10 stage acts as a clock stretcher. The other is that it also gives us the 10% or 50% duty cycle choice. So while ratios that are not multiples of 10 can be set (up to 21,327), the output pulses will have a duty cycle of 1/r, where r is the ratio. For example, a ratio of 321 will result in output siliconchip.com.au Converting a division ratio into an appropriate configuration As mentioned in the text, we only use a prescaler mode of 8 for the 74HC4059 chip because it gives the largest possible range of division ratios and has no disadvantages in this configuration. As the prescaler can be preset, that doesn’t restrict us to division ratios that are multiples of eight. Given a desired ratio, first we check if it is within the possible range of 3-21,327,000. If it is below 3, we make it 3, and if it is above 21,327,000, we clamp it to that value. We then check whether we need one, two, or three divide-by-ten stages based on the value. If the ratio is a multiple of ten and at least 30, we force it to have at least one divide-by-ten stage so we can get a 50% duty cycle output. We can then divide the desired ratio by either 1, 10, 100 or 1000 to determine the division factor necessary for the 74HC4059, which will be a value between 3 and 21,327. The final step is to figure out the 16-bit J value that produces that ratio and load it into the shift registers. J1-J3 are the prescaler preset value, between 0 and 7, calculated simply as the desired ratio modulus eight (ie, what is the remainder after dividing the ratio by 8). We then divide the ratio by eight and, if the result is 1000 or more, bit J4 must be 1. If J4 = 1, we subtract 1000 from the remainder. We can then determine the value for bits J13-J16 by dividing the remainder by 100 again. We take the remainder from that and divide it by 10 to get the J9-J12 value, then the remainder is the value for J5-J8. There’s just one remaining trick, which is that when we do the divisions by 100 and 10, we might get a value of more than 15, which is not possible to fit into four bits. In that case, we simply use the value of 15 but add the excess back to the remainder so that it flows into the later calculations. That’s necessary because of the odd ‘extended range’ trick needed to use the full set of possible division values of the 74HC4059. It’s a result of it being a BCD device but having 4-bit registers that can be programmed to any value between 0 and 15, not just 0 to 9 like a regular BCD device. pulses with a duty cycle of 1/321 or 0.31%. That’s fine for feeding to most frequency counters, using as a trigger pulse for an oscilloscope and so on. But there may be applications that such short pulses are not suited to. Since we have a microcontroller in the circuit, we can feed the output of the divider to it and have it count the pulses over a fixed time to give us an onboard frequency counter capability. The micro has no crystal but it synchronises its clock to the USB host, which likely does, so it should be reasonably accurate. So that you can use this device without it being tied to a computer, once the ratio is set, it can be stored in internal flash memory. In that case, it will be automatically restored each time it is powered up after that until it’s changed again. So you can plug it into a computer, set the ratio, unplug it and take it somewhere else to use it. It just needs a 5-12V DC supply to operate away from a computer. Circuit details The full circuit of the new Australia's electronics magazine Programmable Frequency Divider is shown in Fig.1. If you compare it to the one from May 2024, you will see that the power supply and circuitry around IC6, the comparator that acts as an input signal amplifier, is identical. The one difference in the power supply is that I didn’t have room to easily fit a 2-pin header as an alternative to the barrel socket (CON5) on this PCB. The arrangement of IC1-IC5 is also essentially the same, except that IC3’s configuration inputs are now driven by microcontroller IC10 and two 74HC595 serial-to-parallel shift registers (IC8 & IC9). Also, rather than a fixed signal chain of CON1 → IC6 → IC1 → IC2 → IC3 → IC4 → IC5 → CON3, we now have IC7b selecting whether IC3’s input clock comes from IC6, IC1 or IC2. The other half of that device, IC7a, also selects whether the output signal fed to IC5d/ e/f comes from the output of IC3 or IC4. IC7 itself is controlled by microcontroller IC10 via its S0 & S1 inputs. With S0 & S1 both low, Ya0 and Yb0 are selected, so all three 74HC4017s are bypassed and only the 74HC4059 February 2025  81 provides frequency division. This is for division ratios from 3 to 21,327 but not multiples of 10 (except for 10 and 20) because, as explained earlier, we ideally want IC4 to be involved so the 10% and 50% duty cycle outputs have the expected duty cycles. For the ratios 3-29 or 31-21,327 that are not multiples of 10, IC4 must remain out of the circuit. JP1 must be in the 50% position but the output duty cycle will actually be much lower (1/r, as mentioned earlier). If you put JP1 in the 10% position, you will get an output but its frequency will be divided by 10 compared to the other position. With S0 high but S1 low, Ya1 and Yb1 are selected. IC1 and IC2 are still bypassed (ie, the signal goes straight from IC6 to IC3) but now the output comes from IC4 instead of IC3. This is for ratios from 30 to 213,270 that are 82 Silicon Chip multiples of 10. Above 21,327, only multiples of 10 are available. With S0 low and S1 high, Ya2 and Yb2 are selected. Now only IC2 is bypassed and the range of available ratios is 300 to 2,132,700. With both S0 and S1 high, the signal passes through all four divider chips and the range of available ratios is 3000 to 21,327,000. Microcontroller IC10 The firmware running on IC10 does a few things: ● USB serial communications. It receives and echoes characters typed by the user. When enter is pressed, it parses the command and, if it’s valid, changes the division ratio, saves it to flash or performs a frequency measurement via its RC5 (pin 5) digital input. ● Configuring IC3. When a new division ratio is sent to IC10, it calculates the closest possible ratio that Australia's electronics magazine is actually achievable and figures out what values of J (16 bits) and K (3 bits) are required to achieve it. It then spits out the 16 bits of J one at a time via the SPI-like bus formed by the following digital outputs: RA5 (pin 2, serial data); RA4 (pin 3, serial clock) and RC4 (pin 6, latch enable). The latch-enable function is shared with the S0 control for IC7 to save a pin, since the S0 state doesn’t matter while the ratio is being reprogrammed. ● Performing frequency measurements. Pin 5 is also the 8-bit Timer 0 clock input (T0CKI), allowing IC10 to easily measure the output pulses. 16-bit Timer 1 counts IC10’s internal 12MHz clock pulses simultaneously. The currently set division ratio (which IC10 steps through to find the best one) along with the ratio of the two counts allows it to calculate the input frequency. For more on how it does this, siliconchip.com.au Fig.1: the signal from CON1 is amplified/squared up by IC6, then is divided by 10 by IC1, and again by IC2. IC7b determines which of the original or divided signals arrives at the CP input of IC3. Its division ratio is set using serial-toparallel registers IC8 & IC9 along with signal direct from micro IC10. The output is optionally divided by 10 again by IC4 and then fed to the output buffers. see the panel at the end of the article. In-circuit serial programming header CON6 allows IC10 to be reprogrammed while on the board. This was very useful during development and you may need it if you plan to program the chip yourself. If you purchase a programmed chip from the Silicon Chip Online Shop (possibly as part of a kit), you could leave CON6 off. Input signal conditioning We skipped over this in the description above, partly because it was already covered in the May 2024 article. Here’s a quick rundown. The incoming signal is terminated by a 75W resistor (it could be 50W depending on your preference) and then AC-coupled to the non-­inverting input of high-speed comparator IC6 via a 220W resistor. This resistor limits the current in case the input signal level siliconchip.com.au is too high, in which case the voltage at the non-inverting input of IC6 is clamped to the +5V/0V rails by dual schottky diode D1. The inverting input of IC6 is held at 2.5V, ie, half of the 5V supply due to a pair of 10kW resistors across the 5V supply and a 100nF capacitor to stabilise it and keep the source impedance low. Because both inputs are DC biased to close to the same voltage, only a small signal is required at the CON1 input to cause IC6’s output to toggle and swing between +5V and 0V. Without hysteresis, this would have a tendency to oscillate, as even a bit of noise would be enough to cause that toggling. However, the 10MW resistor from IC6’s pin 6 output to its pin 3 non-inverting input means that the input signal needs to exceed 23.5mV peak-to-peak before the output level will switch. It also means there is no Australia's electronics magazine output from IC6 if CON1 is left disconnected. There will be a small voltage across the 47kW resistor due to IC6’s input bias current, which places an upper limit on the practical value of that resistor. It will form a divider with the 220W resistor but it only reduces the incoming signal level by about 0.5% so it doesn’t really affect the operation. Outputs The two outputs are each driven by three parallel stages of IC5, the MC74VHCT50A hex buffer. This is similar to a 74HC04 hex inverter except that the outputs follow the inputs rather than being inverted, so there is no phase inversion. We need three in parallel for each output in case the constructor chooses 50W termination, in which case the buffers could be driving a load as low February 2025  83 Firmware 16-PIN USB-C SOCKET 100nF 5.1kW 100nF 1 IC4 50% 74HC4017 IC7 74HC4052 1 100nF 100nF 1 1 100nF 1 JP1 1 IC8 74HC595 IC5 ‘74HCT50A 10% IC7 74HC4052 75W AMS1117 REG1 1 100nF 75W 100nF A LED1 IC8 74HC595 IC10 PIC16F1455 5.1kW CON4 1 1 100nF IC2 74HC4017 D1 IC3 74HC4059 CON5 220W 10MW IC9 74HC595 CON6 1 USB-prog. Freq. Div. IC1 74HC4017 100nF 10kW 75W 10kW CON1 In 1 47kW IC6 1nF 100nF The firmware is based on a CDC USB/serial implementation that IC5 ‘74HCT50A 10% IC9 74HC595 100nF If a USB-C cable is connected via CON4, it powers the circuit directly. 5.1kW pull-down resistors are provided on the A5 and B5 pins to ask the host to supply 5V. If a plug is inserted in barrel socket CON5, USB power is disconnected from the circuit to avoid it feeding back into the computer. With 5-12V applied to CON5 (ideally at least 6V), power is fed to low-dropout 5V regulator REG1, which supplies the rest of the circuit. Regardless of the source of 5V, LED1 lights up. Mosfet Q1 is provided in case a power supply is connected to CON5 with reversed polarity. With the correct polarity, Q1’s gate is pulled positive via one or both of the 10kW resistors. Q1 is switched on and it connects the barrel socket ground to circuit ground (its body diode conducts before it switches on fully). Alternatively, if CON5 has the wrong polarity, Q1’s gate is pulled negative, holding it off, and its body diode is reverse-biased, so no current can flow between circuit ground and CON5’s outer barrel contact. Thus, no damage can occur. Zener diode ZD1 protects Q1’s gate from high applied voltages; it is safe up to -30V, at which point Q1’s channel could begin to break down. IC3 74HC4059 Power supply IC4 50% 74HC4017 related power supply components on the underside. That makes assembly a bit easier overall. As before, top and bottom ground planes have been used with plenty of vias, and signal tracks have been kept short to avoid too much distortion of high-frequency signals. The device is built on a double-­ sided PCB coded 04108241 that measures 84 × 35.5mm. Fig.2 shows the components on the top side, while Fig.3 shows those on the underside. We suggest fitting all the SMD parts to one side of the board, followed by the other, then the through-hole parts. It’s best to start with the top, as more parts are on that side. The passives are all M3216/1206 size at 3.2 × 1.6mm and all the ICs are SOIC/SOP types with a relatively large 1.27mm lead spacing. The only slightly tricky party is the USB socket, as it has a row of fairly closely spaced pins. It has two mounting pins that go into through-holes, so insert it first and push it down fully, then tack-solder one of the small end pins. Check that all the pins are lined up over their pads; if not, remelt that solder and gently nudge it one way or the other until they are all lined up. Then spread a thin layer of good-quality flux paste over all the pins, clean the soldering iron tip, add some fresh solder and drag it over the row of pins. You Construction may need to add more solder if you JP1 USB-prog. 100nF Assembly of the device is 1similar run out partway. 0kW Freq. Div. 75W 0kW 5W to the May 2024 version, except 1that If a few joints get too 7much solCON1 In 1 47kW there are a few extra chips onboard. der, resulting in a bridge to adjacent IC6 1nFsimilar, D1 pins, The layout of the PCB is also remove the bridges 1using a bit 1 1 1 1 100nF 100nF CON6 2IC6 20W 1and 0MW 75W CON2 although last time, comparator flux paste and an 100nFapplication 1 1more 1 0 0 n F 0 0 n F 100nF 1 1 A IC10 Lsliding ED1 its associated components were on the of clean solder wick, it away CON5 PIC16F1455 1 10from 0nF K underside and most of the other5.1parts the pins when it starts to draw kW CON3 were on the top. up the solder. 1 1kW 16-PIN 5.1kW USB-C This time, IC6 has been moved off the flux residue CON4 Sto 04108241 using a 10Clean 0nF OCKET the top, keeping all the signal com- solvent TOP OF Band OARuse D a magnifier and good ponents in a row from left to right, light to check that the solder joints with just the regulator and a few on all the pins look good and there IC2 74HC4017 provides the serial console used to configure the device. Added to that is code to set up the inputs and outputs and program the dividers in response to commands sent from a computer via USB. It also incorporates a frequency measurement routine that lets the board act as a basic frequency counter from 300Hz up to about 70MHz. The software code is converted to a HEX file using Microchip’s XC8 compiler. There is a free version of that compiler that lacks size optimisation (-Os). There is also a free 60-day trial of the Pro version that includes that feature. Because the PIC16F1455 only has 8k words of flash, we had to use the -Os option to fit all the features we wanted. Since we supply a compiled HEX file, you don’t need the compiler. You can either purchase a pre-programmed PIC or load the HEX file yourself. But if you want to modify the code with the full feature set, you will need the Pro version of the compiler. Another option is to remove the line “#define XC8_PRO” near the top of the file “main.c”. That will remove the L, H and P commands (see below) but the code will then compile with the free version of XC8 v2.5.0 (using -O2 optimisation) and still fit in the available flash. Those commands are not critical to the device’s operation. IC1 74HC4017 as 100W. In that case, the output current requirement at 5V is 50mA, right at the limit of two stages <at> 25mA each. The output load will be higher at higher frequencies due to the characteristic impedance of the output cable, approaching 150W (50W source + 50W cable + 50W termination). Still, at low frequencies, we have to assume that the cable impedance is close to 0W. CON2 K 1kW CON3 1m F 100nF 10kW 10kW 100nF ZD1 04108241 TOP OF BOARD 100nF Q1 UNDERSIDE OF BOARD Figs.2 & 3: most of the components are SMDs (on the large side) and virtually all mount on the top of the PCB. Those on the underside are basically just the power supply. You could use vertical SMA connectors if you wanted to; CON2 is not strictly required, as CON3 is the main output. CON2 usually provides a squared-up version of the input signal. 84 Silicon Chip Australia's electronics magazine AMS1117 REG1 1mF 100nF siliconchip.com.au are no more bridges. If so, solder the mounting posts into their holes to give it mechanical strengths. Now move onto the ICs, soldering them similarly. But first, a word of warning. I was very proud of how neatly I soldered the ICs onto the board, until I realised that I had put them all in with pin 1 at upper left – that’s not how the board is designed! Many of the ICs have their pin 1 towards the bottom of the board, so I had to use a hot air station to remove and then resolder them. Don’t make that mistake! In each case, place the IC, check its orientation (!), tack-solder one pin, check the alignment of all the pins and adjust if necessary. Then add flux paste, solder the remaining pins and remove any bridges. Once all the ICs are in place, clean off the flux residue, as it’s much easier to do it in stages. Chemtools’ Kleanium Deflux-It G2 Flux Remover is our preferred solvent but pure alcohol will also work. With all the ICs in place, solder diode D1 next, then all the passives on the top side of the board, referring to Fig.2. There is just one 1nF capacitor on the top of the board, next to CON1; all the other capacitors on this side are 100nF types. The resistors will be printed with codes indicating their values; see the parts list if you are unsure about that. Next, use a DMM on diode test mode to carefully probe the ends of the SMD LED until it lights up. The red probe will be on the anode (A) and black on the cathode (K), so use that information and Fig.2 to orientate it correctly before soldering it in place. Give the PCB another clean to remove flux residue, then flip it over. Parts List – Programmable Frequency Divider 1 double-sided PCB coded 04108241, 84 × 35.5mm 3 right-angle or vertical through-hole SMA connectors (CON1-CON3) 1 SMD USB Type-C power plus USB 2.0 data socket (CON4) [GCT USB4105] 1 PCB-mount DC barrel socket (CON5; optional) 1 5-pin header, 2.54mm pitch (CON6; optional, programming IC10 in-circuit) 1 3-pin header, 2.54mm pitch (JP1) 1 jumper shunt (JP1) 1 6-pin stackable header (only needed for programming IC10 in-circuit) Semiconductors 3 (CD)74HC4017(M96) CMOS Johnson decade counters, SOIC-16 (IC1, IC2, IC4) 1 (CD)74HC4059 high-speed CMOS programmable divide-by-N counter, SOIC-24 (IC3) 1 MC74VHCT50A hex CMOS non-inverting buffer, SOIC-14 (IC5) 1 TLV3501AID rail-to-rail high-speed comparator, SOIC-8 (IC6) 1 74HC4052 dual CMOS 4-to-1 analog multiplexer, SOIC-16 (IC7) 2 74HC595 8-bit serial-to-parallel shift registers, SOIC-16 (IC8, IC9) 1 PIC16F1455-I/SL 8-bit microcontroller programmed with 0410824A.HEX, SOIC-14 (IC10) 1 AMS1117-5.0 or compatible 5V 1A low-dropout regulator, SOT-223 (REG1) 1 AO3400 30V 5.8A N-channel logic-level Mosfet or equivalent, SOT-23 (code XORB) (Q1) 1 SMD LED, SMA/M3216/1206 size, any colour (LED1) 1 BZX84C5V6 5.6V 1% tolerance zener diode, SOT-23 (code YX) (ZD1) 1 BAT54S dual series schottky diode, SOT-23 (code KL4 or L44) (D1) Capacitors (all SMD M3216/1206 size 50V X7R unless noted) 1 1μF 13 100nF 1 1nF Resistors (all SMD M3216/1206 size 1%) Complete Kit (SC6959, $60): 1 10MW (code 106 or 1005) includes all components 1 47kW (code 473 or 4702) listed in the parts list except 4 10kW (code 103 or 1002) the programming header. 2 5.1kW (code 512 or 5101) 1 1kW (code 102 or 1001) 1 220W (code 220 or 220R) 3 49.9W (code 49R9) or 75W (code 75R or 75R0) (to suit impedances) Underside parts Solder REG1 as shown in Fig.3, then ZD1 and Q1, being careful not to get them mixed up as they will look similar (if you do, check the markings and compare them to the codes in the parts list). There is one 1μF capacitor on this side; the rest are 100nF. Both resistors are 10kW types. Clean off the flux residue on the underside, then finally move on to the through-hole parts. Those are CON1CON3, CON5 and JP1. You may also want to fit pin header CON6 if you need to program IC10 in-circuit. Note that you could leave SMA connector CON2 off if you don’t need or want You can get away with just fitting the USB socket to power the Programmable Frequency Divider. If you choose to do so, make sure to fit a wire link as shown in Fig.2. siliconchip.com.au Australia's electronics magazine February 2025  85 the extra output that usually provides a buffered, squared-up version of the input signal (or with its frequency divided by 10 or 100 for higher programmed division ratios). If you only want to power it using the USB socket and you will leave CON5 off, you need to solder a wire link as shown in red on Fig.2 or there will be no ground connection for the USB socket. If fitting CON5, do not fit that wire link. The underside of the PCB is primarily occupied by the power supply components. Testing Assuming IC10 is already programmed, the easiest way to test the board is to connect it to your computer using a USB-C cable. If your computer detects a new USB serial port and LED1 lights, things are looking good. If not, unplug it quickly and check for faults like bad solder joins, especially on the USB socket and IC10. If the board isn’t behaving, common problems to look for are solder bridges, pins where the solder hasn’t adhered to the PCB pad below, or incorrectly orientated ICs (we did warn you!). If the serial port does appear, use a program like Tera Term pro to connect to it (the baud rate is unimportant). Type “s” and press Enter/Return and you should get a similar status report: J=292 K=4 P=10 ratio=1000 Scope 2: the CON2 (green) and CON3 ‘50% duty cycle’ (yellow) outputs for a 15.5MHz 100mV sinewave input and with a 155:1 division ratio. Because it isn’t a multiple of 10, the output pulses from CON3 are not 50% duty cycle but instead are short (1/155 or 0.65% duty). Despite that, the ‘scope has no trouble measuring the frequency. The default division ratio is 1000 but you can change it with the ‘r’ and ‘w’ commands. If you have an oscilloscope with a built-in waveform generator (or separate signal generator), or a signal generator and frequency counter, hook them up to CON1 & CON3 and verify that the output frequency from CON3 is 1/1000th that of the input at CON1. If there’s no output signal, carefully check all the parts on the board for correctness and good soldering. If there’s an output but the frequency is wrong, that suggests a problem with the soldering of either IC3, IC8, IC9 or possibly IC7/IC10. If the ratio is correct, hook it up to a computer via USB and try changing the ratio to values like 3, 30, 30,000 and 300,000 by typing “r”, then the ratio, then pressing enter. The new ratio should be applied immediately. Scope 1 shows both outputs for a 20MHz input and 1000:1 division ratio. You can see that the 50% duty cycle output is nice and square and very accurate, while the CON2 output Australia's electronics magazine siliconchip.com.au Scope 1: the CON2 (green) and CON3 50% duty cycle (yellow) outputs for a 20MHz 100mV sinewave input and with the default 1000:1 division ratio. 86 Silicon Chip has frequency matching the input signal. Scope 2 shows the ‘50% duty cycle’ output for a 15.5MHz input and division ratio of 155:1. Each positive pulse is the length of one input cycle. So in this mode, the higher the division ratio, the lower the duty cycle. Usage You can get help on the available commands by typing “h” and then pressing enter (see below). Besides the ratio-setting command “r” mentioned immediately above, the most useful commands are “w” <Enter> (which saves the current ratio as the default at power-up) and “m” <Enter> which measures and then displays the input frequency. Low-frequency measurements may take a second or so. Listed just below are 16 measurements we made in a row of a 20MHz source: – 19996544 – 19999168 – 19999472 – 19997136 – 19999040 – 19997824 – 20005056 – 20001968 – 19998656 – 19998048 – 19997424 – 19999568 – 20001184 – 19998960 – 19996192 – 19999360 You will see that they vary a little above and below the exact frequency but they are surprisingly close given that the board has no crystal and the measurement method is fairly basic. The mean of those measurements is 19,999,100Hz (-0.011%), while the standard deviation is 2137 (0.015%). The “p” command lets you set the J, K & prescaler (1/10/100/1000) values directly but you generally shouldn’t need to use that. There are some combinations that you can set with that but not the “r” command but we don’t think they are that useful; we just included it for completeness. More details on these (and other) comSC mands follows. Typing ‘h’ (or ‘H’ or ‘?’) and pressing ◀ Enter should display a help message only that’s an abbreviation of this list. We have also added a short description for each command. siliconchip.com.au How IC10 measures the input signal frequency accurately To measure the input frequency, we use the programmable divider to divide it by a factor r, then measure the ratio of the result (Fout) to the 48MHz USB-­ derived clock (Fusb). We do that by counting the number of Fout pulses and Fusb pulses over an identical period. The actual period is not relevant, except that it determines the precision of the measurement. Let’s say the pulse counts are Pout and Pusb. We can then calculate the input frequency as Fin = Pout x r ÷ Pusb. However, as the PIC16F1455 is a low-end device with relatively little flash and RAM, we don’t have space for floating-­ point calculations, so it’s a bit trickier than just performing a multiplication and division. 8-bit Timer 0 counts pulses from the divider, while 16-bit Timer 1 counts the 48MHz USB clock pulses. Timer 0 is initiated at 255 and Timer 1 is initialised at 0 but is inactive. When the first pulse comes from the divider, Timer 0 rolls over to zero and triggers an interrupt that enables Timer 1. Both timers then count until Timer 0 rolls over again, at which point they are both paused. There is a slight delay between the positive edge of the divider output being received and Timer 1 starting/stopping, but the code is designed for the delay to be the same in both cases, so it cancels out. The number of times Timer 1 rolls over while the timer is active are counted, effectively extending Timer 1 from 16 bits to 24 bits, required to get an accurate result. The resulting 24-bit value is the number of 48MHz clock pulses that occurred between the 1st and 257th pulse from the divider. We can then calculate the division ratio times 48 million (the number of USB clock pulses per second), divide it by the number of actual USB clock pulses counted, then multiply by 256. Just before starting the counting, the division ratio is set to 80 as that is the highest multiple of ten that avoids a 32-bit integer overflow in these calculations. If we find the 24-bit Timer 1 counter overflows during this measurement (after about 350ms), that means the frequency is below 58.6kHz. In that case, we drop the divider ratio to 3 and restart the measurement. That allows us to measure down to 2.2kHz. If it still overflows, we change the number of Timer 0 clock pulses to measure over to one and try again. That would theoretically let us measure down to 9Hz, although measurements get pretty inaccurate below 300Hz due to hardware limitations. We perform 16 measurements and average them to get a more stable reading, although the number of measurements is reduced to four or one if the frequency is determined to be on the low side (eg, close to one second per measurement). Otherwise, making that many measurements would take too long. L load ratio from flash memory this is automatically done at power-up, but if you have changed the ratio and wish to reset to the default, you can run this command M measure frequency it’s most accurate over the range of about 1kHz to 50MHz but will work from 300Hz up to the upper limit, which is typically around 77-80MHz at room temperature P Jx Ky Pz set J/K/P to x/y/z valid ranges for values are: J[0-65535] K[0-7] P[1,10,100,1000] the J and K numbers are fed directly to IC3, while the P (pre/ postscaler) value determines the configuration of the 74HC4052 and thus which 74HC4017s (if any) are bypassed Rx set ratio to x valid range for values is: R[3-21327000] if the exact ratio given is not available, the closest possible ratio is used. Remember that ratios that are not multiples of 10, or ratios of 10 or 20, will give shorter output pulses and the 10% duty cycle output ratio will not be correct S show status this reports the J, K, P values and ratio. These are also reported after changing or loading the ratio W write ratio to flash memory the current ratio is stored and will be used at power-up in the future (the initial default is 1000). High-endurance flash is used, so it should not wear out after even 100,000 writes Australia's electronics magazine February 2025  87 SERVICEMAN’S LOG Another busman’s holiday Dave Thompson I recently travelled to Australia, the spiritual and physical home of Silicon Chip magazine. Sadly, I did not get to stop in and meet the people I have been working with for many years because my wife and I were headed to Western Australia, which is literally on the other side of the continent! One day, I’ll make it there but for now, we had a pressing need to get to Fremantle and visit some of my wife’s relatives, many of whom emigrated there after World War 2. They are all very elderly now and that was one of the reasons to get there and touch base with them. Fortunately, we made it in time, and all was well. Although Australia and New Zealand share a lot of history and have many things in common, visiting Australia is always like stepping into an alternate reality for me. Many things there are just done differently, and the philosophy among the people is somehow very different. This is more obvious when going to the many states across the vast space that is Australia – for example, the people in Darwin are typically different from the people in Melbourne or Sydney. Likely this is because many of the original immigrants brought their own cultures and customs to their new homes, wherever they decided to settle in this vast country. Fremantle has a large population of ex-pat Croats, and my wife has four aunts and many other relatives still living there. We were going to visit one aunt in particular who is ailing, which made it a very pressing and poignant trip. Where we live in Christchurch, New Zealand, there are hardly any people from Croatia. So for her to walk into a deli or a market in Freo, or in one case, get into a taxi, and speak Croatian to the driver, is a real plus for her. 88 Silicon Chip That cultural balance aside, there are many other subtle differences between our two countries, or at least things that I noticed. In parts of Europe, for example, most people who build a house use a standard type of shutter/door arrangement for all their windows and doors. They likely come in a few specific sizes, and homes are constructed to suit those sizes. A case of shutter envy I have often wondered why we don’t have those shutters and doors here in New Zealand. They are brilliant, with many features that enable opening them in many different ways and even shutting them to complete blackout level. Not only are these shutters and doors very secure, and almost impossible to open from the outside, they are versatile enough to let the outside world in without compromising security. Anyway, you are likely wondering what all this has to do with the Serviceman’s Curse – which I am sure you knew was going to make an appearance sooner or later. I can run to another country, but I can’t hide! It seems I cannot travel anywhere in the world without having to fix something, or even think about fixing something. All this talk of shutters and doors and what-have-you brings me to the point. A lot of homes – many of which are so-called spaghetti mansions (built in the style of Mediterranean houses) – in Freo have the same hardware installed as I saw in Europe. No doubt the people who emigrated here brought this stuff with them. This hardware just doesn’t exist in New Zealand, more’s the pity because I would have this installed in my house in a heartbeat. I suppose I could import it, but the cost would be prohibitive. These things weigh a tonne, and as I’d need a house-sized lot, it was not really an option for us to import them here. They are obviously available in Western Aussie, though, because many of the houses I saw there had them. Usually, these shutters are manually operated, with a canvas-type ribbon on a spindle that can be pulled either way to raise or lower the shutters. It’s a simple system that has likely worked for a hundred years. Australia's electronics magazine siliconchip.com.au Items Covered This Month • A trip over the Tasman • Repairing a bulging iPhone 7+ • A shocking experience • The dangers of lightning Dave Thompson runs PC Anytime in Christchurch, NZ. Website: www.pcanytime.co.nz Email: dave<at>pcanytime.co.nz Cartoonist – Louis Decrevel Website: loueee.com Now, though, we have this electricity thing to make life easier. No more pulling on ropes, ribbons or strings; now we can just hit a switch and the shutters open or close. Obviously, this requires motors and actuators to make it all happen, and over time these have been introduced and have replaced the manual methods of yesteryear. A fault rears its ugly head So, one of the places we visited has these electrically operated shutters. There was a switch on the wall inside the lounge, which looked like a light switch, that operated the shutters, up or down, depending on the position of the switch. Except the switch didn’t work properly, and the owner complained that it often didn’t open or close the blinds properly. He commented that often, he would have to toggle the switch several times to actuate the shutters, and that it was becoming more and more of a problem. To my serviceman’s mind, I immediately thought that either the switch or the actuator was the problem. I know what you’re thinking, is it the switch or the motor? I’m way ahead of you; it could be either! So here I am, seemingly now on a busman’s holiday, trying to figure out what’s going on with this shutter system. Of course, the owner is telling me not to worry about it, that I’m a guest, and only here for dinner, but what would any self-respecting serviceman do? Looking the other way isn’t really an option. I can’t sit at this guy’s dinner table and eat his very well-cooked food knowing that there is something not working properly. I mean, it is the Serviceman’s Curse, not the Serviceman’s Gift! The first thing after dinner was to check out the other shutters and see how they worked. All operated normally; it was just this one in the main dining room that didn’t. As it was the most used, it likely wore out quicker than the rest. Each shutter has a covered part at the top where the motor/actuator and the rest of the gubbins live. These covers were easy enough to get off as they were just screwed on and have a weatherproof seal to keep the worst of the rain out. As most of the shutters were installed under the eaves of the house and were well out of the way of the weather, it was kind of moot, but of course the seals had to be there. So, during dinner, all I could think about was this problem. I thought the problem must surely lie with the switch. When it worked, it worked well, and the shutter descended and opened up once it was going. I went around the house siliconchip.com.au and tried all of them – this one in the dining room did feel a little spongy. Just less precise in its operation. Since they’d been installed 25 years ago, it is normal to assume that something may have worn out. My guess was the switch, rather than the motor because it didn’t feel ‘right’. All the others around the house were crisp in their actions and just felt right. I offered my professional opinion that the switch was the problem and that we should change it for a new one. This was not going to be a problem, as these switches are a standard item and available from the various window and shutter retailers dotted around the landscape. Our host said he would take me to one of these places the following day, so at least we could enjoy a nice meal that night without the Curse intervening! Time to switch the switch He was good to his word, and we soon sourced a new switch. Now it was just a matter of putting it in without killing myself. Fortunately, the electrical systems are very well-thought-out and simple. We also have fuse boxes and breaker panels in New Zealand, but they seem far less standardised than the ones in Australia. The house is an older-style brick place, I’m guessing built in the 1960s, and the power breaker panel is easily accessible and well-labelled. In New Zealand, we just guess which breaker goes where and hope we don’t get zapped! One of the good things about renovating this house I own now before we moved in was that I could map the entire electrical system and produce a diagram showing what breaker controls what circuit and how everything is connected. I’m not sure why sparkies don’t do that here – or maybe they do, and I just haven’t seen it. I mean, I have seen breaker panels with those old black and gold stickers on them showing hot water or outside lights or whatever. Still, it seems to me that many homes – at least the ones I’ve lived in – have had bits added or removed over time and many times the stickers no longer refer to the correct circuits. It must be a real headache for electricians to walk into a place and have to work out what goes where. This could be down to the cowboy culture here, but I didn’t see that in Western Australia, at least, not in the house I was visiting. Everything was labelled and sectioned off in a proper and easily accessible cupboard, and I was pleasantly surprised. Most breaker panels in the homes I’ve lived in are set high up near the roofline and required a stepladder or at least a chair to gain access. Perhaps the theory was that putting Servicing Stories Wanted Do you have any good servicing stories that you would like to share in The Serviceman column in SILICON CHIP? If so, why not send those stories in to us? It doesn’t matter what the story is about as long as it’s in some way related to the electronics or electrical industries, to computers or even to cars and similar. We pay for all contributions published but please note that your material must be original. Send your contribution by email to: editor<at>siliconchip.com.au Please be sure to include your full name and address details. Australia's electronics magazine February 2025  89 it out of reach was the safest method to ensure idiots like me don’t mess with it! This one in WA was right in front of me and I could just open it. Luxury! So, we had the switch and now we had to disable the active circuit that the shutter was on. Despite the labelling and easy access, we still had to do some trial and error to ensure we killed the right one. It would be embarrassing in the extreme to end up frying myself at our host’s house! What would the neighbours think? The next problem was that all my tools are thousands of miles away across the continent and the Tasman Sea – as if that little bit makes a difference. My host said he had some tools in the garage, and I was free to use any of them. Tools for fools Great! Until I checked the tools. These were the kind of things I would find at a $2 shop or maybe a car boot sale at one of the markets Australians love so much. I have to admit, I too was seduced by the markets. The Fremantle Market is huge and a joy to walk through. If the thousands of other tourists are anything to go by, they all love it as well! But, and here’s a big but, the tools I see on sale there are the single-use type. I’m sure you know this level of excellence – you buy a Phillips screwdriver, then try to undo a screw with it and it strips like it was made of Plasticine. Have these manufacturers never heard of hardening? These were the kind of tools my host had in his garage. I guess if I was very careful I might be able to use them to change a switch plate, but, well, you never know with these things. The guy who installed it likely used a proper screwdriver and smoked those screws so tightly that I’d never get it undone using a waxworks screwdriver like this one. Like any serviceman, I need tools, so the day after we got the switch we went back to the same place and bought two screwdrivers, a flat head and a Phillips head driver. 90 Silicon Chip You used to be able to service an entire car with just these two tools (OK, maybe also a shifter, or Crescent as we call it here), so changing a switch plate shouldn’t be an issue. I also bought a mains power detector, one of those things that looks like a pen but beeps and flashes its LED when near a mains circuit. It always makes me feel happier working on wiring when I don’t hear those things beeping. I have owned a few over the years, but the early ones are a bit dodgy now, and I don’t really trust them anymore, so this will be a nice addition to my tool set. I will leave the drivers with the owner of the house – I already have several decent drivers, and in the future he can make use of them. So, with the assurance that the circuit was dead and there was no chance of me being cooked along with dinner, I removed the dodgy switch and simply replaced the old one with the new. That just involved pulling the power leads from the faulty one (which, as I assumed, were really tightly fixed) and putting them into the new switch. I powered up the circuit with the switch hanging off the wall – yes, I know, a dangerous practice, but in my defence, I am a cowboy from New Zealand after all, so I tried it before buttoning it all back up. It worked perfectly. That was good news. I didn’t really want to be disassembling an electric shutter mechanism in the break between the main meal and dessert! I replaced the switch assembly, which of course fits perfectly because people do things properly in Australia, rather than sometimes multiple different types of switches in New Zealand that use different mounts and standards. I guess we really are the wild west out here. In some ways, that can be a good thing, but standards are what make the world go around, so it was nice to see them being used in Australia. I am, of course, talking about Western Australia. Perhaps things are different in the other states. I don’t know, so I will rely on others to put me right on this. Anyway, the shutter now works well (and without lots of cursing), and the host was generous in offering me a nice dessert with some beautiful wine to finish with, so it all worked out in the end. Australia's electronics magazine siliconchip.com.au I love Australia and have visited there many times all my life. Admittedly, there have been long periods between my visits, but I have family there and a love for the country. I wish I could visit more, but really, fixing everything there would be a real challenge for me. So perhaps it is better I let all the amazing servicemen already there do it, and then I can retire, and maybe pop over to see how things are going! iPhone 7+ repair My youngest son’s iPhone 7+ was bulging badly due to the failing battery swelling. This same thing happened to my Samsung Galaxy tablet; that repair was featured in the October 2020 Serviceman’s log (on page 65; siliconchip.au/Article/14609). This iPhone was originally bought by my younger daughter in 2016, so it was now eight years old. Until now, it had not required any repairs. The other problem with the phone was that it was saying that it did not have a SIM, even though one was present. I thought this might be related to the bulging. I first looked on YouTube to see if there were videos on replacing the battery. Finding a few, I selected the one that had the best tutorial. Then I ordered a new battery, tools and a The bulging iPhone battery (left) and a photo showing how it was removed new screen protector from eBay (the exist- from the case (right). ing screen protector was badly cracked). The parts arrived, but there was no screen seal, so I had to the screen seal, which wasn’t quite wide enough for the order that separately. With everything on hand, I set about phone, but I managed to get it in place successfully. So it dismantling the phone. was finally time for reassembly. I first removed the two pentalobe screws at the bottom I reconnected the screen, then the battery and replaced of the phone and then carefully prised up the screen. This the two shields. One particular trilobe screw caused an job was made easier by the fact that the battery had lifted enormous amount of trouble; it refused to screw in and it on both sides, but had not broken it, which would have kept flicking out and vanishing. I lost it six times in the added considerable cost to the repair. process, with it landing outside the phone the first few With the screen free, I opened it up on the right-hand times, then inside the phone. side like a book and used a box to hold it while I worked I decided to try a different screw in that location and I on removing it. I removed the two shields with a trilobe had success with it, so I moved the troublesome screw to screwdriver, then flicked out the connectors for the battery where I had removed the replacement screw and this time and screen. I could then put the screen aside and work on it screwed in successfully. It is unclear why this screw was removing the battery. giving me so much trouble, as it was the same size as the The battery is removed by first prising up the adhesive at other one. [It may have been slightly bent by the bulging its end and then pulling the adhesive out carefully while battery – Editor] not breaking it. There are three adhesive strips that have With the screen and battery connected, I switched the to be removed in this manner (see the photo). phone on before assembling it, to make sure that it worked, With the adhesive removed, the battery can be lifted which it did. It was now searching for the network but not free of the phone and preparations made to install the finding it. new battery. The new battery did not come with adheEither the phone had a fault, which I thought unlikely, sive strips, so I cut two lengths of double-sided tape to or the SIM was faulty. I turned the phone off, removed secure it in place. the SIM, cleaned it and put it back in, but it still did the It is very important to connect the battery before adhering same thing. it to the phone, to make sure that it is lined up correctly. I put the SIM from another phone into the iPhone 7+ If it were secured first and the connector does not line up and it immediately found the network, so the phone was with the logic board connector, that would be a big problem. in working order. Putting the SIM from the iPhone 7+ into Once the battery is secured, it is disconnected again for the other phone caused it to come up with the message installing the screen seal. I ran into some difficulties with “Invalid SIM”. So the SIM was definitely faulty. siliconchip.com.au Australia's electronics magazine February 2025  91 I fully fitted the screen, pressed it down firmly and carefully around the edges and put the two Pentalobe screws back in the bottom of the phone. I then removed the old cracked screen protector, cleaned the screen and installed the new screen protector. The phone had a case which was not in very good condition, but as it happened, my wife had found a new case at an op shop for $1, so with that, the repair was complete. We just needed a replacement SIM, which my wife picked up at Officeworks when she was nearby. With the new SIM now on hand, I rang the carrier to go through the process of changing the number over to the new SIM. After the process was completed, the consultant said it would take 1-4 hours for the new SIM to become active. However, as soon as I inserted it and switched the phone on, it was active. My son was very happy to have his phone back and now working well with its new battery. B. P., Dundathu, Qld. A shocking experience! This shows the basic capacitor discharger I made, which I should have used right at the beginning of the repair! Also see the Capacitor Discharger project in the December 2024 issue (siliconchip.au/Article/17310). 92 Silicon Chip I had a bad electric shock the other day. I hadn’t suffered one for years, so complacency had obviously set in. A friend had brought in his electric motorbike charger and battery. The bike can be used on motorways, so the battery is huge, along with its associated switch-mode power supply unit/charger. The switch-mode power supply unit (SMPSU) was giving no output, so I took it apart. My friend assured me he hadn’t plugged it in for a week. Looking inside, I found that it was a common SMPSU problem: bad lead-free soldered joints around the enamelled wire from the ferrite transformer. This usually happens when the enamel hasn’t been fully removed before soldering. Also, lead-free solder has inferior wetting properties and its brittleness results in cracking from the high-­ frequency vibration due to magnetostriction in operation. I soon set to work, scraping off the burnt flux with a scalpel around the joints to get a good look. BANG! I got a massive DC belt from one arm to the other, very nasty, like an old Fender valve amp HT rail but worse. The scalpel was nowhere to be seen; luckily it wasn’t embedded in my friend’s head! I shouted, “that felt like 350 volts!”. I got my meter out and shakily measured between the pin I was scraping and the chassis. I thought (belatedly) that I had better discharge the main smoothing capacitors, of which there were three in parallel. So, stupidly, I got my nice insulated Bahco Ergo pliers out and shorted the pins. BANG! It blew one of the tips off. 4500µF of capacitance charged to 350V is a lot of energy (E=½CV2 so 275J)! Still shaking a bit, I continued and fixed the joints. I wasn’t going to be beaten by this modern ‘disposable’ electronics. Having fixed the bad joints, I soldered a bleeder resistor of 39kW 5W across the caps and switched it on. The LEDs lit up and it gave the correct 80V DC output at 10A; perfect. My friend thought the whole thing was most entertaining! It’s a good job it worked. He’s now enjoying his bike, and I found the scalpel stuck in the skirting board a week later. Morals of the story include: • Don’t assume something is discharged, even if the client says it hasn’t been switched on for a week! Australia's electronics magazine siliconchip.com.au • Don’t use a metal Swann Morton scalpel for repair work; use a plastic-handled one instead. • Don’t assume all SMPSUs have bleeder resistors wired across the main smoothing capacitor bank. Even if it has, the resistor may have become open circuit. This circuit didn’t have one because the continuous dissipation would be high and reduce its efficiency. • Measure the voltage across big capacitors as soon as you open the case and before you start working on the PCB. If they are charged, discharge them slowly with a bleeder resistor attached to insulated test probes. • Remember the ‘left hand in pocket’ rule; don’t make an easy current route through your heart. If you must hold the metal enclosure or chassis while working on it, physically clamp it or insulate your hand. Even relatively mild shocks involving the heart can lead to cardiac arrhythmias. • There’s nothing more embarrassing than getting a small shock and wetting yourself, then getting a massive one because you are standing in a pool. I know because I once did it in front of a load of students! It became a standing joke; I defused it by having a spare pair of underpants in the first aid box. J. R., Llandrindod Wells, Wales, UK. What lightning can do Mention was made a while back about the damage lightning can do. Here is my experience. Back in 1960 during my apprenticeship, I was called to a TV fault in a country home. On arrival, the owner told me what had happened. Lightning struck a power line down the road, it got into the house and blew all the fuses in the switchboard (they were actually fuses in those days). It also got to the TV antenna, and he showed me how the 300W ribbon had sprayed the fibro wall with molten copper. I went to the TV set and saw the ribbon had melted off the input terminals and was dangling in midair. I took the back off the set and saw that the on/off switch on the back of the volume control had been vaporised, and the mains wires were also dangling in midair. After replacing the volume control and 300W ribbon, the set was functional but the picture was snowy. Further investigation showed the input balun in the tuner was burnt out. After ordering another one and putting it in, the set was back to normal. I doubt if a modern TV would be as repairable as this one was after a lightning strike. SC T. V., Morayfield, Qld. siliconchip.com.au Australia's electronics magazine February 2025  93 SILICON CHIP .com.au/shop ONLINESHOP HOW TO ORDER INTERNET (24/7) PAYPAL (24/7) eMAIL (24/7) MAIL (24/7) PHONE – (9-5:00 AET, Mon-Fri) siliconchip.com.au/Shop silicon<at>siliconchip.com.au silicon<at>siliconchip.com.au PO Box 194, MATRAVILLE, NSW 2036 (02) 9939 3295, +612 for international You can also pay by cheque/money order (Orders by mail only) or bank transfer. Make cheques payable to Silicon Chip. 02/25 YES! You can also order or renew your Silicon Chip subscription via any of these methods as well! The best benefit, apart from the magazine? Subscribers get a 10% discount on all orders for parts. PRE-PROGRAMMED MICROS For a complete list, go to siliconchip.com.au/Shop/9 $10 MICROS $15 MICROS ATmega328P ATtiny45-20PU PIC10LF322-I/OT PIC12F617-I/P 110dB RF Attenuator (Jul22), Basic RF Signal Generator (Jun23) ATSAML10E16A-AUT High-Current Battery Balancer (Mar21) 2m VHF CW/FM Test Generator (Oct23) PIC16F1847-I/P Digital Capacitance Meter (Jan25) Range Extender IR-to-UHF (Jan22) PIC16F18877-I/P USB Cable Tester (Nov21) Active Mains Soft Starter (Feb23), Model Railway Uncoupler (Jul23) PIC16F18877-I/PT Dual-Channel Breadboard PSU Display Adaptor (Dec22) Battery-Powered Model Railway Transmitter (Jan25) Wideband Fuel Mixture Display (WFMD; Apr23) PIC12F617-I/SN Model Railway Carriage Lights (Nov21) PIC16F88-I/P Battery Charge Controller (Jun22), Railway Semaphore (Apr22) PIC12F675-I/P Train Chuff Sound Generator (Oct22) PIC24FJ256GA702-I/SS Ohmmeter (Aug22), Advanced SMD Test Tweezers (Feb23) ESR Test Tweezers (Jun24) PIC16F1455-I/P Auto Train Controller (Oct22), GPS Disciplined Oscillator (May23) Railway Points Controller Transmitter / Receiver (2 versions; Feb24) PIC32MX170F256D-501P/T 44-pin Micromite Mk2 (Aug14), 4DoF Simulation Seat (Sep19) Battery-Powered Model Railway TH Receiver (Jan25) PIC32MX170F256B-50I/SP Micromite LCD BackPack V1-V3 (Feb16 / May17 / Aug19) PIC16F1455-I/SL Battery Multi Logger (Feb21), USB-C Serial Adaptor (Jun24) Advanced GPS Computer (Jun21), Touchscreen Digital Preamp (Sep21) Battery-Powered Model Railway SMD Receiver (Jan25) PIC32MX170F256B-I/SO Battery Multi Logger (Feb21), Battery Manager BackPack (Aug21) USB Programmable Frequency Divider (Feb25) PIC32MX270F256B-50I/SP ASCII Video Terminal (Jul14), USB M&K Adaptor (Feb19) PIC16LF1455-I/P New GPS-Synchronised Analog Clock (Sep22) $20 MICROS PIC16F1459-I/P K-Type Thermostat (Nov23), Secure Remote Switch (RX, Dec23) ATmega32U4 Wii Nunchuk RGB Light Driver (Mar24) Mains Power-Up Sequencer (Feb24 | repurposed firmware Jul24) ATmega644PA-AU AM-FM DDS Signal Generator (May22) 8-Channel Learning IR Remote (Oct24) PIC16F1459-I/SO Multimeter Calibrator (Jul22), Buck/Boost Charger Adaptor (Oct22) $25 MICROS PIC16F15214-I/SN Digital Volume Control Pot (SMD; Mar23), Silicon Chirp Cricket (Apr23) PIC32MX170F256B-50I/SO + PIC16F1455-I/SL Micromite Explore-40 (SC5157, Oct24) PIC16F15214-I/P Digital Volume Control Pot (TH; Mar23), Filament Dryer (Oct24) PIC32MX470F512H-I/PT Stereo Echo/Reverb (Feb 14), Digital Effects Unit (Oct14) PIC16F15224-I/SL Multi-Channel Volume Control (OLED Module; Dec23) PIC32MX470F512H-120/PT Micromite Explore 64 (Aug 16), Micromite Plus (Nov16) NFC IR Keyfob Transmitter (Feb25) PIC32MX470F512L-120/PT Micromite Explore 100 (Sep16) PIC16F18146-I/SO Volume Control (Control Module, Dec23), Coin Cell Emulator (Dec23) $30 MICROS Compact OLED Clock & Timer (Sep24), Flexidice (Nov24) PIC16LF15323-I/SL Remote Mains Switch (TX, Jul22), Secure Remote Switch (TX, Dec23) PIC32MX695F512H-80I/PT Touchscreen Audio Recorder (Jun14) PIC32MZ2048EFH064-I/PT DSP Crossover/Equaliser (May19), Low-Distortion DDS (Feb20) STM32G030K6T6 Variable Speed Drive Mk2 (Nov24) DIY Reflow Oven Controller (Apr20), Dual Hybrid Supply (Feb22) W27C020 Noughts & Crosses Computer (Jan23) KITS, SPECIALISED COMPONENTS ETC USB PROGRAMMABLE FREQUENCY DIVIDER (SC6959) (FEB 25) NFC PROGRAMMABLE IR KEYFOB (SC7421) (FEB 25) COMPACT HIFI HEADPHONE AMP (SC6885) (DEC 24) CAPACITOR DISCHARGER KIT (SC7404) (DEC 24) Complete Kit: includes all components (see p85, Feb25) Complete Kit: includes all required items, except the cell (see p67, Feb25) Complete Kit: includes everything except the power supply (see p47, Dec24) Includes the PCB and all components that mount on it, the mounting hardware (without heatsink) and banana sockets (see p36, Dec24) PICO COMPUTER $60.00 $25.00 $70.00 $30.00 (DEC 24) For full functionality both the Pico Computer Board and Digital Video Terminal kits are required, see page 71 in the December 2024 issue for more details. - Pico Computer Board kit (SC7374) $40.00 - Pico Digital Video Terminal kit (SC6917) $65.00 Separate/Optional Components: - PWM Audio Module kit (SC7376) $10.00 - ESP-PSRAM64H 64Mb SPI PSRAM chip (SC7377) $5.00 - DS3231 real-time clock SOIC-16 IC (SC5103) $7.50 - DS3231MZ real-time clock SOIC-8 IC (SC5779) $10.00 FLEXIDICE COMPLETE KIT (SC7361) (NOV 24) MICROMITE EXPLORE-40 KIT (SC6991) (OCT 24) DUAL-RAIL LOAD PROTECTOR (SC7366) (OCT 24) PicoMSA PARTS (SC7323) (SEP 24) Includes all required parts except the coin cell (see p71, Nov24) Includes all required parts (see p83, Oct24) Hard-to-get parts: includes the PCB and all semiconductors except the optional/variable diodes (see p73, Oct24) Hard-to-get parts: includes the PCB, Raspberry Pi Pico (unprogrammed), plus all semiconductors, capacitors and resistors (see p63, Sep24) COMPACT OLED CLOCK & TIMER KIT (SC6979) Includes everything except the case & Li-ion cell (see p34, Sep24) $30.00 $35.00 $35.00 $50.00 (SEP 24) $45.00 siliconchip.com.au/Shop/ DISCRETE IDEAL BRIDGE RECTIFIER (SEP 24) DUAL MINI LED DICE (AUG 24) AUTOMATIC LQ METER KIT (SC6939) (JUL 24) ESR TEST TWEEZERS COMPLETE KIT (SC6952) (JUN 24) DC SUPPLY PROTECTOR (JUN 24) Both kits include the PCB and everything that mounts to it (see page 83, Sep24) - All through-hole (TH) kit (SC6987) $30.00 - SMD kit (SC6988) $27.50 Complete kit: choice of white or black PCB solder mask (see page 50, August 2024) - Through-hole LEDs kit (SC6849) $17.50 - SMD LEDs kit (SC6961) $17.50 Includes everything except the case & debugging interface (see p33, July24) - Rotary encoder with integral pushbutton (available separately, SC5601) Includes all parts and OLED, except the coin cell and optional header - 0.96in white OLED with SSD1306 controller (also sold separately, SC6936) All kits come with the PCB and all onboard components (see page 81, June24) - Adjustable SMD kit (SC6948) - Adjustable TH kit (SC6949) - Fixed TH kit – ZD3 & R1-R7 vary so are not included (SC6950) USB-C SERIAL ADAPTOR COMPLETE KIT (SC6652) (JUN 24) WIFI DDS FUNCTION GENERATOR (MAY 24) Includes the PCB, programmed micro and all other required parts Short-form kit: includes everything except the case, USB cable, power supply, labels and optional stand. The included Pico W is not programmed (SC6942) - Optional laser-cut acrylic stand pieces (SC6932) - 3.5in LCD touchscreen: also available separately (SC5062) 10MHz to 1MHz / 1Hz FREQUENCY DIVIDER (SC6881) (MAY 24) ESP-32CAM BACKPACK KIT (SC6886) (APR 24) Complete kit: Includes the PCB and everything that mounts to it, including the 49.9Ω and 75Ω resistors (see page 38, May24) $50.00 $10.00 $17.50 $22.50 $20.00 $20.00 $95.00 $7.50 $35.00 $40.00 Includes everything to build the BackPack, except the ESP32-CAM module *Prices valid for month of magazine issue only. All prices in Australian dollars and include GST where applicable. # Overseas? Place an order on our website for a quote. $100.00 $3.00 $42.50 Vintage Radio The TRF-One AM Radio based on a vintage IC By Dr Hugo Holden In April 1969, Electronics Australia published a radio design using the then-new LM372 AM radio integrated circuit (IC). 55 years later, the design is still valid, although the chip can be somewhat difficult to obtain. Despite that, intrigued by the design, I decided to build a modern version. T he single IC radio has always been a source of excitement and intrigue for radio constructions. The notion that nearly all the work can be done inside a single chip package is very appealing. This did not escape the attention of Jim Rowe in 1969, when RF-­capable ICs were making their debut. The siliconchip.com.au result was his “Micro-Plus” radio receiver design, published in the April issue of EA that year. The idea of a TRF (tuned radio frequency) radio is as old as the notion of radio itself. It involves a tuned resonant circuit consisting of an inductor and a capacitor; in a radio application, it is typically tuned by Australia's electronics magazine a variable capacitor. The tuned frequency range is usually the medium wave (MW) band, typically from 530kHz to 1600kHz, sometimes to 1700kHz. The most basic form of a TRF radio was the crystal set. In that case, the tuned circuit’s output was simply rectified by a diode to recover the February 2025  95 Fig.1: the LM372 IC from the late 1960s contains 14 NPN transistors, nine diodes and 17 resistors. It was intended to be the IF gain stage, detector and AGC circuit of an AM radio, but someone realised an antenna could be coupled directly to the input for MW reception. EA’s Micro-Plus radio used just eight components besides the LM372, battery and earphones (most of them capacitors). transmitted radio carrier’s amplitude modulation (AM). That audio signal could be sufficient to drive a high-­ sensitivity earphone or a crystal earpiece without any active amplification, so no power supply was needed! If a power supply was available, amplification could be added to the circuit to get a higher volume level, eg, to drive a loudspeaker. Later, multi-stage TRF radios were designed with high selectivity; then superhet radios came along with excellent selectivity and from that point on, TRF sets fell out of favour. I wanted to revisit the design of a single-gang variable-capacitor tuned The LM372 was not designed as the crux of an AM medium-wave TRF radio, but was pressed into service for that application. Rather, it was intended as an IF (intermediate frequency) amplifier with AGC (automatic gain control) – see Fig.1. However, it turned out that little needed to be added to the chip to make it function as a complete radio. Other MW band-single IC radios have been designed based on the ZN414 IC. Also, single-chip FM radios came along using the Phillips TDA7000, including popular radio kits sold by Dick Smith Electronics in the 1990s. The LM372 came in a TO-99 metal can package and has three internal functional blocks (see Fig.1). The gain stage typically amplifies the signal by 2360 times (67dB), while the precision detector stage has a gain of three times Photo 1: I made the radio’s case from phenolic material (left) and white Bramite (right). The latter was an Australian product that was no longer manufactured. Here the panels have already been cut to size, with the holes drilled and countersunk. Photo 2: the phenolic base with the rubber feet, spacers & other hardware. The two wires emerging from the base go to the AA cell holders underneath. 96 Silicon Chip TRF circuit, perhaps because I built these as a boy and had good success with them. I made several superhet radios as an adult, including those based on PLLs (phase-locked loops), and some FM (frequency modulation) radios too. Still, I retain a fondness for those early TRF sets. National Semiconductor’s LM372 IC Australia's electronics magazine siliconchip.com.au Fig.2: like EA’s 1969 Micro-Plus design, I have coupled a ferrite rod winding to the pin 2 input of IC1 with a series capacitor. The 1969 design was pretty minimalist, using just one active device (an NPN transistor) besides the LM372 IC, while mine adds a vintage op amp and two transistors for more overall gain and more power delivered to the speaker with less battery drain. or 10dB. So, a 50μV signal input modulated by 80% will produce an audio output signal of around 280mV RMS, or 800mV peak-to-peak. The AGC stage has an enormous control range of 60dB, with a threshold of 50μV. Therefore, using this IC as the basis of a TRF AM radio, the output level could be expected to be reasonably constant even if the signal level from the antenna increased from 50μV to 50mV. That means, tuning across the MW band, weak and strong signals would come in at a fairly uniform volume, more so than your typical AM radio. The LM372 has long been discontinued, but I found some for sale on eBay, so I snapped them up. They are presently hard to find, but a few are still for sale on eBay. At the time of writing, this listing offers two units (see Screen 1): siliconchip.au/link/abtp This IC has an input impedance of around 3kW, much lower than the ZN414. So when used in this application, unlike the ZN414, it requires a tap on the main resonant circuit, or a small coupling coil, to avoid damping the main tuned circuit. Designing a new circuit The circuit I designed around the LM372 IC is shown in Fig.2. As it is based on the same chip, it bears some similarities to the Micro-Plus from EA, April 1969, but it is my own original design. The main difference is that the Micro-Plus used a single-­ transistor Class-A amplifier whereas I have incorporated a preamplifier stage based on an op amp (IC2) plus a more powerful and efficient push-pull Class-AB amplifier based on NPN transistor Q1 and PNP transistor Q2. For a small battery-operated radio, it is always important to consider the power consumption. The LM372 can’t drive a speaker directly, so I decided to use a vintage Fairchild 741H op amp, also in a TO-99 metal can package, to provide a further voltage gain of 10 times. It drives a complimentary emitter-­ f ollower transistor output stage with simple diode biasing. Also, I decided to settle for a modest power output of 150-180mW (depending on whether I used the 32W or 40W speaker) so the output transistors would not require heatsinks. A 470μF capacitor stops DC being applied to the speaker. The 741H IC, 2N3053 NPN and 2N4036 PNP transistors are also available from eBay sellers. The physical bodies of the metal TO-5 cased 2N3053 and 2N4036 transistors act as heatsinks for the transistors inside them. They are better Photos 3 & 4: at this stage, I had soldered all the passives and sockets to the PCB, and then by mounting these high-quality metal AA cell holders on the underside of the base, the battery can easily be replaced when it goes flat. Once the cells are installed, a metal bar goes across them so they can’t fall out. siliconchip.com.au Australia's electronics magazine February 2025  97 Screen 1: it is challenging to find LM372s for sale these days, but there are a few around. This listing on eBay is probably your best bet (siliconchip.au/link/ abtp), but only two are available. The price is not bad, considering the original price and how long these have been obsolete. than epoxy-cased transistors in this respect. The temperature rise of each transistor body at full continuous sine wave output power is 10-13°C above ambient. In normal use listening to the radio, they never get noticeably warm. The manufacturer did not recommend the two 47W resistors in series with the LM372 input pins 2 (RF input) and 3 (gain stage input). Still, reading the Electronics Australia article, they had some difficulty with HF stability. So I decided to add them as a precaution. I also paid attention to the design of the PCB tracks around the input pins of the LM372. I provided double RF bypassing on the supply rail with high-quality 100nF axial ceramic capacitors. 1N5819 schottky diode D1 is included in case somebody installed the battery cells backward, so the LM372 and LM741 ICs would not be destroyed. I had vintage 40W and 32W speakers to test to see if they were suitable, along with a vintage National tuning dial that was made in 1943. I made the PCB with iron-on film, etched with ferric chloride. I added eyelet tags to connect wires to it and 0.9mm gold-plated pins and single connectors to couple in the signal from the ferrite rod’s coupling coil. Ultimately, I removed the tags and just used the eyelet part for the PCB connections. I stuck with all axial-leaded parts to give it a vintage theme. It pays to be mindful of the quality and appearance of the components. For example, the green 100nF 100V ceramic capacitors I used are high-quality vintage parts made by Corning Glass Works. I also used some ‘tropical fish’ capacitors to throw in a splash of colour. The IC and transistor sockets are high-quality types with gold-plated pins. It is a shame to have to solder to the pins of a very rare part like the LM372, or a vintage 741, for that matter. I found that grounding the body of the LM372 helps improve the stability, because it is such a high-gain arrangement in a very small package. I made a springy earth clamp out of brass that I screwed to one of the variable capacitor mounts with a collar – see Photos 7 & 8. I silver-plated the brass with an interesting product from the UK that is used to restore tea pots with a silvered finish. Mechanical construction In making an original or unique radio, ideally, you want it to look good and be long-lasting. So I never scrimp on materials and spend plenty of time to ensure that cut edges are smooth and polished. I also ensure that all the holes are in the correct positions, with perfect countersinking, so the screw heads sit flush where necessary. I had quite a lot of 10mm-thick brown phenolic material left over from other projects, some perforated aluminium mesh, and some white insulating material called Bramite – see Photo 1. Bramite is a uniquely Australian insulating panel material once used on household fuse boxes. It is practically unobtainable now. It is fantastically heat resistant, incredibly strong and machines well. I buy the 10mm-thick brown phenolic insulating panels from the markets at Akihabara in Tokyo. A local plastics company helped by planing the Bramite panel down to 5.5mm thick. All the hardware in this set is made from either stainless steel or nickel-­ plated brass. The hookup wire is Teflon covered. To keep the holes neat, I marked them with a micrometer edge, then a hand-held spike and started them with a 1mm drill bit in a hand pin chuck. I then drilled 1.5mm pilot holes and checked that everything fit together correctly. To ensure the CS screws that attach the front panel to the base and sides were all in the correct positions, I Photos 5 & 6: the completed radio chassis. The dial visible in the righthand photo is a vintage unit from 1943, while I made the speaker grille on the right from a scrap of perforated aluminium. 98 Silicon Chip Australia's electronics magazine siliconchip.com.au initially glued it together with some small dots of weak glue and used the holes in the panels used as a template to start the drill holes into the 10mm-thick phenolic material, so they all were in perfect registration. I used a metal strap to prevent the batteries from falling out of their holders. These Keystone cell holders (visible in Photo 4) are far superior to the usual Nylon AA holders that often stretch, harden or crack over time. The holders are retained by 4-40 UNC machine screws, with threads tapped the entire thickness of the 10mm thick phenolic base. The rubber feet are door stoppers. I machined spacers from ¼in brass tube that fit inside them, and they are attached to the base with 6-32 UNC screws passing into threaded holes in the baseplate. The variable capacitor is mounted on two nickel-plated brass spacers that attach it directly to the PCB. The vintage 40W loudspeaker was rusty and required rubbing down, treatment with Fertan and re-painting with Holts Auto Spray Paint. When the 365pF variable capacitor is fully meshed and the coil on the rod positioned to tune 530kHz, at the high end, the radio tunes to 2MHz. This Japanese-made variable capacitor does not have an additional trimmer capacitor on it, and there is nothing directly loading that point to add any capacitance there. It was made for the American market, with a ¼in shaft, and its body holes are pre-threaded with 6-32 UNC, rather than the usual metric threads found on Japanese parts. The finished radio is 200mm wide and about 150mm tall, including the rubber feet. At the upper end, by 18kHz, the first change in the amplifier’s output waveform, rather than amplitude loss, is slew-rate limiting by the 741 op amp. Performance The sinewave simply becomes trianThe main problem that a radio with gular, and the amplitude drops as the just one tuned circuit has is reduced frequency rises further. selectivity compared with a superhet This radio could almost be regarded or a TRF type, as they both have more as a hifi AM receiver. A tone correctuned stages. In other words, isolating tion capacitor is required to roll off the a station is harder if it’s close in fre- higher audio frequencies a little for a quency to another station. balanced sound. After some listening In this radio, this concern is some- tests, I found that an 820pF capacitor what offset by the very high-Q ferrite across the 100kW feedback resistor rod coil (shown in Photo 5) and the gave the best result. low loading on this by the coupling While there are better modern op coil, which I spaced away a little from amps than the 741, with output stages the Earthy end of the primary tuned that can swing closer to the supply circuit. Note that the input resistance rails (to gain more power output before of the LM372 is around 3kW. clipping), the internally frequency-­ Also, unlike most TRF radios, this compensated 741 is totally deaf to radio has a very high gain and a phe- radio frequencies and very stable, too, nomenally effective AGC. Weak and so it suits the application well. local stations appear with a similar Using the radio in an outdoor patio volume. Therefore, the performance is area, the 150mW of audio power is super lively, with many stations com- plenty, and it easily receives many AM ing in at a similar volume. stations with loud, crystal-clear outThe rod antenna is also deaf to elec- put with the volume control at about tric field noise. I used 30 AWG wire half or less. The radio exceeded my (0.254mm diameter) for the rod coil. expectations for an AM radio based I tried Litz wire but could not detect on a single tuned circuit. It is a very any difference in the Q compared to pleasant radio to listen to, and I find the 30 AWG wire. The large, high-­ myself using it most days. It was also permeability rod means there are fewer a fun exercise to design and build it! turns on the coil (just 46) than most If you want to build a similar set, you MW transistor radio coils. can download the PCB pattern from Due to the absence of transformer siliconchip.au/Shop/10/394 coupling in the audio stages, the freI also have dimensional drawings quency response of the audio circuit for the case, the PCB component layis flat, being about 3dB down at 50Hz out and some other details in the PDF (not that the small speaker could repro- at www.worldphaco.com/uploads/ SC duce such low frequencies very well). THE_TRF-ONE.pdf Photos 7 & 8: here you can see the spring-loaded grounding clamp I made to ground the TO-99 metal package of the LM372 radio IC. It attaches to the grounded metal post of the tuning gang. siliconchip.com.au Australia's electronics magazine February 2025  99 PRINTED CIRCUIT BOARDS & CASE PIECES PRINTED CIRCUIT BOARD TO SUIT PROJECT WIDE-RANGE OHMMETER WiFi PROGRAMMABLE DC LOAD MAIN PCB ↳ DAUGHTER BOARD ↳ CONTROL BOARD MINI LED DRIVER NEW GPS-SYNCHRONISED ANALOG CLOCK BUCK/BOOST CHARGER ADAPTOR AUTO TRAIN CONTROLLER ↳ TRAIN CHUFF SOUND GENERATOR PIC16F18xxx BREAKOUT BOARD (DIP-VERSION) ↳ SOIC-VERSION AVR64DD32 BREAKOUT BOARD LC METER MK3 ↳ ADAPTOR BOARD DC TRANSIENT SUPPLY FILTER TINY LED ICICLE (WHITE) DUAL-CHANNEL BREADBOARD PSU ↳ DISPLAY BOARD DIGITAL BOOST REGULATOR ACTIVE MONITOR SPEAKERS POWER SUPPLY PICO W BACKPACK Q METER MAIN PCB ↳ FRONT PANEL (BLACK) NOUGHTS & CROSSES COMPUTER GAME BOARD ↳ COMPUTE BOARD ACTIVE MAINS SOFT STARTER ADVANCED SMD TEST TWEEZERS SET DIGITAL VOLUME CONTROL POT (SMD VERSION) ↳ THROUGH-HOLE VERSION MODEL RAILWAY TURNTABLE CONTROL PCB ↳ CONTACT PCB (GOLD-PLATED) WIDEBAND FUEL MIXTURE DISPLAY (BLUE) TEST BENCH SWISS ARMY KNIFE (BLUE) SILICON CHIRP CRICKET GPS DISCIPLINED OSCILLATOR SONGBIRD (RED, GREEN, PURPLE or YELLOW) DUAL RF AMPLIFIER (GREEN or BLUE) LOUDSPEAKER TESTING JIG BASIC RF SIGNAL GENERATOR (AD9834) ↳ FRONT PANEL V6295 VIBRATOR REPLACEMENT PCB SET DYNAMIC RFID / NFC TAG (SMALL, PURPLE) ↳ NFC TAG (LARGE, BLACK) RECIPROCAL FREQUENCY COUNTER MAIN PCB ↳ FRONT PANEL (BLACK) PI PICO-BASED THERMAL CAMERA MODEL RAILWAY UNCOUPLER MOSFET VIBRATOR REPLACEMENT ARDUINO ESR METER (STANDALONE VERSION) ↳ COMBINED VERSION WITH LC METER WATERING SYSTEM CONTROLLER CALIBRATED MEASUREMENT MICROPHONE (SMD) ↳ THROUGH-HOLE VERSION SALAD BOWL SPEAKER CROSSOVER PIC PROGRAMMING ADAPTOR REVISED 30V 2A BENCH SUPPLY MAIN PCB ↳ FRONT PANEL CONTROL PCB ↳ VOLTAGE INVERTER / DOUBLER 2M VHF CW/FM TEST GENERATOR TQFP-32 PROGRAMMING ADAPTOR ↳ TQFP-44 ↳ TQFP-48 ↳ TQFP-64 K-TYPE THERMOMETER / THERMOSTAT (SET; RED) PICO AUDIO ANALYSER (BLACK) MODEM / ROUTER WATCHDOG (BLUE) DISCRETE MICROAMP LED FLASHER MAGNETIC LEVITATION DEMONSTRATION MULTI-CHANNEL VOLUME CONTROL: VOLUME PCB ↳ CONTROL PCB ↳ OLED PCB SECURE REMOTE SWITCH RECEIVER ↳ TRANSMITTER (MODULE VERSION) DATE AUG22 SEP22 SEP22 SEP22 SEP22 SEP22 OCT22 OCT22 OCT22 OCT22 OCT22 OCT22 NOV22 NOV22 NOV22 NOV22 DEC22 DEC22 DEC22 DEC22 JAN23 JAN23 JAN23 JAN23 JAN23 FEB23 FEB23 MAR23 MAR23 MAR23 MAR23 APR23 APR23 APR23 MAY23 MAY23 MAY23 JUN23 JUN23 JUN23 JUN23 JUL23 JUL23 JUL23 JUL23 JUL23 JUL23 JUL23 AUG23 AUG23 AUG23 AUG23 AUG23 SEP23 SEP23 SEP23 OCT22 SEP23 OCT23 OCT23 OCT23 OCT23 OCT23 NOV23 NOV23 NOV23 NOV23 NOV23 DEC23 DEC23 DEC23 DEC23 DEC23 PCB CODE 04109221 04108221 04108222 18104212 16106221 19109221 14108221 09109221 09109222 24110222 24110225 24110223 CSE220503C CSE200603 08108221 16111192 04112221 04112222 24110224 01112221 07101221 CSE220701 CSE220704 08111221 08111222 10110221 SC6658 01101231 01101232 09103231 09103232 05104231 04110221 08101231 04103231 08103231 CSE220602A 04106231 CSE221001 CSE220902B 18105231/2 06101231 06101232 CSE230101C CSE230102 04105231 09105231 18106231 04106181 04106182 15110231 01108231 01108232 01109231 24105231 04105223 04105222 04107222 06107231 24108231 24108232 24108233 24108234 04108231/2 04107231 10111231 SC6868 SC6866 01111221 01111222 01111223 10109231 10109232 Price $7.50 $7.50 $5.00 $10.00 $2.50 $5.00 $5.00 $2.50 $2.50 $2.50 $2.50 $2.50 $7.50 $2.50 $5.00 $2.50 $5.00 $5.00 $5.00 $10.00 $5.00 $5.00 $5.00 $12.50 $12.50 $10.00 $10.00 $2.50 $5.00 $5.00 $10.00 $10.00 $10.00 $5.00 $5.00 $4.00 $2.50 $12.50 $5.00 $5.00 $5.00 $1.50 $4.00 $5.00 $5.00 $5.00 $2.50 $2.50 $5.00 $7.50 $12.50 $2.50 $2.50 $10.00 $5.00 $10.00 $2.50 $2.50 $5.00 $5.00 $5.00 $5.00 $5.00 $10.00 $5.00 $2.50 $2.50 $5.00 $5.00 $5.00 $3.00 $5.00 $2.50 For a complete list, go to siliconchip.com.au/Shop/8 PRINTED CIRCUIT BOARD TO SUIT PROJECT ↳ TRANSMITTER (DISCRETE VERSION COIN CELL EMULATOR (BLACK) IDEAL BRIDGE RECTIFIER, 28mm SQUARE SPADE ↳ 21mm SQUARE PIN ↳ 5mm PITCH SIL ↳ MINI SOT-23 ↳ STANDALONE D2PAK SMD ↳ STANDALONE TO-220 (70μm COPPER) RASPBERRY PI CLOCK RADIO MAIN PCB ↳ DISPLAY PCB KEYBOARD ADAPTOR (VGA PICOMITE) ↳ PS2X2PICO VERSION MICROPHONE PREAMPLIFIER ↳ EMBEDDED VERSION RAILWAY POINTS CONTROLLER TRANSMITTER ↳ RECEIVER LASER COMMUNICATOR TRANSMITTER ↳ RECEIVER PICO DIGITAL VIDEO TERMINAL ↳ FRONT PANEL FOR ALTRONICS H0190 (BLACK) ↳ FRONT PANEL FOR ALTRONICS H0191 (BLACK) WII NUNCHUK RGB LIGHT DRIVER (BLACK) ARDUINO FOR ARDUINIANS (PACK OF SIX PCBS) ↳ PROJECT 27 PCB SKILL TESTER 9000 PICO GAMER ESP32-CAM BACKPACK WIFI DDS FUNCTION GENERATOR 10MHz to 1MHz / 1Hz FREQUENCY DIVIDER (BLUE) FAN SPEED CONTROLLER MK2 ESR TEST TWEEZERS (SET OF FOUR, WHITE) DC SUPPLY PROTECTOR (ADJUSTABLE SMD) ↳ ADJUSTABLE THROUGH-HOLE ↳ FIXED THROUGH-HOLE USB-C SERIAL ADAPTOR (BLACK) AUTOMATIC LQ METER MAIN AUTOMATIC LQ METER FRONT PANEL (BLACK) 180-230V DC MOTOR SPEED CONTROLLER STYLOCLONE (CASE VERSION) ↳ STANDALONE VERSION DUAL MINI LED DICE (THROUGH-HOLE LEDs) ↳ SMD LEDs GUITAR PICKGUARD (FENDER JAZZ BASS) ↳ J&D T-STYLE BASS ↳ MUSIC MAN STINGRAY BASS ↳ FENDER TELECASTER COMPACT OLED CLOCK & TIMER USB MIXED-SIGNAL LOGIC ANALYSER (PicoMSA) DISCRETE IDEAL BRIDGE RECTIFIER (TH) ↳ SMD VERSION MICROMITE EXPLORE-40 (BLUE) PICO BACKPACK AUDIO BREAKOUT (with conns.) 8-CHANNEL LEARNING IR REMOTE (BLUE) 3D PRINTER FILAMENT DRYER DUAL-RAIL LOAD PROTECTOR VARIABLE SPEED DRIVE Mk2 (BLACK) FLEXIDICE (RED, PAIR OF PCBs) SURF SOUND SIMULATOR (BLUE) COMPACT HIFI HEADPHONE AMP (BLUE) CAPACITOR DISCHARGER PICO COMPUTER ↳ FRONT PANEL (BLACK) ↳ PWM AUDIO MODULE DIGITAL CAPACITANCE METER BATTERY MODEL RAILWAY TRANSMITTER ↳ THROUGH-HOLE (TH) RECEIVER ↳ SMD RECEIVER ↳ CHARGER 5MHZ 40A CURRENT PROBE (BLACK) DATE DEC23 DEC23 DEC23 DEC23 DEC23 DEC23 DEC23 DEC23 JAN24 JAN24 JAN24 JAN24 FEB24 FEB24 FEB24 FEB24 MAR24 MAR24 MAR24 MAR24 MAR24 MAR24 MAR24 MAR24 APR24 APR24 APR24 MAY24 MAY24 MAY24 JUN24 JUN24 JUN24 JUN24 JUN24 JUL24 JUL24 JUL24 AUG24 AUG24 AUG24 AUG24 SEP24 SEP24 SEP24 SEP24 SEP24 SEP24 SEP24 SEP24 OCT24 OCT24 OCT24 OCT24 OCT24 NOV24 NOV24 NOV24 DEC24 DEC24 DEC24 DEC24 DEC24 JAN25 JAN25 JAN25 JAN25 JAN25 JAN25 PCB CODE 10109233 18101231 18101241 18101242 18101243 18101244 18101245 18101246 19101241 19101242 07111231 07111232 01110231 01110232 09101241 09101242 16102241 16102242 07112231 07112232 07112233 16103241 SC6903 SC6904 08101241 08104241 07102241 04104241 04112231 10104241 SC6963 08106241 08106242 08106243 24106241 CSE240203A CSE240204A 11104241 23106241 23106242 08103241 08103242 23109241 23109242 23109243 23109244 19101231 04109241 18108241 18108242 07106241 07101222 15108241 28110241 18109241 11111241 08107241/2 01111241 01103241 9047-01 07112234 07112235 07112238 04111241 09110241 09110242 09110243 09110244 9049-01 Price $2.50 $5.00 $2.00 $2.00 $2.00 $1.00 $3.00 $5.00 $12.50 $7.50 $2.50 $2.50 $7.50 $7.50 $5.00 $2.50 $5.00 $2.50 $5.00 $2.50 $2.50 $20.00 $20.00 $7.50 $15.00 $10.00 $5.00 $10.00 $2.50 $5.00 $10.00 $2.50 $2.50 $2.50 $2.50 $5.00 $5.00 $15.00 $10.00 $12.50 $2.50 $2.50 $10.00 $10.00 $10.00 $5.00 $5.00 $7.50 $5.00 $2.50 $2.50 $2.50 $7.50 $7.50 $5.00 $15.00 $5.00 $10.00 $7.50 $5.00 $5.00 $2.50 $2.50 $5.00 $2.50 $2.50 $2.50 $2.50 $5.00 USB PROGRAMMABLE FREQUENCY DIVIDER HIGH-BANDWIDTH DIFFERENTIAL PROBE NFC IR KEYFOB TRANSMITTER FEB25 FEB25 FEB25 04108241 9015-D 15109231 $5.00 $5.00 $2.50 NEW PCBs We also sell the Silicon Chip PDFs on USB, RTV&H USB, Vintage Radio USB and more at siliconchip.com.au/Shop/3 ASK SILICON CHIP Got a technical problem? Can’t understand a piece of jargon or some technical principle? Drop us a line and we’ll answer your question. Send your email to silicon<at>siliconchip.com.au How does the VSD work without speed sensing? Reading through the recent VSD article (Variable Speed Drive; November & December 2024; siliconchip.au/ Series/430), I was wondering about speed sensing. How will this work with heavy starting or large variable loads? If the slip gets too large, an induction motor won’t be doing much. Shouldn’t you measure the slip to control the torque? How does your controller deal with dynamic loads where a large slip would stall the motor? Or does it only work in load bands where the slip stays in range? (A. H., via email) ● Variable speed drives for induction motors do not generally use shaft speed sensing to measure slip (except for some high-precision servo types). If the software is intended to control torque rather than speed like the Silicon Chip VSD, a complex algorithm is used to estimate torque/slip based on an internal model of the induction motor to be controlled. You can read more about this by searching for “induction motor vector control”. The VSD described in the magazine operates in an open loop manner and does not sense motor torque or slip. At moderate to high speed, the VSD will most likely trip on overcurrent if the motor is overloaded. At very low speeds, you may be able to stall the motor if the stall current does not exceed the VSD’s maximum ratings. This VSD does not impart any special capabilities to the motor as far as load is concerned – it will only work within its normal load ratings. During construction I “blew” the thermal fuse, and had to replace it (with the necessary heatsinks attached). This was a challenge due to the proximity of the surrounding components. I decided to replace it on the opposite side of the PCB, where there is plenty of space, and used a wine bottle cap of water as a heatsink, giving me plenty of time to solder it securely. (J. A., Townsville, Qld) ● The vent fan can be as simple as the specified small fan mounted to the enclosure with appropriate cutout, or if you want, you can 3D print the housing we designed. The location we used was close to the main heater controller. We admit that we did not provide detailed drawings for this. As you will note from the plastic box, there are no right-angles anywhere, making an accurate drawing rather difficult. Even if we made one, constructors would have no solid reference to measure from. We mounted the controller, then ‘eyeballed’ the location of the vent with the following aims: • avoiding the mounting holes for the controller; • keeping it reasonably low, though this is not super critical; • choosing a flat spot on the box. We used long bolts to fix it, although glue would also work. Those fuses are very sensitive to soldering. I blew one too. The original draft of the article described my equivalent to your wine cap full of water: a rather less hygienic pair of fingers, which I licked first. If that fuse ever blows again, it will be for your safety, but I don’t think it will. 3D Printer Filament Dryer fan & vent queries Where to find test clips I am constructing the Dryer project in the Bunnings container and would for tiny ICs like some advice on the location and construction of the vent and associated fan, details of which are scarce in both articles (October-November 2024; siliconchip.au/Series/428). siliconchip.com.au In the “USB Mixed Signal Logic Analyser” article, on page 63 of the September 2024 issue (PicoMSA; siliconchip.au/Article/16575), there is a photo of a set of “female DuPont Australia's electronics magazine style mini probe clips” that appear to be smaller than the ones I have. My clips work OK with SOIC chips like the 74LV74, but they seem a bit risky for chips that have a finer pin spacing. Would the clips in the photo be capable of safely attaching to chips such as the fine pitch TSSOP-28 CS4398CZZ as used in the CLASSiC DAC (February-May 2013; siliconchip. au/Series/63)? If so, where can I obtain some? (D. J., Umina Beach, NSW) ● Richard Palmer, the author of that article, responds: The only significant factor for the test clips used for the PicoMSA is that they will mount onto the 2.54mm spaced male headers on the logic analysers. Beyond that, the choice is up to the constructor. The clips I included as an example will work OK with SOIC chips (with a 1.27mm pin pitch), but not really with SSOP/TSSOP packages that have much smaller 0.65mm or 0.5mm pin spacings. Test clips are available from eBay that will apparently work with finepitch chips, eg, eBay 322087457819 A DIY alternative I have heard suggested, but never used, is a blob of BluTack with needles poked through it! When I’m debugging fine-pinned chips, I usually look for test points away from the chip. If a convenient test point isn’t available, I clear off some of the protective coating from a connected trace and solder a short length of wire-wrap wire (Kynar) or other fine wire. It’s straightforward to attach clips to the ‘hedgehog’ created. DC motor control for a milling machine I have a few technical questions relating to the 180-230V DC Motor Speed Controller from your July & August 2024 issues (siliconchip.au/ Series/418). I operate a Sieg X3 milling machine, but it has not run since a lightning strike near my workshop a fortnight ago. It uses a 600W 3.2A 240V DC motor that is running OK on a test February 2025  101 supply. The mains input supply is present, but there is no DC output from the control board, which I have sent away for repair. I have begun building your 180230V DC Motor Speed Controller as backup or a possible upgrade. My first question relates to the “Emergency Stop” (terminals 7 & 8 of CON1). The Sieg had a stop button wired in a similar part of the circuit. Is there an advantage in placing the Emergency Stop in this part of the circuit rather than in the mains supply to the whole circuit and machine? I guess it’s handy in a motor overload situation where the circuit can remain live while the problem is quickly rectified, but an emergency stop on the main supply would be better in case of electrical failure, smoke, fire or pump malfunction. I tend to operate the machine using the Fwd/Stop/Reverse switch and eventually discovered the main power indicator light had broken, thus there was no green light to remind me to turn off the main switch when I locked up in the evening. As I now know, part of the circuit was still live and susceptible to damage from the lightning strike (I had the same problem with the workshop radio!). So I’ve installed a big red button and green light on the main supply to the machine. I’m unsure if I need the other stop button in the circuit for any situation. My second question relates to the “Restart” function. If I’m doing a production job, milling 25 brass trunnions, or drilling 50 similar holes on x-y coordinates in a large part, I want to find the optimum setting for the speed and operate the machine using the Fwd/Stop/Rev switch. Can I do this, as I could on the Sieg, without having to go back and reset the speed each time, which would add time, cost to the job and an opportunity for errors? My third question relates to relay RLY3. I would like to add an extra pole to the relay to operate the mains-­ powered coolant pump simultaneously with the motor. I guess I could add another relay. Do you have any suggestions on the neatest way to do this? A friend just introduced me to the magazine. I was an old Electronics Australia subscriber; I love it and your work. (D. T., Castle Hill, NSW) 102 Silicon Chip ● The emergency stop connection requires only low-current switching, enabling it to be used as a safety cutout should, for example, shields not being in place before starting. A high-­ current emergency stop switch could be used in series with the mains supply instead, but it must be rated to handle the full motor current. The restart function requiring the speed pot to be brought fully anti-clockwise before the motor can start is a safety function so the motor does not suddenly restart, possibly at full speed, if a current overload clears or the emergency stop button is released. We do not recommend bypassing this feature. Instead, you could consider using a sticker and pen to add a mark near the speed pot to indicate the ideal speed setting for a specific job. That should allow you to bring it back up to the required speed pretty quickly each time you start it. Adding an extra relay to effectively provide another contact for RLY3 would be easy enough. Just connect the extra relay coil across the CON3 terminals. It needs to have a 12V DC coil. Changing Roadies’ Test Oscillator output The Roadies’ Test Oscillator project article (June 2020; siliconchip.au/ Article/14466) shows how to wire an XLR to TRS cable to give an XLR output connector. I need to make a TRS to RCA cable instead. I have looked on the internet but the results I find are confusing. What is your recommended wiring? (R. M., Melville, WA) ● For that project, the two leads that are shown connected to the XLR connector at pins 2 and 3 can instead connect to each terminal of the RCA connector. It doesn’t matter which way around they go. Various motor speed control options I built one of the original Induction Motor Speed Controllers (April & May 2012; siliconchip.au/Series/25) and have used it with a few small tools in the workshop, like a scroll saw and a miniature table saw. Lately, I’ve been considering adding variable speed to the wood lathe. The motor I picked for my lathe some 40-odd years ago was a scrap Australia's electronics magazine washing machine motor and it has served me well. Unfortunately, it’s a capacitor start type, as confirmed by the label: 230VAC, 3.5A, 1/3 HP, 1425RPM, CAP ST. The Mk1 (and new Mk2) VSD articles say capacitor start is not an option for speed control, so I thought no more of it. However, it would be possible to isolate the centrifugal switch and use an external switch (eg, a PIC and a relay) with some extra logic. I have the motor apart at the moment for servicing, which got me thinking. The centrifugal switch opens at some particular RPM; the article mentions 70% of full speed. I don’t know how critical that RPM is. The external switch could be opened at a certain speed and, via another relay, it could also switch the motor power source from the mains socket to the speed controller. The external switch would not close until the motor stopped. Do you have any idea of the minimum RPM at which the centrifugal switch (internal or external) can be opened? Would anything undesirable happen if/when the controller takes over? The speed setting of the controller will not likely be the same as the motor’s speed when the power source is switched. I also took note of the DC Motor Speed Controller from July 2024 (siliconchip.au/Series/418). As it happens, I’ve dismantled a couple of treadmills recently and have their motors. The large one at the back of the photo (shown above) is from a well-built treadmill and is stated to be 1.75HP. Curiously, the noticeably smaller one in front, from an obviously cheaply made treadmill, is stated to be 2HP. Yeah, right. The electronics of both stopped working but the motors seem OK. I could make one of your DC speed continued on page 104 siliconchip.com.au MARKET CENTRE Advertise your product or services here in Silicon Chip FOR SALE FOR SALE USED ITEMS FOR SALE Retired Silicon Chip staff member Jim Rowe is trying to find good homes for the following items: 1. A Sony VPL-CSI LCD data and SVGA video projector ($100). 2. A Teac PC-10 portable stereo cassette recorder with ‘Dolby System’ and AC power pack ($75). 3. A Chinon 506-SM-XL Super-8 sound camera ($50). 4. A Pioneer VSX-D506 5-channel amplifier with a Dolby Digital decoder, and 100W output from each channel ($100). 5. An AKG D19C dynamic wideband cardioid microphone ($50). 6. An LG BP125 Blu-Ray player ($75). 7. A Toshiba SD-2500 DVD player ($40). 8. A Hantek DSO-2250 USB PC oscilloscope, with two 100MHz channels, plus an operating manual and a small software CD ($50). All of the above are available to be picked up from my home in Arncliffe, Sydney. Also available are quite a few mini file drawers with electronic components such as capacitors, resistors, transistors, ICs, LEDs and diodes, etc. These I’d be happy to give away if someone would be prepared to call and take them away. Please contact me by email to jimrowe<at>optusnet.com.au if any of the above is of interest. LEDsales KIT ASSEMBLY & REPAIR LEDS, BRAND NAME AND GENERIC LEDs, filament LEDs, LED drivers, heatsinks, power supplies, kits and modules, components, breadboards, hardware, magnets. Please visit www.ledsales.com.au DAVE THOMPSON (the Serviceman from Silicon Chip) is available to help you with kit assembly, project troubleshooting, general electronics and custom design work. No job too small. Based in Christchurch, New Zealand, but service available Australia/NZ wide. Email dave<at>davethompson.co.nz PMD WAY offers (almost) everything for the electronics enthusiast – with full warranty, technical support and free delivery worldwide. Visit pmdway.com to get started. KEITH RIPPON KIT ASSEMBLY & REPAIR: * Australia & New Zealand; * Small production runs. Phone Keith: 0409 662 794 keith.rippon<at>gmail.com FOR SALE: 600 used radio valves: 7-pin & 9-pin, condition not tested. The lot for $100 + postage. For more info email: Dieter Dauner, VK2EDD ddauner<at>bigpond.net.au PCB PRODUCTION PCB MANUFACTURE: single to multilayer. Bare board tested. One-offs to any quantity. 48 hour service. Artwork design. Excellent prices. Check out our specials: www.ldelectronics.com.au Advertising in Market Centre Classified Ad Rates: $32.00 for up to 20 words (punctuation not charged) plus $1.20 for each additional word. Display ads in Market Centre start at $82.50 per column centimetre per insertion. All prices include GST. Closing date: 5 weeks prior to month of sale. To book, email the text to silicon<at>siliconchip. com.au and include your name, address & credit card details, or phone (02) 9939 3295. WARNING! Silicon Chip magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should be considered dangerous or even lethal if not used safely. Readers are warned that high voltage wiring should be carried out according to the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed or injured while working on a project or circuit described in any issue of Silicon Chip magazine. Devices or circuits described in Silicon Chip may be covered by patents. Silicon Chip disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. Silicon Chip also disclaims any liability for projects which are used in such a way as to infringe relevant government regulations and by-laws. Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Competition & Consumer Act 2010 or as subsequently amended and to any governmental regulations which are applicable. siliconchip.com.au Australia's electronics magazine February 2025  103 Advertising Index Altronics.................................27-30 Blackmagic Design....................... 7 Dave Thompson........................ 103 DigiKey Electronics....................... 3 Emona Instruments.................. IBC Jaycar............................. IFC, 51-54 Keith Rippon Kit Assembly....... 103 LD Electronics........................... 103 LEDsales................................... 103 Microchip Technology.............OBC Mouser Electronics....................... 4 OurPCB Australia.......................... 5 PCBWay......................................... 9 PMD Way................................... 103 Radio Valves - Dieter Dauner.... 103 Silicon Chip Back Issues........... 25 Silicon Chip Binders.................. 79 Silicon Chip Pico BackPack...... 78 Silicon Chip Shop........ 71, 94, 100 Silicon Chip Subscriptions........ 31 The Loudspeaker Kit.com............ 8 Used Gear - Jim Rowe.............. 103 Wagner Electronics..................... 93 Notes and Errata Maxwell’s Equations, November 2024: on p91, Gauss’ full name is incorrectly given as Henrich Gauss instead of Carl Friedrich Gauss. Watering System Controller, August 2023: a bug in the WiFi stack used in the original WebMite firmware can cause spurious reboots of the Controller. We recommend you update to the latest firmware version (released January 2025) which fixes this problem. Next Issue: the March 2024 issue is due on sale in newsagents by Thursday, February 27th. Expect postal delivery of subscription copies in Australia between February 26th and March 14th. 104 Silicon Chip controllers for the larger motor, although that would be more expensive than modifying a capacitor start motor (which might not work anyway). Any thoughts? (J. C., Auckland, New Zealand) ● Andrew Levido responds: You have a couple of good options. It seems like you have pretty good understanding of motors and understand the risks in the following advice. You probably can use an unmodified capacitor start motor with a VSD as long as you ramp up past the centrifugal switch opening speed and remain above the closing speed thereafter. I believe there is a fair bit of hysteresis built into the centrifugal switch to prevent it from chattering. You would have to measure the two speeds (closed and open) for each motor; I suspect they are pretty variable between models. Alternatively, you could switch the start winding and capacitor out externally with a suitable switch or contactor once the motor has started. A manual switch or a simple timer would probably be enough to control it. Then you could reduce the speed as far as you like without any risk of burning out the start winding (but you would still need to monitor it to make sure it doesn’t stall). The downside I see here is reduced torque at low speeds. I have no idea how much of a problem this would be for a lathe. Still, I suspect once the work is spinning, the torque required to keep it going is much lower than that required to get it up to speed. However, the speed regulation may be poor at low speed – I honestly don’t know. I think you would have to try it and see. I don’t suggest you switch the motor between mains power and the speed controller, as the instantaneous current demand at switch-over could trip the over-current protection. It’d be better to let the motor be spun up gradually by the speed controller with the start winding in-circuit, then switch it out. Also see the letter in the Mailbag section last month from Ian Thompson (p6, January 2025), who has an alternative method for controlling the speed of induction motors with centrifugal switches. Eliminating hum in 12V 20W Stereo Amplifier I got around to building the Compact Australia's electronics magazine 12V 20W Stereo Amplifier kit (May 2010; siliconchip.au/Article/152) from the Altronics K5136 kit. Overall, I’m surprised how good the sound is, and it can drive the two Klipsch speakers I have quite well. The only thing I’m not sure about is that it seems to have a bit of a ‘hum’ at the loudspeakers when switched on, even with the volume low, if there is no input signal. Where does the hum come from and can I eliminate it? I used heavy duty shielding on input cables. I attached the wire across the three potentiometers, they are grounded together and it’s in a metal box. The potentiometers and wire are grounded to the negative 12V DC input as per the instructions. I notice that when I put my finger on the volume potentiometer, the hum almost goes away. Could it be some sorting of grounding problem? (E. M., Hawthorn, Vic) ● If the design had an inherent hum problem, it would have shown up in the performance measurements/graphs published in the article as a poor signal-to-noise ratio or high distortion. So we’re pretty sure it isn’t a circuit problem. Also, it’s DC-­powered, so there’s no reason for mains hum to be present unless the DC supply is poorly regulated. Based on your description, it sounds like it may be being picked up by the potentiometer bodies or the case. First, did you scrape off the passivation from the pot bodies before soldering the wire to them? Please be careful doing this as the dust can be toxic (eg, do that outside and make sure it doesn’t blow towards you). If you didn’t, it’s possible there is no electrical connection to the grounding wire. If the bodies are definitely grounded, it’s likely the hum is coming into circuit ground from the power supply. Do you have a different power supply you can try? Do you have an Earthed metal object (like a desktop computer case) that you can connect to circuit ground via a clip lead, to see whether Earthing it helps? We also suggest checking if the hum is present without any input signal connected, to verify that it isn’t being injected via the signal ground connection from the signal source. Note: E. M. replied to say that Earthing the negative output terminal of the switch-mode power supply eliminated the hum. 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