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Table 1.2: Going Further with the 2N7000 MOSFET Topic operation can be altered by varying the values used in the twin-tee network (see Going further below). Note that popular Spice-based simulation software can often be used to model and optimise the operation of MOSFET circuits, as shown in Fig.1.14. Finally, Fig.1.15 shows a crystalcontrolled oscillator based on a single 2N7000 device. This provides a typical output of 2Vpk-pk and has been tested with quartz crystals between 2MHz and 20MHz. This arrangement forms the basis of the author’s own simple Crystal Checker circuit, which he uses in his workshop. Going further Our Going further table (opposite) will help you locate the component parts and further information that will allow you to quickly progress with your own designs and modifications. It also provides you with background reading that will help you get up to speed with the necessary underpinning knowledge for key topics discussed. GET T LATES HE T COP Y OF TEACH OUR -IN SE RIES AVAIL AB NOW! LE Source Notes 2N7000 MOSFET The 2N7000 is available from many suppliers, including CPC/Farnell, Mouser, RS Components and numerous online suppliers. Data sheets can be downloaded from manufacturers’ websites including ON Semiconductor, Fairchild and Vishay Prices range from a few pence to about 50p depending on the quantity purchased Audio amplifiers For all your audio amplifier requirements, the PE column Audio Out provides a treasure chest of tips, hints, designs and ideas, drawing on Jake Rothman’s decades of experience in teaching, designing and building a huge range of audio circuits. All Audio Out columns are available via back issues at: www.electronpublishing. com Circuit simulation The author’s own book, Electronic Circuits: Fundamentals and Applications (Fifth Edition 2020 published by Routledge 9780367421984) provides an introduction to circuit simulation based on the popular SPICE-based Tina Pro software Tina software can be obtained from www.tina. com. A cut-down version can be freely downloaded from Texas Instruments at: www.ti.com/tool/TINA-TI Distortion A detailed explanation of different types of distortion can be found in Part 8 of Electronics Teach-In 7 www.electronpublishing. com/product/electronicsteach-in-7 Negative feedback For a useful introduction and relevant theory see Part 8 of Electronics Teach-In 7 www.electronpublishing. com/product/electronicsteach-in-7 Transistor characteristics and load lines The author’s own book, Electronic Circuits: Fundamentals and Applications (see above) provides an introduction to transistor characteristics, load lines and amplifiers. Order direct from Electron Publishing PRICE £8.99 (includes P&P to UK if ordered direct from us) EE FR -ROM CD ELECTRONICS TEACH-IN 9 £8.99 FROM THE PUBLISHERS OF GET TESTING! Electronic test equipment and measuring techniques, plus eight projects to build FREE CD-ROM TWO TEACH -INs FOR THE PRICE OF ONE • Multimeters and a multimeter checker • Oscilloscopes plus a scope calibrator • AC Millivoltmeters with a range extender • Digital measurements plus a logic probe • Frequency measurements and a signal generator • Component measurements plus a semiconductor junction tester PIC n’ Mix Including Practical Digital Signal Processing PLUS... YOUR GUIDE TO THE BBC MICROBIT Teach-In 9 Teach-In 9 – Get Testing! A LOW-COST ARM-BASED SINGLE-BOARD COMPUTER Get Testing Three Microchip PICkit 4 Debugger Guides Files for: PIC n’ Mix PLUS Teach-In 2 -Using PIC Microcontrollers. In PDF format This series of articles provides a broad-based introduction to choosing and using a wide range of test gear, how to get the best out of each item and the pitfalls to avoid. It provides hints and tips on using, and – just as importantly – interpreting the results that you get. The series deals with familiar test gear as well as equipment designed for more specialised applications. The articles have been designed to have the broadest possible appeal and are applicable to all branches of electronics. The series crosses the boundaries of analogue and digital electronics with applications that span the full range of electronics – from a single-stage transistor amplifier to the most sophisticated microcontroller system. There really is something for everyone! Each part includes a simple but useful practical test gear project that will build into a handy gadget that will either extend the features, ranges and usability of an existing item of test equipment or that will serve as a stand-alone instrument. We’ve kept the cost of these projects as low as possible, and most of them can be built for less than £10 (including components, enclosure and circuit board). © 2018 Wimborne Publishing Ltd. www.epemag.com Teach In 9 Cover.indd 1 01/08/2018 19:56 FREE COVER-MOUNTED CD-ROM On the free cover-mounted CD-ROM you will find the software for the PIC n’ Mix series of articles. Plus the full Teach-In 2 book – Using PIC Microcontrollers – A practical introduction – in PDF format. Also included are Microchip’s MPLAB ICD 4 In-Circuit Debugger User’s Guide; MPLAB PICkit 4 In-Circuit Debugger Quick Start Guide; and MPLAB PICkit4 Debugger User’s Guide. ORDER YOUR COPY TODAY JUST CALL 01202 880299 OR VISIT www.electronpublishing.com 42 Practical Electronics | January | 2021 PIC n’Mix Mike Hibbett’s column for PIC project enlightenment and related topics Part 3: PIC18F development board W e continue this month with  Flexible – the ability to select which Simplifying soldering the process of creating a development board for the PIC18F processor. In this and the following article we are going to cover the design of the board, ensuring it supports a number of different externally connected technologies: GPS location, digital compass, various display types, speech output, connectivity to a PC connection via USB, Wi-Fi communication, interfacing with Amazon Alexa, servomotor control and analogue input. The focus for this article series will be on making use of the processor, rather than diving into the finer details of internal processor peripherals – a hands-on, practical approach. In the previous article we selected the processor we will be using – the PIC18F47K42 in a 40-pin DIL package. We chose that package because it provides lots of I/O signals yet is very easy to solder (via a 40-pin socket) making it easy to replace if damaged. There is a lot of work to do before we jump in and start designing a PCB. Even before drawing the schematic, we need to be clear in our objectives for this board, writing down some requirements, and check if any of our items might clash with each other and (hopefully) find solutions to those issues. Let’s start by writing down our key requirements:  Easy to assemble – no tiny surfacemount soldered parts. We want this to be easily assembled, by any hobbyist features are fitted at any one time, without major re-work  Re-usable – easy to re-purpose to different projects  PC serial communications interface using an on-board UART-to-USB converter chip, the MCP2221A-I/P  Header connector for an ESP-01 Wi-Fi interface  Header for a Micro SD Media card module  Several three-pin headers for servomotors  Header for a colour touchscreen LCD  FET power switches for external device control  PICkit 4 header for programming/ debugging  Header pins for external devices with power and I2C or SPI buses  Two configurable op amps, for analogue input signal conditioning  32kHz crystal for very low-current operation  Plenty of analogue input and digital I/O headers  Single status LED, and a power LED  Power input from a standard DC power brick. The first requirement is driven by a desire to enable this board to be built by as many hobbyists as possible, avoiding the use of difficult-to-solder surface-mount parts. This is easy to achieve with most components, but the USB and SD Media connectors are only available in hard-to-solder formats. Thankfully, this has been recognised by a number of electronics suppliers who provide very simple and low-cost preassembled carrier boards, as shown in Fig.1 and Fig.2. We will also use a Wi-Fi module with a similar simple header interface (shown in Fig.3). These little boards enables us to avoid the difficulty of soldering ICs that are only available in surface-mount format. Another benefit of providing these as plug-in features is that they are optional – buy them only if you want to use them. On our PCB, USB connectivity to a PC is provided by a special IC, the Microchip MCP2221A-I/P. Our processor does not have USB built in, and the choice to use an external IC rather than choose a processor with a USB peripheral is deliberate – in our experience the USB software libraries provided by Microchip (and other vendors) are very complicated to use. The MCP2221A provides a simple USB-to-UART conversion, so our processor we will connect the IC to one of our serial ports, and the USB cable will look to the PC like a standard serial port. This will make software development Fig.1. USB connector board. Fig.2. SD-Media connector board. Practical Electronics | January | 2021 That’s quite a list, but many of these will be easy to implement, and simply remind us to include certain components in the schematic. Many requirements have a consequence – adding a 32kHz crystal, for example, means that two GPIO pins become unavailable. Fortunately, however, there are many GPIO pins, so that won’t be an issue for us. Fig.3. Wi-Fi interface board. 43 comes pre-loaded with software that is easy to use and communicates with our processor over one of the UART interfaces (we have two UARTs on our processor, so simultaneous Wi-Fi and USB connectivity will be possible.) Power supply design Fig.4. Plug-in option boards. on both the PC and the processor simple. Plus, the MCP2221A can run at 3.3V or 5V, so it’s very flexible. An SD Media card communicates over an SPI interface, so we will route one of the SPI buses to it. SD Media cards operate at a voltage of 3.3V maximum, so bear that in mind if you are thinking of running your board at 5V. An external adaptor to level-shift the signals will be required if you want to use an SD Media card for data storage and run the board at 5V simultaneously. This will be true for the Wi-Fi interface too. The downside of using standard components in a design like this is that the PCB will larger than the more usual Microchip development boards, which use tiny surface-mount components. This is not really a big issue – the board is for development purposes, so easy soldering and easy access to components is more important than size. Plug-in options Although this will be a general-purpose development board, we do have in mind some external devices that we would like to be able to connect to it. Fig.4 shows the current selection from our lab – An LED matrix, touch-input colour LCD screen, GPS module and a bag of over 30 different random sensors that were purchased cheaply on eBay. We will provide links for the parts used in upcoming articles, as we make use of them. Needless to say, these will just be examples, you are free to attach whatever you want to your board. Wi-Fi interface Choosing a Wi-Fi interface board presented some interesting challenges. The board has to be cheap, easy to purchase, easy to solder and easy to use. We settled on a relatively old device, the ESP-01, which is based on the ESP8266 processor. This is such a popular design that it has become available from dozens of different suppliers and is incredibly cheap – as low as £3 including delivery, if you do not mind the long delivery times from Asia. The module 44 With those thoughts behind us, it’s time to make our first key design decisions – how will we power the board, and how will we direct the processor’s 36 I/O pins to onboard and external connections? We’ll start with the power supply design. So, first, we must decide at what voltage to run the board: 3.3V or 5V? These two common operating voltages have for years been a bone of contention for us, too often we have tried to pair a 5V external device with a 3.3V processor development board (servomotors with a Raspberry Pi for example) or a 3.3V module with a 5V processor board (a GPS module with an Arduino, for example.) It’s been such an issue over the years that we have decided to design a board that can operate at either voltage, selected by a simple jumper. The key ICs in our design have been selected as ones that can operate at either voltage, so we only need a power supply design that can be switched between the two voltages – and we can do that with an LM317 regulator and a few resistors. We will use a beefy TO220-style regulator, as powering some external devices, particularly servo motors, may bring our current-consumption requirements up to around 500mA or so. Plus, we make a mental note: leave space around the regulator for heatsinking, the LM317 is a linear regulator, and will get warm at these higher currents. We could have gone with a much more efficient switching-regulator design, but that would add complexity. We will typically be powering this board from a wall socket power supply, so power supply efficiency is not a design requirement. Studying the datasheet of the LM317 we spot that is has a requirement for a minimum 3V input to output voltage differential, so to generate a 3.3V or 5V output will require an input power supply running at a minimum of 8V DC. 9V and 12V DC power supplies are common and cheap, so the regulator choice works well. We will include a power supply isolation header pin, so 3.3V or 5V can be supplied to the board directly, if desired. This will enable the testing of low-power designs, or allow the addition of a more efficient power supply should you want to use a battery. A barrel jack will be accommodated to support plugging in a standard DC power brick. There are several different standards for barrel jack connectors (different pin diameters) so we have to make a decision on which to go with – we chose the 2.5mm P I C 18 F 4 7 K 4 2 MC LR / R E3 1 4 0 R B 7 R A 0 2 39 R B 6 R A 1 3 38 R B 5 R A 2 4 37 R B 4 R A 3 5 36 R B 3 R A 4 6 35 R B 2 R A 5 7 34 R B 1 R E0 8 33 R B 0 R E1 9 32 VDD R E2 10 31 VSS VDD 11 30 R D7 VSS 12 29 R D6 R A 7 13 28 R D5 R A 6 14 27 R D4 R C 0 25 26 R C 7 R C 1 16 25 R C 6 R C 2 17 24 R C 5 R C 3 18 23 R C 4 R D0 19 22 R D3 R D1 20 21 R D2 Fig.5. PIC18F47K42 processor pin-out. version, as that seemed to be the most common size in our lab. Header pins and solder pads will also be provided for power input, as these cost nothing but add flexibility to the choice of power connection. External header pins We have left the most complex design decision until last. The processor, shown in Fig.5, has 36 I/O pins – how will we connect these? Thankfully, the pins are highly configurable; internal peripherals such as SPI, I2C and UART interfaces are all able to be programmed to appear on any pin from a selection of the 36 pins available. While this simplifies circuit design and PCB layout, it does add complexity during software design. With a general-purpose development board design, where we make all pins available, this task is very complicated – but that task can be simplified by writing up a spreadsheet for the pins, indicating where each one will go. The approach taken here is that multiple headers are provided for generic SPI and I2C devices, several servomotor headers (using the standard header layout) and specific headers for the Wi-Fi, USB and Micro SD Media boards. Two FET-controlled power output headers are provided, and then all I/O pins will go to 0.1-inch pitch header strips. Even the I/O signals that are optionally connected to other headers on the board will be routed to the generic I/O headers. Three I/O pins will not be used. PORTE.3 is shared with the master reset pin MCLR, which we will keep as a reset pin. PORTC.0 and PORTC.1 are multiplexed with the external crystal inputs, which will be taken up with our 32kHz crystal, for ultra-low power operation. To finish off, two op amps will be implemented on the board, but left unconnected, with headers to allow user choice of where they connect. Practical Electronics | January | 2021