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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 Dual Battery Lifesaver mods for 24V Silicon Chip is a great magazine. What changes would be necessary to use the Dual Battery Lifesaver from the December 2020 issue (siliconchip. com.au/Article/14673) with a 24V battery? (J. O’G, Allambee, Vic) • You would need to use a different regulator as the S-812C33 is only rated for 16V at its input. The only suitable regulator we can find in a TO-92 package is the AP7381-33V-A, but it has a different pinout from the S-812C33. It could be adapted to the PCB footprint with a bit of lead-bending. The capacitor ratings would also need to be increased to 50V. The Mosfet ratings are 30V, so they should be just adequate. Other than the above, you would just need to change the dividers to suit the higher thresholds. Isolation transformer no good for fast signals In the November 2020 issue (p110), a reader inquired about “Adjusting Mosfet dead time with a scope”. Could this be done by inserting a step-up transformer in place of the shunt resistor? Alternatively, could a ferrite clamp transformer be used to measure the dead time current magnitude and shape? The current meter style clamp I bought at Jaycar has a voltage-to-current conversion table printed on it. This meter does not seem to be available any more; perhaps you could publish a project to build one. (N. B., Taylors Lakes, Vic) • A transformer in that role would affect the measured signal phase, and would be unlikely to have enough bandwidth to accurately reproduce the signal, which has fast rise and fall times. So a transformer probably couldn’t be used to measure the Mosfet dead time accurately. Ferrite current clamp meters have the same problem, whereas a resistive shunt does not cause a phase shift and siliconchip.com.au will not affect the rise and fall times, even at low levels. The measurement limitation then becomes the scope’s bandwidth and sensitivity, with many scopes being fast enough and sensitive enough to make this sort of measurement. As for your idea of a DIY Clamp Meter, we published such an article in the September 2003 issue (siliconchip. com.au/Article/3884). That design is still valid and all the parts used are still available, although you will have to find another source of the 50A alligator clip as the original DSE is no longer around. Questions about AWA ribbon mics I have a pair of AWA ribbon microphones that are identical in every respect to RCA 44BX models, and I’d like to know if they were made under licence by AWA or someone else or just rebadged. I’ve corresponded with a guy in the USA who reconditions these; apparently, his father worked for RCA and designed the 44BX. He’s fairly sure they’re just rebadged. I recall them being advertised in Electronics Australia in the mid-70s when 4VL was selling off some surplus studio gear. It stuck in my mind as my brother was an announcer there around that time. Years later, a friend told me he bought them from that ad and subsequently gave them to me. I’ve loaned them out a couple of times as stage props, but on both occasions, they were used as live mics. My brother worked at QTQ9 for a period and had their techs check them out, and they considered they were still up to broadcast specs. I have no use for them and would like them to be used by someone who does. The price will be enhanced if someone can confirm that RCA built them, so I hope someone can fill in the gaps. (B. M., North Ryde, NSW) • We don’t know the answer to this, but maybe one of our readers does. Australia’s electronics magazine Using D1 Mini BackPack with 2.8in screen I happen to have a 2.8-inch TFT touchscreen on hand, so I am using it to build the D1 Mini BackPack project (October 2020; siliconchip.com.au/ Article/14599). I have wired it all up and loaded the code etc. Only part of the screen has a display, and the keyboard buttons respond but they are not aligned. For example, if I press “s” I get a “c” etc. Are there other software settings and changes I need to do to get a 2.8-inch screen working with this project? (S. O., Sydney, NSW) • As we said in the article, our software is designed for the 3.5in display, which uses a different driver and has a different resolution to the 2.8in display (480x360 compared to 320x240). If it was simple to make the software work with both displays, we would have done so, but it is not, and it will involve more than just changing settings. The driver and initialisation code will have to be changed to suit the ILI9341 controller that is used in the 2.8in display. Also, all the graphics, touch and user interface will have to be adjusted to work at the lower display resolution. That means numerous changes throughout the code. We are sure that it is possible to get it to work, but it will involve significant changes to the demonstration sketch. D1 Mini BackPack not showing weather info I am building the D1 Mini BackPack; thanks for an interesting project. I have gone through the software installation, and everything seemed OK until I got to the OpenWeather part. I made a free account and got the API key straight away. I entered that into the script and loaded it into the ESP8266. When I switched it on, the screen came to life, and I entered my WiFi info and then selected my location. But all I get on the screen is my area, WiFi IP address and signal January 2021  107 strength stating OK (green), plus the local time. But no weather information ever comes up. I am wondering if anybody has had the same problem. (R. S., Epping, Vic) • If it is showing the current time, that means that it is connecting to your WiFi OK and has internet access. We don’t know why it isn’t connecting to OpenWeatherMap. There’s a lot more information available through the Serial Monitor, so we suggest that you open that and check what messages appear. Your API key should be enough for the weather function to work. Vintage Radio cabinet restoration I always enjoy reading the Vintage Radio section of your magazine. Associate Professor Graham Parslow and Ian Batty do a great job presenting the details of the restoration process. The October 2020 edition depicts a marvellous 1940 AWA Radiola, and it made me wonder about how the cabinet is restored to such fine condition. Have you ever published details on that subject? (T. V., Ivanhoe East, Vic) • We do sometimes have descriptions of cabinet restoration in Vintage Radio articles, but not that often. One recent example is the article on restoring a 1946 STC model 512 radio in the August 2017 issue (siliconchip.com. au/Article/10764), also by Graham Parslow. That article had around ten paragraphs describing the cabinet restoration. More recently, there was the Vogue radio restomod by Fred Lever (November 2019; siliconchip.com.au/ Article/12101). Sometimes the cabinet does not require extensive restoration; it might have been restored before it came into the writer’s possession. And sometimes they simply don’t cover the restoration in any great detail. Pocket Oscillator resets on input selections I purchased your Shirt Pocket Oscillator kit (September 2020; siliconchip. com.au/Article/14563) and put it together over a day or two. When I finished it, I powered it up only to be presented with a blank screen. I could see that the chip was getting power, so I tried reprogramming it and it came up with the welcome screen, then 1000 appeared on the screen. 108 Silicon Chip But when I try to change the frequency or use the rotary encoder, the ATtiny85 resets and returns to the welcome screen. The output from the Oscillator looks OK on my DSO, as does the signal from the rotary encoder. Did I receive an ATtiny chip that wasn’t programmed? Or do I have something weird going on with my unit? I have carefully checked my construction and it appears to be all correct. I wonder if you have struck this problem and if there is an obvious fix for it. (T. MacC., Bathurst, NSW) • It does sound like you received an unprogrammed chip, but we are mystified how that could happen. We checked all of our programmed chips in stock and they appear to all have been programmed correctly. We think this was an isolated incident as we have not received any other similar complaints. As for it resetting after you have programmed the chip, the designer (Andrew Woodfield) replies: It is almost certainly due to a failure to program the fuse bits, and in particular, the high fuse bit 7. If it is possible to verify the chip after programming, then that bit was definitely not programmed correctly. In the Pocket Oscillator design, the RSTDISBL bit must be set to 0. The default for this fuse bit is 1, in which case, any high-to-low transition on the RST pin (pin 1), where the rotary encoder is connected, will reset the chip. The fix is easy: program the fuse bit as described in the article, using any normal Atmel/Microchip programmer such as one of the super cheap USBASP programmers. Once this has been done, you can no longer reprogram the chip because the chip will not respond to the reset signal from the programmer at the start of programming. The only way to program a chip once it has been flashed with RSTDISBL=0 (or with an incorrectly configured clock source) is to use a high-voltage programmer. cycle lead-acid battery that cost over $200, and I want to keep it on float charge most of the time and use it for an electric outboard motor. Would your Universal Battery Charge Controller (December 2019; siliconchip.com.au/Article/12159) be the way to go? I note that you had a Deep Cycle Charger (November 2004; siliconchip.com.au/Series/102), but thought that might be a bit dated by now. (G. C., Toormina, NSW) • Yes, the December 2019 Charge Controller is suitable for float charging. The November 2004 charger would also work, but you might find it hard to source some of the parts now. I recently had a deep-cycle battery that gets infrequent use, fail while being kept on float charge. That was with a standard battery charger and a regulator added on to maintain the float voltage. I have purchased another deep December 2019; siliconchip.com.au/ Series/339) and testing it set to 0V with all pots turned low, I measure -0.417V at the output. The article says if it is not 0V, to check for faults and I’m wondering How to use Silicon Chip short links At the bottom of page 13 in your November 2019 issue, there is a reference to an article numbered 8124. How do I find this from your home page? (B. H., Pacific Pines, Qld) • It took me a while to figure out which page of which issue you are referring to. I think it is the web link at the bottom of page 13 in the November 2019 issue (siliconchip.com.au/ Article/8124). All you need to do is type the link address (in this case, “siliconchip. com.au/Article/8124”) into the address bar on a web browser and press Enter. It will then take you to the appropriate page. The situation is the same with our short links to external websites, which are formatted like siliconchip.com.au/link/abcd The links will also work with a www. in front, but we leave that part out to keep the links as short as possible. That’s also why we don’t prefix the links with http:// or https://, which is technically required to make them proper URLs. But most web browsers default to assuming the HTTP protocol, so we don’t include that part of the links either. Bench supply output slightly negative Using Charge Controller sits I’m at the point of final testing of the for float charging 45V 8A Linear Power Supply (October- Australia’s electronics magazine siliconchip.com.au where to look. I also think the temperature reading is a couple of degrees higher than actual and I’m wondering if there’s a way to calibrate that. (S. B., Banyo, Qld) • Assuming that you haven’t reversed the output terminals, the schottky diode should clamp any negative voltage at the output near the output terminals (the one that was confusingly marked D5 or D6 in different places). A non-zero positive reading could indicate a circuit fault, but that diode is the only thing that could cause a negative voltage to slip through. We suspect that it will go away once you complete the calibration. The displayed temperature simply comes from a table in the file “thermistor.h”, so to fully recalibrate the temperature readings, you would need to alter that table and recompile the software. Otherwise, try changing the value of the 9.1kW resistor between pin 1 of CON7 and the +12V rail. That will only allow you to shift the temperature at one point, so it might become inaccurate in other places, and it might also affect how the fans respond and the thermal shutdown point. Trouble with touchscreen on Arduino I purchased the PCB to adapt the 3.5in LCD touchscreen to an Arduino in the May 2019 issue (siliconchip. com.au/Article/11629) along with an ILI9488-based 3.5in screen. The graphics test and SPI display demo sketches work correctly. However, the SPI touch calibration loads the serial monitor display correctly, then blanks the screen and doesn’t respond to serial inputs or screen touches. Also, the SPI shield demo with touch failed to verify (compile) giving the error message “cannot declare variable ‘c’...” from line 11. All of these sketches place a lot of technical bookkeeping in the main program. Any suggestions to get these programs running would be appreciated. Silicon Chip magazine is always a great read! (D. H., Nelson, NZ) • The only difference we can see between our setup and yours is that you are using the older Arduino 1.6.13, while we are using 1.8.5. We are sure that this is the cause of the “cannot declare variable ‘c’...” error, and probably the other failures as well. Please siliconchip.com.au upgrade your Arduino IDE to version 1.8.5 and then try again. The reason we put everything in the main program is to make it easier for people to modify the software and see how it works. Wide-range LC Meter pitfalls I recently built the Wide Range Arduino based LC meter (June 2018; siliconchip.com.au/Article/11099). I am having some difficulty getting it to work, and wanted to ask if there was any errata published for this project which might help me work out what is wrong. (B. C., UK) • There are no known problems with the article or the design, but there are a few common pitfalls which can prevent it from working, and can be frustrating to track down. The two biggest problems that we have seen from constructors are: 1) Using a relay which has a different pinout, coil voltage or integral diode compared to the one we used. Partly this is because our suggested sources (Jaycar/Altronics) theoretically sell only suitable relays of the type we specified. Still, other types are available elsewhere, and it is easy to get them mixed up. Suppliers might sometimes have 12V relays in their 5V bins, so you need to check! The usual symptom of incorrect relays is that the display will work, but the results will be wrong, or the unit won’t calibrate correctly. If all the relays are heard to be clicking when operating the device, then there’s a good chance that is not the problem. 2) Variants of the I2C LCD controller having a different address. The code notes (at line 14) that the controller could be in the range 0x20-0x27, but there are variants which have addresses from 0x38-0x3F. If the default address of 0x27 doesn’t work, try changing it to 0x3F. If the display is not working, there’s a good chance this is the reason. This was noted in an erratum that we published in September 2018. Any number of other construction errors could also show either of these symptoms, but it is worth checking the above first. Finally, we note that one constructor reported that the unit started working after changing the comparator IC for another one. Australia’s electronics magazine Maximite or Micromite? I just bought the “Maximite BackPack” and have assembled it and have tried it on a terminal. It will connect and give answers to print 1/7 etc and the LCD screen lights brightly. But it will not run the OPTION LCD command; it just says “command not recognised”. Can you suggest anything to help? I thought of reflashing the HEX file but am not really sure how. (R. M., Ilkley, Qld) • We haven’t published a Maximite BackPack. Are you sure it is a Maximite and not a Micromite? Please send us a copy of the terminal text (commands and responses) so that we can check them. Also send the results of this command (which prints the software version): PRINT MM.VER The Micromite has no OPTION LCD command, only OPTION LCDPANEL so perhaps this is the root of your problems. By default, the screen will light up white if it is not initialised. If you want to try reflashing the HEX file, refer to the Microbridge article from May 2017 (siliconchip.com.au/ Article/10648). Running SC200 amplifier from ±63V Can I power the SC200 amplifier (January-March 2017; siliconchip. com.au/Series/308) from a ±63V DC supply? (R. R., Melbourne, Vic) • We definitely don’t recommend doing that if you are going to drive 4W speakers, as you are likely to blow the output transistors at high output levels. We still don’t think it’s a great idea with 8W speakers, but you might get away with it. Check your speaker impedance curves (if you have access to them) to verify that their lowest impedance is not too low at any given frequency (ideally, no lower than about 6W). Hopefully, that will keep the output transistors within their safe operating areas. You will have to change the 63Vrated capacitors to 80V or 100V types, as your supply is likely to have peaks above 63V. We don’t think any of the other parts would need to change. So basically, if you are willing to risk the output transistors and have 8W January 2021  109 speakers without any very low impedance dips, you could consider trying it. But we cannot guarantee that it will work as the design was not verified with supply voltages above 60V DC. Speed Controller cuts out at higher currents A few months, I built the High Power DC Motor Speed Controller (January & February 2017; siliconchip.com.au/ Series/309) with some modifications. I raised the switching frequency because the motor was too noisy. I am using this controller for one Minn Kota 12V trolling motor. I have used the motor on my fishing boat with this speed controller, and I’m happy with it. But I have one problem that I can’t solve. Without load, I can adjust the speed from minimum to maximum without a problem. When loaded, once the speed potentiometer is halfway and the current reaches around 16A, the motor suddenly stops for one second and then runs again, then stops again. If I reduce the speed a little bit, the motor runs normally. When it stops for a fraction of a second, it also lights up the red LED. My battery is a 100Ah lithium type and can provide more than 50A without problems. I assumed the back-EMF was the problem but adjusting the trimmers does not solve it. (A. D., via email) • This is probably due to the lowvoltage shutdown setting. At 16A, the battery voltage drops below the low voltage threshold, and the voltage increases when the motor is switched off or the speed is reduced. Adjust low-voltage threshold trimpot VR3 for a slightly lower cut-out voltage, so that the battery does not reach the cut-out voltage during normal operation when charged. Uses for ‘electronic transformers’ Has Silicon Chip magazine ever published any applications for the ubiquitous electronic transformer? Are they suitable and safe for inclusion into various power supply projects? I have been one of your keen readers for many years; I used to read Electronics Australia since the 1960s and made many of the kits. (E. U., Castle Hill, NSW) 110 Silicon Chip • We haven’t used the switchmode 12V AC output ‘transformers’ in any projects (typically used for driving halogen or LED downlights). However, we used the older style 12V AC halogen transformers in a battery charger project in April 2013 (siliconchip.com. au/Article/3759). We would not recommend using the switchmode transformers in this application, as they were connected in parallel for more current, and the electronic versions may be damaged when paralleled. Electronic transformers can usually be used as a 12V AC supply. They may fail or shutdown if connected to a bridge rectifier and filter capacitor to derive a DC supply. They are mainly suitable for the purpose they were designed for, ie, supplying 12V AC to a resistive load such as for LED lighting. CDI module needed for use in jet skis I want to rebuild some retro 1980s & 1990s stand-up jet skis with Kawasaki JS550 twin-cylinder, two-stroke engines. These all had CDI systems originally, but it is difficult and expensive to get original parts, so I am thinking of building replacement CDI systems for them. As I understand it, they have a wasted spark arrangement, where both plugs are both fired every half revolution. On the later models, they realised that a rev limiter was needed for when the impeller cavitates and the engine spins on no load, so that would be a good feature to add. I am considering your Replacement CDI Module For Small Petrol Motors from May 2008 (siliconchip.com.au/ Article/1820), but I see that you have also published other ignition systems, including a Multi-Spark CDI in December 2014 & January 2015 (siliconchip. com.au/Series/279). Can you recommend which option is best for me, and what smart/programmable controls could be added to such as a rev limiter, multi-spark, timing advance etc. As the HV coils and CDI were all sealed inside a single factory original unit, what is your recommended coil arrangement? There seem to be several aftermarket twin coils for sale online, but how would I choose? Their Australia’s electronics magazine only specifications are primary and secondary impedance values. (L. C., Donvale, Vic) • The Kawasaki magneto ignition includes a high-voltage generator coil to produce around 300V to charge the capacitor in the CDI, and another trigger coil to fire the ignition. The May 2008 replacement CDI unit should be suitable. This CDI will work with many ignition coils, so the choice is not critical, and ideally, you should use the two high-tension outputs to drive both spark plugs. We don’t recommend that you use so-called “sports coils” as these can develop very high voltages and could break down when used with a CDI. The primary resistance of the coil is an indication of whether the coil is suitable. Choose one rated at 3W or more (for a 12V coil). That means that the charged (or saturation) coil current would be less than 5A (assuming a 14.4V supply from the battery). Many sports coils have a much higher charging current. The multi-spark CDI is more for converting a standard ignition that has conventional triggers such as reluctor, Hall effect or optical and powered via a battery supply. We have published two rev limiter designs which you could use, one in April 1999 (siliconchip.com.au/ Article/4589) and one February 2008 (siliconchip.com.au/Article/1753). Substituting amplifier output transistors I am building a pair of 500W Power Amplifier modules (August-October 1997; siliconchip.com.au/Series/146), the article specifies 12 MJL21193/4 output transistors. I have been looking around and found the MJL1302/ MJL3281 which have almost identical specifications. Their safe operating area (SOA) is the same, but the 1302/3281 maximum collector voltage is slightly less at 200V compared to 250V; the maximum collector current is 15A vs 16A. The gain-bandwidth products of these transistors are quite different 4MHz vs 30MHz. Can I substitute the MJL1302/3281s or should I spend more and buy the MJL21193/4? If they can be substituted, are any modifications required? (L. K., Wanganui, NZ) continued on page 112 siliconchip.com.au • While the MJL21193/94 transistors from ON Semiconductor are recommended, they are now obsolete and difficult to get. We recommend the ON Semiconductor NJW21193/94 transistors instead. You could use MJL1302/MJL3281. As you mention, they have a higher cut off frequency. This might or might not be a problem. They may give better performance, but it’s also possible that the amplifiers could oscillate. If you find the DC fuses are blowing for no reason, try increasing the value of the 100pF 500V compensation capacitor. However, as that value is quite high, it should be OK. According to the circuit, you need seven of each type of transistor, regardless of which types you use. Power supply cable stripe polarity I use figure-8 speaker cable (eg, Jaycar WB1703) in many projects. Should the black stripe for polarity identification be used for positive or negative? I originally thought black indicates negative, but then I noticed that most plugpacks have the striped side of the wire indicating positive. Now I’m thinking that as positive is the wire I want to be identified, perhaps I should always use the black striped wire for positive. I use the wire for motors and Arduino projects, but if anyone was connecting speaker wire to the black and red terminals on a speaker box, surely the wire with the black stripe would go to the black terminal. Is there a standard for this? (J. B., Benalla, Vic) • If you have red and black wires, then usually red would be positive and black would be negative. But it’s a bit more tricky when you have a stripe. Sometimes you have a white stripe, sometimes a red stripe and sometimes a black stripe (and possibly other colours). Usually, the stripe is used to indicate positive, but that certainly is confusing when the stripe is black. Ultimately, it doesn’t matter as long as you are consistent so that there is no confusion. As you say, plugpacks tend to use the stripe for positive (usually a white stripe, though), so it would make sense to follow that convention. Probably the best solution would be to use Jaycar Cat WH3057, WH3087 or similar cable which has red and black insulation for the two wires in the cable. Flashing lights wanted for model railway I am trying to find a railway crossing flashing light kit, or at least a PCB for it. I am sure I have seen something like it in past magazine issues. I have searched your site without success. Could someone point me in the right direction? (P. C., via email) • We published a two-lamp flasher circuit (January 1998; siliconchip. com.au/Article/4748). You can download its PCB pattern from the following page: siliconchip.com.au/ Shop/10/2362 Jaycar also sells a kit for that project, Cat KJ8070. This design runs from 12V and so is suitable for 12V SC lamps. Advertising Index Altronics...............................89-92 Ampec Technologies................. 19 Dave Thompson...................... 111 Digi-Key Electronics.................... 3 Emona Instruments................. 101 Jaycar............................ IFC,53-60 Keith Rippon Kit Assembly...... 111 LD Electronics......................... 111 LEDsales................................. 111 Microchip Technology.............. IBC Mouser Electronics...................... 7 Ocean Controls......................... 39 Rohde & Schwarz.................. OBC SC Micromite BackPack............ 47 Silicon Chip Binders............... 106 Silicon Chip PDFs on USB..... 111 Silicon Chip Shop.................. 100 Silicon Chip Subscriptions....... 88 The Loudspeaker Kit.com......... 99 Tronixlabs................................ 111 Vintage Radio Repairs............ 111 Wagner Electronics..................... 5 Notes & Errata Balanced Input Attenuator for the USB SuperCodec, November-December 2020: the photo shown halfway down the lefthand column on page 71 of the December 2020 issue, showing the wiring to the power connector, is incorrect. The positive (red) wire should be shown going to the bottom-most pin in the socket, with the black (negative) wire to the top. Also, in the circuit diagram on pages 50 & 51 of the November 2020 issue, the centre (ground) pin of CON3 at upper right should only be connected to the junction of the two zener diodes, the negative end of the 100µF capacitor next to switch S1 and the negative ends of all relay coils. The junction between this ground and the other grounds in the circuit is on the main SuperCodec board. Two LED Christmas Stars, November 2020: in the parts lists on page 41, there is no such part as a 75HC595. It should read 74HC595 instead. D1 Mini LCD BackPack with WiFi, October 2020: in the circuit diagram (Fig.1), the connections to pins 7 & 8 on the LCD module via CON1 are swapped. The drain of Q1 should go to pin 8 (LED) while pin 7 is the display SCK line and also connects to pin 10 on the LCD module and on to the D5 pin of MOD1. The February 2021 issue is due on sale in newsagents by Thursday, January 28th. Expect postal delivery of subscription copies in Australia between January 27th and February 12th. 112 Silicon Chip Australia’s electronics magazine siliconchip.com.au