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Hang problem with mixture meter I recently purchased and built the Digital Fuel Mixture Meter, as published in the September & October 2000 issues. It has a problem in which at power-up, it will sit for a few minutes, reading (L) for low and no matter what input level you put into it (0-1V), it will hang. This problem often goes after two minutes but may remain in a hang state for longer. It is not yet in a vehicle and is being fed constant 13.5V and a constant input at 0.5V. (F. N., via email). • This is indeed an unusual problem. It is possibly caused by pin 4 of Zero voltage switch for wall oven Some 15 years ago I redesigned our kitchen wall oven to something akin to ‘burst firing’ electronic control of the heat­ing elements. I used a combination of an Analog Devices AD595 Ther­ mocouple Amplifier with a ramp-driven 14-pin CA3059 zero voltage switch IC to control a suitably-sized Triac. It has proved to be accurate to ±1° and quite reliable. Recently though, the CA3059 failed and I have not been able to locate a replacement. Harris and then OnSemi manufactured them but discontinued about a year ago. I was wondering if you could help with the name of a supplier. (J. N., Tauranga, NZ). • We do not know of a substitute for a CA3059 but if you want a zero voltage switching power control circuit, have a look at the Heat Controller published in the July 1998 issue. This used the IC1 being open circuit, either within the IC socket or where soldered to the PC board. Alternatively, the crystal may be a slow starter, causing the software to latch up as it will not be reset properly. Try using smaller value capacitors from the crystal to ground at pins 15 and 16. Values ranging from 10pF to 15pF should be suitable. The other problem could be with the power supply you are using. If this supply rises slowly to 12V at switch-on, the circuit could exhibit the problem you experience. Try connecting the circuit to the power supply terminals after it is powered up. 1988 and October/November 1998. We can supply the 1998 back issues for $8.80 each, including postage and the 1988 articles as photo­stats, again for $8.80 each, including postage. Braking system for a wheelchair readily available MOC3041 zero voltage switching Triac optocoupler. We can supply the July 1998 issue for $10 including airmail postage. I am a Year 12 student starting my Major Design Project. My project is a wheeled walker for the disabled. My current need is for a braking system and an easier way of going about it. I want to create a one-off braking system using electronics and two motors with a simple on/off sequence and a button on each handle to easily apply the brakes. What do you suggest? (D. G., via email). • We suggest you build a speed control with inbuilt braking. Have a look at the 50A speed control published in the May 2000 issue. We can supply this issue for $8.80, including postage. Audio frequency power meter wanted Notes & Errata I am in need of an audio frequency power meter. I’m think­ing of something that reads in dB and mV and takes an electrical input (rather than including a microphone). It needs to be accu­rate across the audio spectrum. While the option of a microphone input would be nice, this is a considerably harder proposition, since the characteristics of the microphone would have to be compensated for and accurate calibration would probably require specialist equipment. (A. M., via email). • We have published two AC milli­ voltmeters which would suit your application, in August/September RIAA Preamplifier, March 2002: the 10µF across the ±15V rails should be 35V not 16V. Smart Card PIC Programmer Interface, February 2003: the clock connection from the card socket should go to pin 12 of the 18-pin socket, not pin 16. PortaPAL Portable PA Amplifier, February and March 2003: there is a circuit error concerning both the microphone MIC1 and MIC2 inputs. The circuit shows two 10kΩ resistors in parallel connecting from pin 2 to ground whereas one of these resistors should connect to pin 3 instead. The SC PC board is correct. 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 Trade Practices Act 1974 or as subsequently amended and to any governmental regulations which are applicable. 92  Silicon Chip www.siliconchip.com.au