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Ask Silicon Chip

Engine immobiliser for diesels

Are the various kits about based on your December 1998 Engine Immobiliser Mk2 (ie, Jaycar KC-5255) suitable for diesel engines? It appears to me that this circuit only works with coil-equipped cars. If it won’t be suitable, is there an alternative? (W. M., via email).

  • As it stands, the immobiliser is set to kill the spark from the ignition coil. To make it work on a diesel you would have to make it kill the signal to the injectors or turn off the fuel pump to the fuel rail. Probably the easiest method would be to turn off the fuel pump using a suitably rated relay driven by the output transistor Q1.

Low-powered surround sound system wanted

Can you suggest a project for a reasonably low-powered 20-50W/channel amplifier for a surround sound system. (J. E., via email).

  • Have a look at the 20W amp module in the December 1993 issue and/or the 40W/50W module in the March 1994 issue; also the twin 50W module in the February 1995 issue. All modules are presently available from Altronics. We can supply the back issues for $7.70 each, including postage.

White LED torch modifications

White LED torch modifications I constructed the White LED Torch (described December 2000) from a Dick Smith Electronics kit. It works brilliantly. I have two questions on it. Is it possible to run the LED at 20mA+ from a 1.2V rechargeable cell with minor changes to the circuit? This would make the unit more useful on long trips away from services. Can it run, with modification, from a 3V source, allowing incorpo­ration into a combined radio/torch setup? (H. J., via email).

  • The White LED Torch can be operated from 1.2V by shorting out resistor R1. This will increase the current drive to the LED. Essentially, the current is limited by the DC resistance of inductor L1 since this determines the saturation current. Winding your own using the Xenon tube transformer former will enable you to increase the LED current when using a 1.2V supply. You would need to use 250 turns of 0.25mm enamelled copper wire (rather than the 0.16mm wire specified) on the bare former.
    You could run the torch from 3V but resistor R1 should then be increased to 4.7W.

Fridge interference to audio system

I have been having trouble with loud ‘pops’ in my stereo system when the fridge switches off. The system includes the "Electronics Australia" Pro Series One power amplifier and Pro Series Two preamplifier and a High Definition Audio device (ex Jaycar) installed between preamp output and power amp input.

I have installed some varistors on the 240VAC input side of the power amplifier and poly switches on the output side of the amplifier. The fridge has an electromechanical timer installed to control the compressor & defrost cycle (I think).

These ‘pops’ don’t always occur but are definitely coming from the fridge through the mains and into my stereo amplifier. Can I damp the contacts of the timer in the fridge with a series RC network to remove the spike that I assume occurs when they go open circuit? (C. L., via email).

  • Connecting your audio gear via a mains filter might be a cure but we suspect that the amplifier or its preamp are picking up interference radiated from the mains wiring each time the fridge clicks off. Whether or not a ‘pop’ occurs depends on when in each cycle of the 240VAC mains waveform the fridge actually switches off. There is probably little point in trying to suppress interference at the switch – we expect it already has these components.

    You will have to do some detective work to figure out how the interference is getting into your system. For example, is it getting in via the speaker leads? You can test this by discon­necting the speakers and listening to your system via the head­phones.

    Is it getting in via the preamp? Disconnect the preamp and see if clicks are still picked up. Then you might find that you can cure the problem by winding the offending leads through ferrite toroids. In the worst case though, it may be necessary to modify the input and output stages of the amp and preamp to completely stop the problem.

    Some designers exacerbate these problems by designing their circuits with excessive bandwidth.

Trouble with quiescent current setting

I’ve been testing one of the output modules for the Ultra-LD 100W Amplifier (described in the March, May & August 2000 issues of SILICON CHIP) and I am having trouble with the quiescent current. When I measure the voltage across the 220 5W resistor with VR1 fully anticlockwise, the lowest voltage I get is 0.68V. If I leave the DMM attached for a few minutes, the voltage rises to 9.3V then sort of cycles between that and 8.3V.

Is this because I haven’t attached the heatsink yet? There are great differences in the temperatures of each pair of the MJL devices. Out of each pair, one is too hot to touch, the other is lukewarm or cold.

The voltages across the 1.5 resistors stablilise at 13-14mV, not 7.5mV. Is this all because I don’t have the heatsink attached? (D. M., via email).

  • You must have the heatsinks attached before you can make any sense of the quiescent current adjustment. What happens is that Q10, the Vbe multiplier, monitors the temperature of the output transistors and adjusts the bias accordingly. Q10 can’t work properly if the heatsink is not fitted.

Hands-free phone amplifier wanted

I find that I am being placed "on hold" when I phone a large number of organisations. Why not offer a small amplifier to rest your phone ear piece on to hear when the called number is back to you or better still, a complete hands-free amplifier and microphone to listen and talk while your hand phone rests on it? It could be mains or battery-powered and activated when the phone hand piece is placed on the unit and shut off when the phone is removed. (N. S., via email).

  • Have you seen our Hands-Free Speakerphone design presented in the September 1988 issue? This could fit your needs. We can supply a photostat copy of the article for $7.70 including post­age & GST.

Solar power for big digit clock

I would like to power the Big Digit Clock (described in March 2001) from a solar power supply. I need to know the current and consumption of the clock in order to make the power supply. Can you help me? Is there anything else about the clock that I need to change? (M. S., via email).

  • The clock can be run using solar cells and a battery. The circuit will draw a maximum of 400mA or 4.8W at 12V. In low light, the display will draw less current, dropping to around 100mA (1.2W) at night. So on average, the clock will draw about 300mA (3.6W) depending on the length of daylight hours.

A-V transmitter has poor range

A while ago I bought an Audio/Video Transmitter (described in July 1999) from Jaycar. I have finally got around to building it. I have two problems though, and hope you can help. After building the kit, I tested it using my DVD player. It works fine if I don't take it any more than about 2-3 metres away! Any more than this and I start to lose the signal and get poor reception. I have tried adjusting the trimpots and still get the same re­sult. I have also tried to connect up a CCD camera using the power output and video input but get no results. I purchased the camera from Dick Smith Electronics and testing shows that the power is getting through and there is some sort of signal. Why won't it work? (J. R., via email).

  • Check that the 3-terminal regulators have not been trans­posed. We referred to this in Errata published in February 2001. The 5V regulator (7805) should be nearest to IC2 and the 12V regulator (7812) closest to diode D2. Check that you have these correct. There should be 12V available for the video camera.
    The transmission range does seem to be very poor. This suggests that one of the amplifiers, IC1 or IC2, is not working. This could be because one is connected incorrectly, an open-circuit connection to one of the pins or there is a short. Also check that the connection passing into the RCA socket for the modulator output (see the Jaycar notes on connecting the wire directly rather than using the plug) is not shorting to the case of the modulator.
    Plug the camera directly into your TV or VCR to check if it is working correctly, before trying to get it operational on the transmitter.

Using Dr Video with NTSC signals

Is the Dr Video stabiliser published in the April 2001 issue only applicable to PAL signals or is it universal? Specifi­cally, can I use it on my DVD player when outputting NTSC sign­als? (S. B, Wollongong, NSW).

  • The timing for NTSC signals is close enough to PAL to allow the circuit to work fairly well. However, some of the latest NTSC releases on DVD have an ‘improved’ version of Macrovision which isn’t easy to remove. So the results on these DVDs may be disap­pointing.

Where to get BASIC

  • We occasionally get queries about where to get BASIC for use with various past projects. As some readers may be aware, GWBA­SIC/QBASIC interpreters were supplied free until DOS version 6. The original IBM PC even had a ROM-based interpreter (BASICA).
  • We’ve had a good look at what’s available and can thorough­ly recommend FirstBASIC, available as shareware ($US25 to reg­ister) from http://www.powerbasic.com
  • You’ll find it listed on their down­loads page.

PIC NiMH charger wanted

You recently published a NiCad battery charger for power tools (April 2001). How about a similar project for NiMH batter­ies? The only difference I'd need is 12VDC input so I could use it in the car. I'd be using it to charge four AAs for my digicam while on holiday. (A. G., via email).

  • There should be no reason to change the design at all apart from a change to the 3.3kW sensing resistor to cope with the lower voltage from your battery pack. However, we are not certain whether the PIC's code has the resolution to cope with the re­quired dV/dt end-of-charge detection. NiMH cells have a very similar charge profile to NiCads but they peak at around 1.8V. You need to be aware also of the C value and note that NiMH cells like to be charged slightly below 1C (about 90%). If the batteries are around 1450mAH then 1W should be OK for the current limiting resistor.

FM SCA decoder/multiplexer

I was wondering if you have ever published a design for a circuit to decode the SCA (subcarrier auxiliary) channels that are transmitted by some of the FM broadcasters. I am not really interested in using it for this purpose, but I want to build a multiplex system designed around the same principle.

I have several wideband (100kHz) radio links that I want to feed to up to six narrow-band (0 to 3.5kHz, phone quality) audio channels. I thought that by modulating subcarriers, as they do with SCA, I could achieve the results that I want. I planned to use subcarriers at, 25kHz, 35kHz, 45kHz, 55kHz and so on. (P. D., via email).

  • We published an ACS Adaptor (same thing as SCA) in the January 1988 issue. We can supply a photostat copy of the article for $7.70 including postage.

Charging circuit for a strobe lamp

I am trying to solve a puzzle, on how to charge a 680F capacitor quickly, up to 320-340V DC, suitable for a high-speed disco strobe. The existing circuit is an AC-to-DC convert­er, which then steps it up to 500V DC using a Mosfet and an inductor. It’s way too slow and inefficient.

I know that by using a high-voltage diode and some sort of current limiting device, you can charge a capacitor to 320V from the 240VAC mains supply. I have experimented with various devices to achieve this, from a fluorescent light ballast to a light bulb to resistors and even winding my own inductor.

I need this circuit to repeatedly charge this capacitor quickly, providing the 320V DC needed to fire the large Xenon bulb of the strobe. The trigger circuit etc is the easy part which I can do myself. (J. W., via email).

  • How fast do you want the strobe to be? We published a beat-triggered strobe in the August 1998 issue which would run at up to 20 flashes per second. Anything faster tends to be perceived as continuous light so there is not much point.
  • Our design used two 470 5W resistors in series with a bridge rectifier running directly from 240VAC. It is quite fast enough and if you use higher charge currents and faster flash rates the limiting factor becomes the ripple current ratings on the discharge capacitor. We can supply the August 1998 issue for $7.70 including postage.

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.

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