Turbo timer false triggers

I recently bought and put together one of the Turbo Timer kits described in the November 1998 issue and it works well except for one thing. Instead of activating the relays only when the ignition is turned off, they are activated as soon as the ignition is turned on and then it switches itself off after the set time.

This means that to activate the timer after going for a long drive, you have to turn the car off, then turn it back on (timer activates straight away) then take the keys out.

I’ve checked and double-checked the layout. Help. (J. M., via email).

Some vehicles do not drop the ignition voltage fast enough to trigger the Turbo Timer circuit. You can improve this by increasing the 2.2μF capacitor connecting to pin 2 of IC1 to a much larger value, say 22μF.

Coil ballast not required I have just purchased one of the High Energy Ignition (June 1998) kits to install into a Mini Clubman. This car is not fitted with a ballast resistor in the coil circuit. Do I just ignore any reference to it on the circuit diagram? I eventually will install a Hall Effect device into the distributor. Can I install all of the components for each of the trigger devices and use only the currently relevant input/output connections or do I have to select and install one type only and make changes when I wish to change to a different method of triggering? Any advice you can give me on these questions will be appreciated.(K. P., via email). Ignore the reference to the ballast. You should only install those components you need for the relevant input. If all components are installed, point A on the circuit will never be able to be pulled low by the relevant input.

Foot switches for the guitar preamp

I am building the 2-Channel Guitar Preamplifier and Digital Reverb from the November 2000 to January 2001 issues and I have a few questions.

I’ll be adding a few effects boards such as distortion, twang and sustain (from Jaycar’s "Short Circuits III"), the SILICON CHIP Mic/Audio Compressor and the recent Tremolo Unit. I plan to build these effects boards into the same case as the preamp and bring the in/out switches to the front panel as well as to sockets on the back for remote foot switches.

Rather than bring the circuit wiring to the foot switch sockets and to the foot switches and risk picking up hum and other noise in the leads, I intend to use mini or DIL relays to do the switching inside the preamp case and have the foot switches switch only the relays.

My question is, after the rather lengthy explanation, is there any possibility of noise or other interference coming from the relays? Are there any precautions I should take? Also, with all these extra boards (and consequential possible sources of noise), will there be much of an advantage in using the Ultra-LD amplifier module or would its predecessor, the 125W Plastic Power module (April 1996) be OK?(J. W., via email).

We would not recommend the Ultra-LD amplifier for use with the guitar preamp. It amounts to overkill. Instead, use the plastic power amplifier which can drive 4-ohm loads.

You can use relays for the foot switch functions. Use a separate power supply for the relays so that when the relays are switched they do not induce noise into the audio. The separate power supply will need a separate transformer.

Modifying the 8-channel mixer

I was thinking of building your 8-channel mixer, as published in the November 1996 issue but have a few questions. Can I substitute the OP27 with the LM627, available from Dick Smith Electronics? Also the SSM2017 preamplifier chip is really difficult to source and as I don’t require eight microphones, mostly line inputs, can I simply connect a line signal source directly to the main VR1 pot prior to IC2 in the schematic, as the LM833 can handle direct line levels? Or can you suggest a workaround?

I was hoping to add a gain control feature. Is this possible? (E. Z., via email).

The LM627 can be used in place of the OP27 as it is an equivalent device. The SSM2017 is available from Altronics, Cat Z-2800.

You can apply line levels to the input Main level control via a coupling capacitor. This is not recommended though because of the uncertain drive impedance of the input source.

Adding a gain control is best achieved with the SSM2017 control at pins 8 and 1. Connect a 10kΩ pot in series with a 330Ω resistor between pins 8 and 1 for a nominal 25dB gain control range.

Alternatively, a gain control can be added with a potentiometer used for the gain resistors between pins 1 and 2 of IC2a. Connect one end of the pot to pin 1 and the other to ground; the wiper connects to pin 2. Some form of upper gain limit would be necessary by adding a series resistor between the end of the variable resistance and ground. When using a 10kΩ pot, the existing 2.2kΩ fixed resistor to ground would be suitable.

Dust causes TV interference

My TV is connected to an outdoor antenna and has poor reception only on ABC on "normal" days. When it is raining, the picture is as clear as a bell. On an indoor antenna, the picture is significantly better than the outdoor antenna on one of those normal days. Heavy overcast conditions also improve reception. I don’t know why this is so. Perhaps you have an answer? (S. N., via email).

It is possible that the interference is caused by dust and dirt on the insulators on adjacent high-voltage power lines. This dirt would be washed off in rainy weather, eliminating any discharge across the insulators and restoring clean reception.

If you can get the ABC on a UHF transmission, it would be well worth the investment to obtain a UHF antenna to solve this problem once and for all. Or you can build one of the SILICON CHIP UHF antenna designs (eg, see the July 1994 issue for a 4-bay bowtie array).

Supply problem in guitar preamp I recently built the guitar preamp described in the November 2000 issue and I have a small dilemma. The power supply I have already has outputs of ±15V (really 14.38V) and when I put the power to the board it drops down to +12.1V and -11.2V and I honestly don’t know why. (D. F., via email). We are not sure whether you are using an existing ±15V supply or the one recommended for the guitar preamplifier. If you are using an existing supply it does not seem to be able to cope with the extra current required from it when connected to the Guitar Preamplifier circuit. You may need to use the recommended power supply. If you are using the recommended power supply, then there does seem to be a problem with its output voltage at no load. The voltage should be closer to 15V than the 14.38V you are obtaining. We would envisage the power supply to deliver at least ±14.8V under no load. Perhaps the regulators are faulty? Voltage droop when connecting the circuit to the supply usually means there is a short circuit somewhere on the PC board. This could be shorted pins on one of the op amps or a fine solder whisker joining tracks. Check with a magnifying glass on the underside of the PC board. Also do a close comparison between the published PC artwork and the copper tracks for solder bridges which should not be there.

Diode for white LED

We are very interested in producing the white LED torch as described in the May 2001 issue but we are having trouble locating the Schottky diode (BYV10-20). We have inquired at numerous electronics outlets including Jaycar, Altronics, Dick Smith Electronics and other outlets in Melbourne.(A. H., via email).

The Schottky diode for the LED torch does not need to be the BYV10-20. You can use the 1N5819 which is available from (Jaycar ZR-1020), (Dick Smith Electronics Z-3250) and (Altronics Z-0040). By the way, the BYV10-20 diode is also available from Farnell. Phone 1300 361 005.

4-ohm loads for class-A amplifier

I have built the 15W class-A amplifier described in the August 1998 issue and am pleased with its performance. The specs state that it is rated at 15W into 8Ω. Can you tell me what it is likely to achieve into 4Ω?

Also is it possible to increase the output by paralleling the output transistors and increasing the power supply?
(S. F., Lake District, UK).

You can operate the class-A amplifier with 4-ohm loads and it will probably deliver about 25-30 watts maximum. The only problem is that once the load current exceeds about 1.9A peak, the amplifier will no longer be in class A. Instead it will be in class AB (albeit with exceedingly low crossover distortion artefacts) for power levels above 7.5 watts.

While we have not looked closely at the biasing, if you wanted to run in class-A up to full power, you would need to double the quiescent current. You would not need to double up on the output transistors but you would need a fan cooled heatsink to cope with the 80-watt dissipation and you would need a bigger power supply.

Charging two sets of AAA cells

I have a Kodak MC3 digital camera and MP3 player. They each use three AAA batteries and buying new disposable batteries all the time is impractical and expensive. I need a charger that can fast-charge two lots of three AAA batteries at a time. Could you suggest a kit that can or a product I could buy?(A. T., Wagga Wagga, NSW).

The Fast Universal Charger published in the June & July 2001 issues will charge 3.6V Nicads or NiMH batteries (ie, it can do three AAA nicads in series) but it could not do two sets at one time because it could not properly monitor the "end-point" voltage of each set.

More IRLEDs for remote control extender

Could you let me know if it is possible to run more than one IRLED on the remote control extender featured in the July 1996 issue. If so, what do I have to do so it still works. I need to do this because I have two units in different places.(R. S., via email).

You can run another IRLED via a separate 220Ω resistor connected to transistor Q1.

Ignition circuit runs hot I have built the High Energy Ignition described in the June 1998 issue. It works well but the case gets very hot. I checked the current draw which was around 4A at idle, dropping to 3A when I revved the engine. All this seems OK but I am just a bit concerned that it runs so hot; I can’t keep my hand on the case.(P. C., via email). Paradoxically, it is the current limiting which causes the circuit to run hot. If you want to check that your circuit is running properly. Short out D3 to disable the current limiting. The case should now run just warm to the touch; if it is still hot, you have a fault. Another possibility is that the case is too close to the engine. It is best to mount the case away from exhaust manifolds and preferably where air can pass over it.

Cooler not cold enough for beer

I have built the Switching Temperature Controller described in the August 1999 issue but cannot get it to regulate the temperature below 10°C; not cold enough for my beer. I am a pommy but not that much. It was built using the Oatley kit. The Esky can cool to -1°C by removing the thermistor from the circuit. Please help!(Andrew, via email).

If the circuit can cool to below 1°C without the thermistor, we suggest you connect a 100Ω resistor in series with trimpot VR1. This will compensate for any tolerance variation in the thermistor.

It seems likely that either there is a wide spread in the thermistor tolerances or a different thermistor has been supplied in the kit.

Modifying a DC-DC converter

In July 1996, you published a design for a switchmode step-up converter for use in charging SLA batteries. This used the Motorola MC34063A controller chip.

I have been endeavouring to modify the design to derive 18V at about 1.5A and with reasonable success, but some aspects still are unclear. I have only been able to download one AppNote pertaining to this chip and it did not provide the info I was seeking.

Similarly, a Motorola linear handbook did not show any series inductor/shunt external switch configurations as you used.

I appreciate that you cannot undertake design modifications for individual readers and that is not what I am chasing. What I would like, however, is a lead on any documentation you used in the preparation of this design.(P. M., via email).

The configuration of the DC-DC converter used in the SLA battery charger is our own circuit adaptation of the MC34063. You won’t find it in the Motorola application notes.

To obtain 18V you will need to adjust the divider at pin 5 so that this pin "sees" 1.25V when the output is 18V. Using a 30kΩ resistor in place of the 22kΩ resistor will give the correct divider ratio.

The inductor value used should be about right to obtain 1.5A.

Fuel mixture display shows lean

I bought the SILICON CHIP Fuel Mixture display kit (from Jaycar) and hooked it to my 1994 Holden Barina GSi but the only LEDs that come on are the red ones. When I start the engine the last LED comes on but as I turn the trimpot fully to the left, only the second last LED lights.

As the car warms up, the last LED comes on and it stays that way. Is the Barina designed differently and do I need to replace the resistors with different values?

There are no solder bridges and all the LEDs are correctly positioned. What can I do? (G. E., via email).

We are not sure what you mean by "only the red LEDs come" on as all the LEDs are red. We assume you mean that the display always shows a lean mixture, with the lower two LEDs lighting. Possibly the problem is with the adjustments of both VR2 and VR3. You need to set them as described in the adjustment section of the article.

Also check that the connection to the EGO has a good ground. Differences between the ground connection for the power supply on the fuel mixture display unit and the sensor ground can cause the display to show lean.

Train detector for outdoor work

I need a model train detector that works outdoors. I tried ultrasonics – the wind defeated that. IR – the sun beats me there. RF (radar) – there are kids about and some people want more.

The only one that I can think that may work is a metal detection type. HF across the rails won’t work with DCC systems and some of us run live steam and battery power.

By the way, the white LED torch has another use. I made one for my niece who goes scuba diving. She did not like the cost of a decent diving torch, so I made one up with a couple of NiMh C cells, 10 5mm white LEDs and an inductive charger (bits and pieces of your previous projects jumbled together), all in an old Dolphin-like (smaller) torch case. I then filled the case, after testing, with casting resin, lens area and all.

She now has one solid, pressure proof and near indestructible diving torch that throws a good beam and lasts a long while. I wonder how long it will be before headlights are made of white LED arrays?(R. P., via email).

We’ve done two train detectors that will work outdoors, in April 1994 (a Hall effect sensor) and a 25kHz system in June 1995. We can supply these issues for $7.70 each, including postage.

Thanks for the information on the diving light.

Dr Video not good on latest Macrovision

I recently constructed two Doctor Video kits from the April 2001 issue and have since tried to use them only to find that although they do help they don’t work fully.

I tried to copy my Star Wars trilogy set so as to keep the originals in perfect condition. I found that it helped stop some of the horizontal tearing in the picture but not enough to stop all of the colour and horizontal problems. Can you help?(P. C., via email).

It sounds like the tape you want to copy has the very latest version of Macrovision which is designed to be much harder to remove than the earlier versions. If so, the Dr Video box won’t be able to give clean enough video.

As far as we know, there isn’t any complete fix for the latest Macrovision – luckily it hasn’t been used on too many movies yet but that may quickly change.

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.