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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. Write to: Ask Silicon Chip, PO Box 139, Collaroy Beach, NSW 2097; or send us an email.

Increasing the rating of the Battery Guardian

Thanks for the Battery Guardian (SILICON CHIP, May 2002). It is exactly what I need for my solar power system but I would like to increase the current available from the circuit to about 20A. Could I increase the size of the heatsink, parallel another Mosfet with Q1 or parallel the entire driver portion? Or is there a better way?

(R. P., via email).

The Mosfet should be paralleled with another STP60NE06 and provided with a heatsink. Also the fuse should be a 3AG 20A type (using 3AG fuse holder clips in the extra holes on the PC board).

Ideally the wider current carrying tracks on the PC board should be made heavier by applying a thick layer of solder over them. Be careful that the tracks are not overheated and begin lifting off the board. An alternative would be to wire between the component leads in parallel with the PC tracks using insulated hookup wire.

Connector confusion in Theremin kit

I recently got my hands on one of your Theremin kits (SILICON CHIP, August 2000) and have more or less finished putting the whole thing together. But I still have one question, regarding the AC adapter.

Because I am located in Canada and could not get the exact model number you stated, I picked up a generic one which will fit the bill. The thing is that I am not sure about the tip polarity required for the Theremin and I can't seem to find the information in the literature provided. Could you help me out?>/P>

(M. E., Fredericton, Canada).

If you have a look at the PC board wiring diagram you will see that the DC socket has the tip connected to positive. Make sure your DC plugpack is the same and if you can't change it, swap the wiring in the DC socket to suit.

Earthing fuel tanks for safety

I was in a garage the other day and was told to put my jerrycans on the ground before filling them. When I asked why I was told that it was to earth them. Sounds reasonable. So I asked what about the fuel tank in my car? The reply was that it was earthed and was OK. The attendant had just been to a safety seminar run by a big fuel company and this is what he was told by the experts. We are now having a running argument.

I say that the fuel tank and the whole car are NOT earthed and the attendant and fuel company say that it is. Who is right?

(M. D., Warwick, Qld).

The important thing is that the fuel tank is grounded to the car - this was a problem with the plastic fuel tanks on Falcons a few years ago because the metal filler was not earthed to the car. The problem is a buildup of electrostatic charge on the fuel tank (or jerry-can).

Anyway, why not put the jerrycans on the ground when filling them? They get heavy otherwise!

Sanwa taut-bandanalog meter wanted

Please can you ask your readers if anyone has a 1983 Sanwa N-501/D analog multimeter they would like to sell. It has a taut band suspension. Thanks a lot.

Mike Sheriff
Phone (02) 9949 2454
email colbox@zip.com.au

Ignition system for a Ferrari

I have a 1979 Ferrari 308. I am looking to change over to a distributorless ignition system. My goal is to use GM type coils and a GM type ignition module to eliminate the distributors. My dilemma is that I do not have a system to control spark advance, and I am not sure of the best method for determining cam or crank position.

This Ferrari has a magnetic type pickup in each distributor, and has two distributors, one for each cylinder bank. I have a rear drive pulley that is attached to the back of on of the cams. If I had a cam position sensor I could locate it here. I want to know if your programmable ignition system (SILICON CHIP, June & July 1999) will solve my problems.

(D. B., via email).

Forget about the PIT module; it is too crude for car use. We would suggest you don't eliminate the distributors but possibly convert them to reluctor or Hall effect pickup. Then build our HEI system (SILICON CHIP, June 1998). That way you retain the necessary spark advance.

Diagnosing motor problems

I know this is a bit out of your area but I would like your opinion. I have a motor which quit while running. Now all it will do is run under no load for a few seconds after switch-on. My hope is that this behaviour indicates failure of the external capacitor rather than the motor windings. The motor name-plate rating is 240V 3A @ 50Hz. The capacitor's legend is too damaged to read. It is a plastic cylinder 7cm long and 3.5cm in diameter.

(F. M., Temagog, NSW).

Whatever is the problem, you should check the capacitor. If you don't have a capacitance meter you can still do a very rough capacitance check using the continuity test on an analog multimeter and then compare it with a known good capacitor. We'd be hopeful too, and would suspect the capacitor.

Smoke precipitator for home fires

I have a suggestion for a project. There was a big conference in Adelaide recently on the dangers of wood smoke. Surely a smoke precipitator such as used in industry is not much different to an ioniser - maybe a good project?

(C. H., Daw Park, SA).

Smoke reduction generally involves a combination of bag chambers and electrostatic precipitators, neither of which are simple or inexpensive. However, wood smoke also has the hazard of creosote and noxious gases. These could probably be removed by a water spray system but again, it is not a simple or cheap system, especially in Adelaide where water is at a premium.

Bridging a Playmaster amplifier

We have just recently purchased a Playmaster Pro-3 Stereo Amplifier from Jaycar Electronics and we were wondering is it possible to link the two output channels together so as to double the output power. We only need one channel and the specifications provided don't mention anything about whether we can bridge the two outputs together.

(S. J., via email).

You need the Bridge Adaptor board published in the June 1985 issue of Electronics Australia. We can supply a photostat copy of the article for $8.80 including postage.

Curing noisy volume controls

I had a clock radio which developed a noisy volume control years ago. I found a quick squirt of WD-40 cured the problem instantly and permanently. It is over five years since I sprayed this particular volume control and it is still silent.

I've used this trick many times since. Someone suggested I ought use an electrical lubricant spray but I tried others and there is just no substitute for WD-40. It really works!

Many volume controls are "specials" and can be very difficult to replace. With the WD-40 technique there is no need to replace them at all.

(R. D., Salisbury Heights, SA.

Thanks for the tip.

Video/audio transmitter kit cannot be found

I have purchased and built the video/audio transmitter kit (SILICON CHIP, July 1999). It works but I am using it on a TV out of the computer graphics card (Geforce 200, 64MB) and the problem is that it won't detect the video/audio module. I can use a TV with the video card with no problems, and then plug in the transmitter and the system works until I shutdown and restart. It then searches for the TV and unable to find it, it then turns off the second monitor.

Is there a way to make the kit detectable by my graphics card?

(M. M., via email).

Your video card is probably looking for a 75Ω load. Try loading the video output with a 75Ω resistor or two 150# resistors in parallel.

Universal preamplifier has insufficient gain

I equipped community radio station 3GDR with three of your excellent universal preamplifiers (SILICON CHIP, April 1994) and was impressed enough to buy a fourth kit and try it out on my Onkyo turntable. Alas, it turns out that I have an Audio Technica moving coil cartridge. The output is way down and the hum level is up. Anything I can do or do I work out another way of using the preamplifier at the radio station?

(B. G., via email).

Alas, you need a moving coil preamplifier. Electronics Australia described one in July 1981. We can supply a photostat copy for $8.80 including postage.

Speed Alarm for a motor home

I have built the Speed Alarm described in the November & December 1999 issues and installed it in my Land Rover Discovery; it works perfectly. I have modified it with leads and sockets so I can also use it in my Motorhome.

I was unable to obtain 0.18mm enamelled wire so I have wound 500 turns of 0.20mm enamelled wire onto a slightly larger plastic bobbin (20mm OD x 9mm ID x 9mm). My question is will it work? I also have the following questions. First, is it necessary to set the speed alarm to read 100km/h before calibrating as per your instruction for calibrating at 60km/h? Second, I am told the speedo in the Motorhome is out by 5km/h. What effect will this have?

(J. L., via email).

You can use a larger bobbin for the coil pickup sensor without any problems. In fact the larger core will enable the speed alarm to register at very slow speeds as well.

If you wish to calibrate the speed alarm at 60km/h you need to set the alarm at 60km/h as well. Calibration at 100km/h requires the speed alarm to be set to 100km/h.

The accuracy of the vehicle speedometer does affect accuracy of the speed alarm. If you know the vehicle speedometer is 5km/h out then you can compensate for this by setting the speed alarm 5km/h different when calibrating. For example, if the vehicle speedometer is known to be showing speeds that are 5km/h faster than the actual speed then the speed alarm can be set to 5km/h less than the speed that it will be calibrated. So if you wish to calibrate at 60km/h as shown on the vehicle speedometer, set the speed alarm to 55km/h before pressing the CAL switch.

Bridging ETI amplifier modules

With reference to the comments on bridging amplifier modules in the August 2002 issue, there was a project and a PC board, the ETI481, a guitar amplifier. Its preamp board enabled the bridging of two of the modules to produce 200W into 8-ohm loads. The PC board provided two out-of-phase outputs, one for each module.

(G. V., via email).

We are aware that ETI published the 481 Guitar PC board which allowed two 100W modules to be bridged together (ETI, June 1977). The only problem is, the combination can only drive an 8-ohm load. It will however, deliver 200W and not 100W as stated in our answer - must have slipped a cog there.

Increased gain for Neon Tube Modulator

I have built the Neon Tube Modulator from the November 2001 issue and it works well on the bench from my stereo amplifier line outputs. However, when I set it up in my car and drive it with the line outputs from the head unit, you have to turn the volume way up to get the Neon Modulator to respond. Is there any way the circuit can be changed to fix this?

(J. E., Wollongong, NSW).

It seems you need a mite more gain from the circuit. We suggest increasing the 3.3kΩ feedback resistor for IC1a to 10kΩW or 15kΩ.

TV pattern generator has no colour bars

I recently purchased a programmable TV Pattern Generator from Altronics. It was described in the June & July 1997 issues of SILICON CHIP. Due to a change in the specifications of the video modulator from the original version Altronics included info concerning modifications to the wiring and overlay to compensate for this change. The changes made were an additional 5.1V 1W zener diode between the 180Ω resistor and REG2 and changes in the wiring points of the modulator using only three of the original four pin outs on the PC board.

My problem is that while the kit works well when displaying the first three patterns (checkerboard, dot and crosshatch + circle). I am unable to display the red raster or colour bars. Also the grey scale is not all that sharp when displaying the full eight bars from white to black. When I switch S4 from grey scale to colour, it tries to change but remains grey scale. Varying VR2 and VC1 made no changes whatsoever to the colour scale when testing it on my small colour TV.

There don't appear to be any obvious solder bridges on the SMD IC10 on the underside of the PC board and all voltages check OK as per the testing procedures specified in the magazine article.

(D. S., via email).

The lack of colour may be due to the delay time set by the 2.2kΩ resistor between pins 8 of IC11b and 3,4&5 of IC11c. Try changing the 10nF (0.01μF) at the IC11c inputs to a larger value.

Also the 270pF capacitor added at IC10's pin 16 input may need to be made slightly larger in value to obtain colour.

Cranking difficulties with Austin A1300

I have a problem with the High Energy Ignition project described in the June 1998 issue. I have installed it in an Austin 1300 which is used infrequently. The circuit produces no spark while the starter motor is cranking the engine. I can only start the engine by towing/rolling down a hill or changing over to the HEI once the engine is warm when the engine will run once the ignition switch is released to the running position. What can I do?

(G. P., via email).

We assume you are using points ignition. The HEI should work even if the battery voltage drops to 6V while cranking. How far does your battery drop? Also check the points gap. Rubbing block wear can reduce the gap to nil. We suggest that you use a larger points gap than specified, to ensure a "clean and fast break" in the points current even when cranking slowly.

LiL Snooper Camera switcher

I am interested in building the Li'L Snooper camera switcher from the June 2001 issue of SILICON CHIP. I have one question though. The cameras to be used with it are both colour. Is this circuit be suitable for colour cameras; one is CCD, the other is CMOS? If not, can you recommend any mods to make it suitable?

(C. L., via email).

The Snooper will work with any camera which delivers a 1V composite video signal.

Low sensitivity in Theremin

I have recently built the Theremin project from the August 2000 issue of SILICON CHIP. It works well but I find the sensitivity and range of the pitch antenna low. This is also the same case with the volume disc but this is not such a problem.

I have earthed myself to the ground plane of the project and this helps but it is still limiting. I was wondering if there is a circuit modification I could do to improve this? This web site www.maxiespages.com has many circuit diagrams but none like your unique design.

(T. H., via email).

Sensitivity to hand movement is dependent on careful tuning of the Theremin adjustments. However, sensitivity is not extreme and is not meant to be. The original Theremins required the hand to be brought very close to the plate or vertical wire for best pitch and volume changes.

Note that positioning of the Theremin is important and it should not be located near to metal surfaces. Also attaching the lid of the Theremin onto the box can alter tuning and readjustments may be required on a trial and error basis.

A larger diameter plate and larger diameter antenna can also improve sensitivity.

How to do bifilar winding

I am an electronics enthusiast in the UK. I recently purchased a kit from Altronics for the 15W class A amplifier and power supply (SILICON CHIP, July & August 1998). However, I am a little stuck over the power transformer. Altronics don't seem to do a 20V + 20V model now so I bought a 100VA 20V + 20V EI transformer over here but it ran very hot. I now have a larger toroidal 18V +18V unit but it runs on the limit of regulator drop out.

In the second part of the project (August 1998) you put extra windings on the 18V + 18V model used for the prototype. These windings were wound bifilar. Could you explain (in reasonable detail please) how I go about adding windings myself - it's the "bifilar" bit I'm not sure about.

(S. F., via email).

Bifilar merely refers to the technique of doing the two windings at the same time; ie, get hold of two wires and wind them on as one. You then terminate them as if they were two separate windings, which is exactly what they are.

Soft start for car headlamps

I recall seeing in "Circuit Notebook", probably in the last 3 years, a simple circuit for `soft starting' 12V car headlamps.

As I use high powered (145/90W) halogen headlamps this `soft start' circuit would be very useful in prolonging the life of these very expensive bulbs.

I have looked up your website (great site and very easy to navigate) but cannot find reference to it. I believe it used a simple Mosfet circuit which did not apply full voltage to the bulbs at switch-on but gradually increased voltage over a period of milliseconds. Again, if my recollection is correct I think there was a relay in the circuit which bypassed the Mosfet once full voltage was reached.

Could you please tell me if you have this circuit and how I may obtain it?

(S. P., via email).

You are probably referring to a circuit which appeared in the October 1997 issue. This was a modification of the 12/24V speed controller (see June 1997) using the "soft start" facility at pin 4 of the TL494. We can supply these issues for $7.70 each, including postage.

Electric guitar preamp modifications

I'm interested in a modification to the guitar preamp from the November 2000 issue. I wish to use one channel for a conventional lead or rhythm guitar and the other channel for a bass.

What I'd really like to do is modify the tone control circuit somewhat to make the turnover frequencies around 500-600Hz for the midrange and 2.5-3kHz for the treble controls. (There's not too much 10kHz out of a bass!)

I feel that by simply altering the values of the capacitors around the mid and treble control pots, I can achieve this. I have done some calculations and come up with the following: Substitute 4.7nF (.0047uF) for the 2.7nF (.0027uF) capacitor across the mid pot; substitute 18nF (.018uF) for the 12nF (.012uF) in series with the mid pot and substitute 4.7nF (.0047uF) for the 1.5nF (.0015uF) in series with the treble pot.

By my calculations this will result in turnover frequencies of around 600Hz and 3000Hz for the mid-range and treble controls respectively. Do you agree with these figures?

G. F., via email).

Your modifications will be suitable. As you have discovered, it is simply a matter of scaling the values to suit your required frequencies.

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.

Notes & Errata

Multi-purpose Fast Battery Charger: June and July 2001. When charging older cells either singly or in series, it is important to ensure that their contacts are clean to prevent voltage drops across these connections.

High resistance connections will prevent the charger from operating correctly as it will detect a high voltage per cell and simply indicate "no Battery".

In addition the connecting leads from the charger to the cell or cells must be rated at 7.5A or more and be no longer than necessary to prevent voltage drops.

K-Type Thermocouple Thermometer/Thermostat, August 2002: The display reading and the thermostat trip point can be affected by RF signals produced by portable and mobile telephones when these are close to the unit. This problem can be cured with the addition of four 100nF (0.1μF) ceramic capacitors and a 1kΩ resistor.

The 1kΩ resistor is placed in series with the probe input connecting to pin 3 of IC1 while one 100nF capacitor connects between pin 3 and pin 4 of IC1. This forms a low pass filter in the input circuit.

The second 100nF capacitor connects between pin 3 of IC1 and ground which is the thicker PC track adjacent to the 10μF capacitor to the left of IC1. The third 100nF capacitor connects between pins 2 and 3 of IC1. The 100nF MKT polyester capacitor connecting between pins 6 and 2 of IC1 (located to the right of IC1 on the PC board) is removed. The fourth 100nF ceramic capacitor connects between pins 2 and 3 of IC2.

To provide for these changes, we have modified the PC board, as shown in this diagram. The modified PC board is coded 04208022 and is available on our website.

Atmel AVR ISP Programming Adaptor (October 2001): The software referred to in the article, avr_isp.zip, is no longer available from the Atmel website. A suitable alternative is "Ponyprog", available for free download from www.lancos.com/prog.html. This program also supports Windows NT/2000 and can program many of the newer AVR devices.

To configure Ponyprog to work with the ISP Programmer, set it up for the "AVR ISP (STK200/300) parallel port interface" as described in the included documentation.

Digital Storage Logic Probe, August 2002: the outputs of the 4N25 optocouplers on the circuit on page 24 should be pins 4 & 5, not 5 & 6. The PC board diagrams are correct in this regard.

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