No attack
or delay

I have recently built the Guitar Limiter as described in the October 1998 issue of SILICON CHIP but the attack and delay functions don’t have any effect at all. The board seems to be in order. I had noticed that the component overlay diagram on page 70 has the capacitor next to a 22k resistor not marked. The circuit diagram shows it to be a 1µF electrolytic. Am I correct on that? (D. F., Salisbury, SA).

The unlabelled electrolytic capacitor on the overlay diagram is 1µF. The only reason that the attack and decay controls would not operate, apart from misplaced components, is that the gain limit control is set too high or the output level is set incorrectly. Make sure that the output level trimpot, VR3, is set as per the instructions on page 73 under "Testing".

Hybrid bridge
voice circuit

I have a son with a severe hearing problem. Has SILICON CHIP ever described a suitable balanced amplifier for use in such an application; ie, adapted to amplifying incoming voice without affecting the outgoing voice? He is using some wheelbarrow size gear put together by his brother from disposals rack-mounted equipment and finds it very helpful but it’s certainly not portable!

This must surely be a widespread problem. I would be grateful for any suggestions that occur to you. (G. B., O’Connor, ACT).

We don’t have a balanced amplifier for phones but you might like to have a look at the Hands-Free Telephone circuit published in the September 1988 issue. It used a special Motorola IC with an inbuilt hybrid circuit. We can supply a photocopy of the article for $7 including postage.

Circuit for
humidity control

Is there a circuit diagram available for a humidity control or where could I find one? (J. D., Mathoura, NSW).

We published a 3-function weather station, incorporating barometer, temperature & humidity, in the April 1993 issue. It used a Philips humidity sensor. We can supply the April 1993 issue for $7, including postage.

Rev limiter for
race circuit car

I have just purchased the Rev Limiter Kit and Ignition Switcher (described in April 1999) to install in my mid 70s 4-cylinder car. It is a dedicated circuit racing car with a 1.6 litre full race engine (carburettors). So this project is abso-lutely ideal for the car.

But I was wondering if it was possible to modify the Rev Limiter to momentarily operate at a set RPM via an additional switch. What I need is to be able to hold the engine at say 4500 RPM (using the Rev Limiter to do so) whilst waiting for the green start light. When the lights go green and therefore the race starts, the engine would be held at the 4500 RPM (or whatever) to enable a perfect start. When the car gets going and has good traction, I would then release this button and the engine would then be able to spin out fully to the set Rev limit.

Would this be possible and what modifications are needed? (Paul, via email).

It is certainly feasible to modify the Rev Limiter to pro-vide a preset limit. You could arrange to have the limit output switchable between pin 1 and pin 7 of IC2 to achieve this effect.

However, while it is feasible we are not sure it would be suit-able for racing starts. The problem is that the rev limit severe-ly cuts back the power and also puts unburnt fuel into the ex-haust system which might also cause a problem if you are really tromping on the accelerator.

Upgrading the
plastic power amplifier

I recently built an amplifier based on the 125W/175W ampli-fier module published in the April 1996 issue of your magazine. I’m very happy with the sound quality but I would like an increase in power output without necessary jumping to the 500W module published in the August, September & October 1997 issues. My idea is to increase the size of the PC board (lengthwise) and add additional wiring, etc to accommodate an extra pair of output transistors. My questions are:

(1). Would adding an extra pair of output transistors allow me to deliver more power to the load (8) and keep within the distor-tion figures quoted (as per the April 1996 module)?

(2). Would I have to do any modifications to the driver or input stages to cope with the additional loading?

My power supply consists of a 500VA toroidal transformer and six (15,000µF 80VW) capacitors. The resulting supply rails are about ±63V. I have also changed the 100µF and 0.1µF capacitors (on the circuit board) from 63V rating to 100V.

Also of note, I’m running the output transistors at the moment at about 30mA each of quiescent current and the main heatsink remains very cool and the heat-sinks on the BF470 (Q6) and BF469 (Q8) are lukewarm at best. Even after running the amplifier at a reasonable volume for an hour or so, both heat-sinks remain relatively cool. (B. F., via email).

If you are only going to drive 8 speakers, there is little point in adding the extra transistors. Your beefed up power supply will probably mean that the amplifier will deliver about 150W/channel and this can be easily handled by the existing transis-tors.

However, if you wanted to drive 4 loads, you would defi-nitely need the extra transistors. There would then be some slight degradation of the distortion figures compared with those published.

Mind you, the increase in power output from a nominal 125W to 150W is only 0.8dB which is inaudible on normal program mate-rial. On balance, we would leave the amplifier as is.

Generating
video crosshairs

I came across your articles on the Colour TV Pattern Generator in the June & July 1997 issues of SILICON CHIP while looking for a requirement I have. I wish to make use of some of those relatively cheap colour CCD miniature cameras for targeting objects in a range of applications. What I need is a device to generate a crosshair pattern superimposed on the camera video that can be retrofitted inside standard video monitors to allow them to be used as fairly accurate sights.

Could the Pattern Generator be configured to perform this function easily with some slight modification to facilitate gen-locking, etc? (K. F., via email).

The generator could be adapted for your application since the checker-board pattern could be modified to provide the crosshair pattern. The project was designed to allow constructors to modify the pattern in Basic on a computer and then reprogram the EPROM. The information supplied in the TV pattern generator articles should be sufficient to allow you to generate your own code for the crosshair.

However the unit would be too large to be housed in a cam-era. The alternative would be to program a micro-controller to produce the patterns. While the TV pattern generator used an EPROM to store all pixels and sync signals in a sequential fash-ion, the microcontroller could be more economical on memory space by having the internal timer decide when to output a pulse.

The crosshair pattern and sync signals are rather basic and so the coding would be only short. The PIC16F84 may be a good contender for this application. Software for developing it is freely available from the http://www.microchip.com site.

We published a simple programmer for it in the March 1999 issue of SILICON CHIP.

Frequency accuracy
of audio generator

I was wondering if the digital readout on the Audio Signal Generator (featured in February & March 1999) was accurate or would I need a frequency counter for loudspeaker testing? (S. B., via email).

For general purpose testing the frequency accuracy is good enough. If you had a signal source of known accuracy and you were willing to tweak the timing components associated with IC11 (the 555), you could get the accuracy to better than 1%.

Car stereo project wanted

I am currently doing a pre-apprenticeship in electrical and we are allowed to build a project of our own choice. I was won-dering if you could suggest a project design for a 300-400W RMS car stereo amplifier? (Justin, via email).

We have not published any 300 to 400W car stereo systems as a DIY project would be more expensive than a built-up unit such as those stocked by Jaycar.

However, we have published a 600W inverter for car sound systems in October & November 1996 (Jaycar kit) and a 350W amplifier module in the August 1996 issue (Al-tronics kit). We can supply back issues at $7 each including postage.

Video-audio transmitter causes picture roll

I recently purchased a video-audio transmitter kit, as described in the July 1999 issue, from Jaycar Electronics. I have a picture coming through onto my monitor but unfortunately it rolls all the time. Can you suggest a cure? (J. B., via email).

The rolling suggests that the sync level is too low for the monitor to lock and maintain a steady picture. This could be due to the levels on VR1 and VR2 not being set correctly. It is recommended that both VR1 and VR2 be initially set fully clock-wise so that the maximum signal level is applied to the modula-tor. The picture may be a little washed out (excessively bright) but the locking problem should be cured. Then wind VR1 back for best contrast without rolling. Then set VR2 for best contrast if necessary.

If the picture cannot be made to lock with full clockwise settings for VR1 and VR2, then it is possible that the 470µF capacitors

Wiring up the
Waa Waa pedal

I recently bought a Waa Waa Effects kit, as described in the September 1998 issue of SILICON CHIP. As I have little experience in electronics I ran into a few snags. How do you wire the slider pot, the pedal and the SPDT slider switch to the circuit?

The power supply is 12V but is it possible to run it off a 9V battery? Could I possibly have run it from a DC plugpack and a battery as well, to make it more versatile? (S. W., via email)

The slider potentiometer is wired with the two end connec-tions to the terminals marked on the PC board overlay as "To VR1". One lug goes to one terminal and the other lug to the other terminal. The wiper of the slider pot goes to the terminal marked "To VR1 Wiper" on the PC board overlay diagram. If you subse-quently find that the Waa Waa effect operates backwards then swap the wires to the "to VR1" terminals.

If you’re not sure which pot terminal is the wiper, it can be found by checking the resistance between the terminals. The two end terminals will not change in resistance as the pot slider is moved from one end to the other. Both the spare terminals are the wiper connection.

The slider switch is mounted on the PC board with the common terminal in the centre. Note that the switch can be a SPDT (single pole double throw) type. If you want to wire the switch to a panel, then simply wire it to the board, maintaining the same connections.

The pedal is connected to the PC board with a stereo jack plug and three-core wire such as a balanced microphone lead. This has two cores and a shield. The shield can be the 0V supply connection. You need to use a stereo socket on the pedal so that the three wires can be connected.

The circuit can be operated from either a 9V battery or a DC plugpack without any further modifications.

 

Repairing a faulty
NAD amplifier

I have acquired an old NAD 3020 integrated stereo amplifier which I would like to use. However I find that the output to the speakers is very distorted and fuzzy even at very low levels. As this amplifier is so old, it is uneconomical to send it to a serviceman and I would like to troubleshoot and repair the amplifier myself.

I am a former Telecom technician (retired) and have been an electronics hobbyist for many years but have no experience what-soever in the art of "audio troubleshooting". Can you recommend a source of knowledge in this area that I could study and/or per-haps offer some tips which may help me to achieve my goal of restoring the sound of this excellent old amplifier? My investi-gations so far indicate the problem to be in the power stage of the unit. (A. L., via email).

We understand that AWA Distribution handle NAD products so you might be able to get a circuit diagram from them. Failing that, we would give the amplifier a close inspection to find any overheated or otherwise faulty components. Then check the base-emitter voltage of every transistor in the circuit. They should all lie in the range from 0.6 to 0.75V. Any that don’t fall in this range are probably faulty or have an associated faulty component. We can’t suggest a suitable reference book.

 

Connecting the high energy ignition system

I’ve just finished constructing the Universal High Energy Ignition (June 1998) to put into my 1979 Toyota Corolla but I am having a little difficulty with the connections. I have tested the unit and adjusted the current limit adjustment as described in the text. The transistor is correctly mounted and is not shorting out on the case.

In the text, it describes connecting the wires as follows: one to the coil negative, +12V from the ignition, earth to the case and then to the chassis, tacho to the tacho (I already have the electronic tacho fitted) and points to the points.

The coil is a standard type with an external ballast resis-tor, connected to a single points-type distributor incorporating a condenser and noise resistor.

This is where I am having troubles. All is connected as above, but the car won’t start. Now do I have to remove the existing wire that goes from the coil negative to the points and use the kit connections instead? If so, and I have tried this, the car doesn’t start! I have also tried it without the tacho connected but that made no difference. Can you please assist me? (T. G., via email).

The existing negative lead from the points should be discon-nected from the points. The points wire is then connected to the High Energy Ignition (HEI) input and the output from the HEI goes to the coil negative. In other words, the wire from the points in the distributor is used to trigger the HEI system and the HEI output now does the switching job for the coil primary that used to be done by the points.

Probably the problem you are having is that the second set of components for the points 2 circuit is in circuit. If you are only using one set of points, remove diode D2 and the 47 5W resistor next to this diode.

The tachometer connection can be made to the tachometer output on the HEI or left in its original position if this oper-ates it correctly.

Programmable ignition timing for CDI system

I recently purchased a Programmable Ignition Timing kit (published in June 1999) from Jaycar Electronics in Perth

I’d like to know if this kit is compatible with a Crane Hi-6 multiple spark capacitive discharge unit. I’m running this device in a Holden VK 308 Commodore with electronic ignition and no vacuum advance.

Do I need to make any changes to the circuitry on the igni-tion timing kit to suit the above system? (S. L., via email).

The Programmable Ignition Timing module could be used with the Hi-6 unit simply by placing it between the points and the Hi-6 ignition system. You would need to build up the input circuitry for the trigger coil on your distributor. The output from this circuit would then connect to the Programmable Ignition and its output would drive the Hi-6 module which would be set up for a points input.

Details on programming the module for various advance curves were provided in the July 1999 issue of SILICON CHIP.

UHF version of Railpower

Being a model railway enthusiast, it is great to see the Railpower project in the October 1999 issue. I have built many of your model railway projects and found them all very successful. I built your last infrared controller of a few years back and have it working very successfully on my home layout.

Your latest effort seems just as interesting and I look forward to the forthcoming editions to see how things progress. I only have one question and that is, would it be possible to have the activation of the controller done by RF rather than infrared? There are RF-activated controllers on the market and these can be a great asset if one is operating layouts at an exhibition. (T. B., Glenroy, Vic).

Yes, you could use the encoder in the remote to drive a UHF transmitter and then have a UHF receiver in the Railpower. You could base it on the UHF system published in the February 1996 issue.

Timing for slot car
drag racing

I have a need to measure times over a few seconds, down to a thousandth of a second, for slot car drag racing. Two lanes need to be timed, false starts indicated and the overall winning car identified at the end of each race. Can this be done with one of the interface cards designed to be plugged into the parallel port of a computer (I would like to use a notebook computer), or does the card need to do the timing and send the information to the software for display?

If you have published anything which may suit my require-ments I would appreciate the information so I can get started. (B. K., via email).

You are correct in your assumption that the interface card cannot be used for the timing function. It could be used to signal false starts etc to the computer. The timing could be done with 2 x 74HC390s driven with one section of a 74HC132 configured as an oscillator. Another section could be used to gate the counter. The 16 counter outputs could be switched in two groups of eight to the interface card inputs with 74HC4066 switches.

Notes & Errata

LED Christmas Tree, November 1999: three 0.1µF monolithic capacitors are required, not two as specified in the parts list. One of these (C6) is not identified on the PC board component diagram but is adjacent to pin 1 of the microcontroller. Also the PC board overlay shows two 15pF capacitors but these should be 27pF as shown on the circuit and parts list.

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.