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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 an email to

Unwanted whistle from speed-controlled motor

I have just built a 10A speed controller kit with four Mosfets for use on a golf buggy. It is mounted in an aluminium enclosure but I have a very annoying audible high-frequency sound coming from the motor.

Is this just a side effect of the switching frequency or is it a curable condition? (J. G., via email).

As you have surmised, the noise is due to the switching of the motor on and off to regulate the speed. You can reduce the noise by increasing the PWM frequency by a small amount.

At present, the frequency is 2kHz. You could try changing the .068mF (68nF) capacitor to a 33nF to double the frequency and reduce noise. However, increasing the frequency too much will actually reduce the motor speed due to the motor’s inductance.

bridge circuit

Sometime in the last 18 months I saw an article in an electronics magazine using a Wienstone bridge circuit. It used a dual op amp (8-pin, not TL0x2 but pin-compatible) amongst its parts. As I rarely look at other publications, I figure it probably was in SILICON CHIP. Can you assist me by telling me what issue it was in if I am guessing correctly? (J. L., via email).

You are possibly confusing a Wien bridge oscillator with a Wheatstone bridge circuit. A Wien bridge oscillator was published in the Circuit Notebook pages of the January 1994 issue. This used a TL072 op amp and a single pot to vary the output frequency. This was not a project article and did not have a PC board.

If you want a complete design for a high-quality audio generator with digital readout, have a look in our February and March 1999 issues.

Remote control
for speakers

After reading your article on the Remote Hifi Volume Control (SILICON CHIP, April 2006), I thought this might be the answer to a problem I have been toying around with for a long time now.

I have two sets of stereo systems (not identical) which I want to use simultaneously to play my music. One is a receiver/amplifier with provision for two sets of speakers – Speakers A (left & right) and Speakers B (left & right). The output is controlled by a single (main) volume control.

What I want to do is put in another volume control so I can control the levels of the two sets of speakers independently.

The other system I have is a 5.1 channel surround sound unit but which I use mainly for stereo sound. It has an output for a subwoofer but is also controlled by a single volume control. In the same principle, I want to add another volume control to operate my subwoofer independently of the front speakers.

Right now, no matter if the subwoofer’s own volume control is on full, if I turn down the main volume, the bass is also diminished. I want the subwoofer to remain at the same level even when the front speakers are fully off.

Next, I want to connect my CD player to both systems through an audio splitter so both will play my music simultaneously. In this instance, I need the opposite: a single master volume control for the whole combined system. Would this be possible without extensive modification of internal circuitry of the amplifiers, using your remote volume control project as external add-ons only? (B. D., Quakers Hill, NSW).

The receiver/amplifier probably has the speaker pairs connected in parallel in the "both" position. This means that there is no way of controlling the volume of the pairs individually unless you do it with a resistive stereo volume control such as units sold by Altronics (Cat. A2305, A2306, A2312, A2313 or A2386).

It’s difficult to comment on the 5.1 channel unit, as there are so many varieties on the market. Assuming it incorporates a separate subwoofer amplifier, then you may be able to find the signal input to that amplifier and control it with a potentiometer. You’d need a copy of the service manual or have a certain level of competency before attempting any modifications.

You could certainly use the remote volume control project as a "master" control by inserting it in the CD player’s signal line.

Electronic wind
vane wanted

Thanks for the low-cost anemometer in the March 2006 issue. I was wondering how to modify it to also provide wind direction. As I have a small yacht, I have been thinking about such a project for long time, as I have not worked out any way to tell which direction the wind is from for night sailing. (K. W., via email).

We published an electronic wind vane with a 16-LED display in the March 2000 issue. You would have to build it as well as the anemometer.

Noise cancelling
in cars

I am wondering if there has been a circuit published that utilises reverse phase noise cancellation. I am looking into reducing the noise in my car by placing microphones outside, to pick up road noise, traffic and engine revs, to then play them back in my car stereo out of phase, to cancel each other out. I would still like to be able to play music through the same speakers so it would have to have a source input, mic input, and filtered output. Do you know of a circuit, or product that would allow me to do this? (J. T., via email).

We published an FM radio intercom for motorbikes in the October and November 1989 issues. This circuit used a noise-cancelling microphone and you may be able to use the same principle in your application.

Queries on PortaPal float battery charger

The March 2003 issue features a 12V SLA float battery charger built around an LM317 adjustable voltage regulator. The designer gives a brief description of the operation of the 317 and mentions that the reference voltage, nominally 1.25V, developed across the output and adjust terminals, will regulate the current through the 120W resistor to 10mA.

This would be the case if the 120W resistor was solely connected between the output and adjust terminals. However, the inclusion of the 1kW resistor, (D11 anode to ADJ pin) alters this.

The 317’s data sheets go some way to explaining this. They include application notes, which feature the 317 configured as an adjustable current limiter. In this configuration, the 120W resistor is referred to as R1 and the 1kW resistor is replaced with a variable resistor. The load would then take the place of trimpot VR7 and the other 1kW resistor on the charger schematic. By adjusting the variable resistor, one can adjust the load current from a maximum value of 1.25/R1 amps down to almost 0mA.

This can readily be demonstrated on the bench. A 121W resistor was used instead of 120W, along with a 1kW variable resistor. A 25W resistor was used as the load; ie, in series with R1. With the variable resistor set at 0W, a measurement of 10.44mA was obtained through the 25W load. As the variable resistor was increased, the 10.44mA measurement started to decrease. A reading of 3.22mA was obtained for a variable resistor setting of 276W.

The designer seems to suggest that because the adjustment pin draws negligible current then practically all of the 1.25V will be dropped across the 120W resistor, yielding a current of 10mA. I don’t believe that’s the case when the 1kW resistor is connected back to the adjust pin. Instead, I calculate the current to be 1.25/(1000 + 120) or 1.11mA. Is this correct? (J. O., via email).

In a typical regulator arrangement where we require more than 1.25V, the LM317 is used with a resistor between the adjust terminal and the output terminal and between the adjust terminal and ground. In our case we use a 1kW resistor and a 500W trimpot (VR7) in series from the adjust terminal to ground.

The resistor between the output and adjust terminal sets the resistor current and hence the output voltage. This occurs as follows. The voltage between the output and adjust terminals is set at a nominal 1.25V and the regulator maintains this with varying load currents. The adjust terminal is simply a voltage monitoring terminal and does not affect the voltages substantially since it draws a maximum of 100mA. The 120W resistor between the output and adjust terminals sets the current at 1.25/120 amps or 10.42mA.

We can usually ignore the 100mA current in the adjust terminal. The current therefore flows through the 1kW resistor and trimpot VR7 from the adjust terminal to ground. The regulator’s output voltage with respect to ground is thus the 1.25V between the adjust terminal and ground plus the voltage developed across the 1kW resistor and trimpot VR7.

If VR7 is set at 0W then the voltage between the adjust terminal and ground will be 10.42mA x 1kW or 10.42V. Adding the 1.25V will give us the total of 11.67V.

If the adjust terminal is not tied directly to the voltage divider formed by the 120W resistor and series 1kW resistor and trimpot VR7 but via a 1kW resistor, then there will be a small voltage drop across this resistor that will affect the output voltage. The drop across the resistor will be a maximum of 100mA x 1kW or 100mV.

This voltage is negligible for our application since we can adjust this out with VR7 when setting the output voltage.

The resistor was included in the adjust sensing leg so that transistor Q2 could be used to control the output voltage to limit the charging current to 1A, as explained in the circuit description. The resistor does not, as we have described above, affect the output voltage significantly.

Connecting a
2.4GHz AV sender

I purchased a 2.4GHz audio/visual sender unit some time ago and only recently connected it to TV sets in two sections of my home. This was prompted by purchase of a new TV set and the staging of the recent Commonwealth Games.

Connecting the sender unit to my new TV set was relatively simple – just insert three plugs into the AV (audio/visual) sockets and connect to power. However, when I began to install the receiver unit to the (much older) remote CRT TV set, I discovered to my consternation that the set was not equipped with AV sockets.

I was somewhat surprised because I was sure that many years earlier I had tried a less sophisticated and inexpensive "rabbit" unit, which connected simply to the same TV set. As it happened, this "rabbit" unit was unsatisfactory in terms of range and the project was abandoned.

I now believe that the "rabbit" output to the remote TV set must have been simply "RF out", replacing the antenna connection. No AV sockets were involved!

So I have the dilemma of a 2.4GHz receiver with three plugs unable to be plugged into my older TV set. I have had a preliminary scan of the circuitry, to evaluate the remote possibility of replicating AV sockets but it all looks too hard. Is there any simple solution to my problem, possibly some sort of interface box accepting the AV plugs from the receiver and converting to a single RF output for connection to my old TV set antenna socket?

Of course one possible solution might be to replace the old TV set with a new one equipped with AV sockets. However, the old set is very far from decrepit and it would be a waste to have to abandon it for the sake of a few sockets. Can you help please? (B. G, via email).

DSE and Jaycar have an AV modulator which will solve your problem. Alternatively, if you have a VCR you can use that to connect the AV signals to your TV.

High power
train controller

I built several model train controllers featured in the April 1997 issue, designed by Rick Walters. The controllers have proven to be very reliable and operate perfectly, giving very smooth and realistic control over the model trains. However, I find that they lack a little in terms of output power, particularly with some of the high draw models that I have.

I was wondering if it is possible to modify the controllers to give an output of around 18-20V and if so, what components would I have to change in order to achieve such an output? (L. G., Penrith, NSW).

It is possible to increase the output voltage of the unit to 18-20V. All you need do is increase the transformer to 15V-0-15V and increase its current rating if your want more current output. However, you will also need much bigger heatsinks on the output transistors, as they will get very hot.

You really would be better off going to one of our switchmode designs which are far more efficient. Have a look at the Railpower design described in the October, November & December 1999 issues of SILICON CHIP.

Looking for kit
radio circuit

I have an "RSC" radio that is not working. I spoke to a fellow in Queensland and he told me that it is a radio kit brought out by "Radio Hobby" magazine. The radio was called "Little General". I am looking for a circuit diagram for it. It is a 4-valve configuration: 6V6GT, 6G8G, 6J8GA and the rectifier I think is a 665. (P. L., via email).

The set most closely matching your valve line-up was described in "Radio & Hobbies" in March 1956, in an article entitled "A Mantel Set From Old Parts". The basic line-up was 6J8, 6C8, 6V6 and 6X5 as the rectifier, although alternative valves were nominated. It doesn’t match the "Little General" sets described in 1956 or 1961.

We can supply a photostat copy of the article for $8.80 including postage.


Frequency Switch For LPG Conversion

I bought a Frequency Switch kit, as featured in "Performance Electronics for Cars", and I need to know if it will do the job I need it for or is there something else to try. I am putting a gas system on my 4x4 and what I need is something to switch it on at 1000 RPM and then switch it off if I go above, say, 3000 RPM but stay on while I am in that range of 1000-3000 RPM.

Now the kit article says that it will switch on a rising or falling frequency. So if I switch it on with rising RPM and then, after going past the trip point, back off, will it turn off straight away or will it wait until it goes under the trip point? Is there a better kit for the job or can this kit be modified to do the job? (G. T., via email).

The only way we can see how you can do this is to build two frequency switches, with one set to switch on at above 1000 RPM and the other set to switch off above 3000 RPM . The contacts from one relay would be connected in series with the second relay; ie, connect the two NO contacts together and use the common contacts to do the switching.

VGA To Component Video Connection

I recently purchased and built the RGB to Component Video Converter kit published in the October 2004 issue. The kit was very professional and easy to assemble.

My intention is to use the converter to allow me to send a video signal from my PC video card to my Sony TV. However, when I tested the card, it appears that there is no sync signal present.

Is there an easy way of adding the vertical and horizontal sync signal present on the VGA output to the component outputs of the converter, either internally or externally? (D. T., Bathurst, NSW).

It’s not entirely clear what you are aiming to achieve. If you want to feed the VGA outputs from your PC video card out to your Sony TV, this won’t work because VGA signals use quite different vertical and horizontal scanning frequencies. Even if you could feed those to the TV, it wouldn’t be able to lock to them and present a stable picture.

However, if your video card does provide a "TV standard" set of RGB video outputs (ie, with a vertical frequency of 50Hz and a horizontal frequency of 15,625Hz) in addition to the usual VGA "monitor" outputs, then using this second set of outputs to feed your TV is more feasible. In this case, you should be able to extract the sync signals from the green (G) video output from the card, using an LM1881 chip in a sync separator circuit like that used in our "Component Video to RGB Converter", as described in the May 2004 issue.

You could use either the composite sync output from the LM1881 (pin 1) to provide the sync signals for the TV, or the signal from pin 1 as the H-sync signal and the signal from pin 3 as the V-sync if the Sony needs two separated sync signals for component video inputs.


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

Passive DI Box, May 2006: the end of the third paragraph in the third column on page 64 states: "The resistor for the ring output also prevents the possibility of the signal from a stereo source being shorted to ground. This could otherwise happen if a mono jack plug is inserted into the ‘thru’ socket".

This is incorrect. It should read as follows: "A stereo source will not be shorted because of the use of a stereo socket and the isolation of the left and right channels via 4.7kW resistors required for stereo mixing. This would not be the case if a mono socket were used instead. Note that the stereo source will be shorted at the ring terminal if a mono jack plug is inserted into the ‘thru’ socket".

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