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.

Measuring mobile phone radiation

Have you ever done a project that measures the radiation from mobile phones/towers?

(F. C., Kensington, NSW)

No we haven't. You need a calibrated yagi or dipole together with a calibrated signal strength meter and possibly a spectrum analyser as well. It is not a simple measurement.

A shocker of a question I have some questions relating to electric shock. Why don't we feel any electric shock when we touch a 1.5V DC battery? Is it the current too small? Why don't we feel any electric shock when we touch 12V DC car battery? Would we feel any electric shock when we touch a 240V DC battery? How does electric shock occur? Does it depend on the strength of the electric current and the conductivity of a human being? What is the minimum current that causes a tingling feeling? (K. W., via email) The severity of electric shock is directly related to the current that flows. A few milliamps causes tingling while 10mA is quite severe and enough to kill a child in some instances. In normal circumstances you need more than about 40V to cause a shock or tingle but where you make really good contact (ie, with both hands moistened and around large metal contacts) you can get a substantial shock with as little as 12V. A 240V battery could easily kill you.

Triac for darkroom timer

Back in April 1995 you published a circuit for a Darkroom Timer. I'm trying to find a replacement for the MAC320A8PF Triac as this device is no longer available. Can you help?

(B. W., via email)

Use the Philips BT137F (F stands for isolated tab). You can buy it from Jaycar.

Some motors don't like low speeds

I have only just read the "Ask Silicon Chip" section of the September 2001 issue. Regarding the 'Power Rating For Speed Controller' question, even though the speed controller may be suitable for continuous use, the motor concerned may object to being used at a reduced speed for a long period.

Motors are designed to run at their rated speed, usually being cooled by their own internal fan. When they run at a reduced speed the cooling airflow is reduced and they can overheat sometimes to the point of burnout.

If the motor concerned is a fixed one it may be possible to install a separate cooling fan (eg, computer fan) to keep it within its rated temperature. I hope this information is of use.

(B. A., via email)

You make a good point although it is one that we have covered before albeit not in the constructional article.

World TV frequencies I need to know the actual TV transmitting frequencies (not channel numbers) for the TV systems in UAE and Saudi Arabia. I have searched high and low. Please help. (M. A., via email) The ideal source for this info is the World Radio & TV Handbook. It is available from Dick Smith Electronics at $47.88; Cat B-2101.

Video sharpening with Dr Video

I've recently bought the kit for the Dr Video and have tried it out. Although the picture was pretty good, it was not quite as good as I would have liked it to be. In the kit instructions it says: "by varying the 330Ω resistor up or down you can change the amount of high frequency video boosting given by the Sharpen switch". What would be the absolute maximum I should vary it or what is the next best resistor value to use? I also have the same questions about the 82pF capacitor.

(P. B., via email).

The limit to video sharpening comes about when you observe more noise and graininess in the picture. So vary the resistor and capacitor with this fact in mind.

Bigger lamp for rev limiter

I have just purchased a kit for the Rev Limiter published in the April 1999 issue. I want to know if it is possible to replace the indicator lamps with relays to operate a much bigger indicator light, say a 21W bulb , as I run a racecar and want an indicator that I will notice.

(T. M., via email)

You can replace each lamp with a 12V relay and a diode, with the anode to the transistor collector and cathode to 0V; ie, the diode is reverse-biased.

Testing frequency response of speakers I'm trying to find out how to measure the frequency response of my speakers. Can you help me? (A. H., via email) Measuring frequency response of speakers is not simple and you would be wise to refer to a textbook on this subject. For the simplest of tests, you will need an audio generator or a CD with test frequencies on it, a calibrated microphone and an AC millivoltmeter with a wide bandwidth.

Radfax decoder software

Can you please supply the software for the Radfax decoder featured in the November 1989 issue of SILICON CHIP? I have contacted the Author, Michael Delahunty, but he can no longer supply it, due to a past computer crash.

(A. M., Reservoir, Vic)

We do not have the software either but perhaps a reader can send it to us, whereupon we will send it to the author (and to you).

How to stop rust on camera screws

I have a question that you may be able to answer. I have an older video camera that had a problem with the zoom motor. Minor problem, easy to fix, however, after removing the original factory screws, and putting them back again, the screws have 'oxidised' and are starting to rust very quickly.

I had the same problem with the internal mechanism screws of my VCR. Is this a tamper-proof feature? Is there any way to prevent the screws from going rusty?

(A. P., via email)

Our guess is that the screws originally had some protective clear enamel over them which you have disturbed, allowing corrosion to proceed. Possible tip: put clear nail varnish over the screws that have been disturbed.

Surround sound amplifiers I recently built the surround sound decoder and digital delay board from the May 1999 issue of "Electronics Australia" and would like to build suitable amplifiers for the centre and rear surround channels. The rear channel usually has two speakers in parallel which would result in a 4Ω load. My main amplifier is capable of 50 watts RMS per channel. Could you advise me of the power output required for the rear and centre channels that would be a suitable mix for my current main amplifier? (Y. C., via email). Have a look at the 50W module described in the March 1994 issue of SILICON CHIP. Kits are available from DSE, Jaycar, etc. We can supply the March 1994 issue for $7.70, including postage.

Glue gun temperature control

I am using a 10W 240VAC glue gun to make candles and I find it gets far too hot for the wax I am using. What is the best way to control the temperature?

(A. Q., Adelaide, SA).

Have a look at the article entitled "Power Control With a Light Dimmer" in the October 1996 issue. Although this article described power control for soldering irons, the idea is equally applicable to glue guns.

Leakage on engine immobiliser

I am enquiring about the Engine Immobiliser described in the December 1998 issue. The immobiliser board has about 0.4MΩ across collector and ground of Q1 when in the idle state. This is enough to affect the points and not allow starting when the unit is not turned on.

Everything works well with the overall kit and all is OK once the standing 12V is removed from the board. Is there a correction for this or do I need to look at my own handiwork?

(B. M., via email)

Try shorting the base and emitter of Q1. If this eliminates the 0.4MΩ, it suggests that you may have leakage in the earlier stages or across the PC board itself.