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Reflector for LED torch

I would like to build my own version of the 6-LED torch from the May 2001 issue of SILICON CHIP. I would be using a single "D" size Nicad and manufacturing my own robust aluminium housing on my lathe. However, is it really necessary to use a torch type reflector? From my experience a LED throws most of its light forward. Or is it because they use these reflectors for the convenience of having a battery case and switch, etc?

(R. G., via email).

There is no real need to use a reflector in the LED torch.

Speed control for ceiling fan

I have a ceiling fan with a 3-speed controller. Lately the fan has started to run more slowly on the slowest speed than it did previously. I have often wondered how these beasts work. The controller seems to consist of a switch embedded in an epoxy block and it doesn't really seem to get very hot so how does it dissipate the energy on the slower speeds ? Even if it has more than four poles, there are only two wires going to the fan, precluding some sort of pole-switching arrangement.

(I. L., via email).

It seems highly likely that your existing speed controller is a tapped inductor. In the January 1990 issue we showed how to use a Triac light dimmer in series with one of the taps on the inductor to achieve a much better and wider speed control. The dimmer board used then is no longer available but you could use the same approach with a standard light dimmer with snubber components (50μH choke and .01μF 250VAC capacitor) added as shown in the speed control circuit. We can supply the January 1990 issue for $8.80 including postage.

Tuning the Theremin

I've assembled a Theremin kit from Jaycar Electronics (based on your August 2000 article) and have run into a little difficulty. I followed the tuning instructions in the directions. The telescoping antenna, which is supposed to control the pitch, appears to have no effect, regardless of how T2 is tuned.

While experimenting with tuning the four transformers, I found in some cases the volume plate affects the pitch, but it appears to be tuned properly at present to affect only the volume. I inspected the PC board and found no obvious soldering flaws. The plate is wired to the hole on the left side of the PC board and the antenna to the upper-right, as shown in Fig. 5.

Would you please give me some pointers on what I should look for?

(D. B., via email).

The voltages on each FET should be checked first. Connect a multimeter to ground (0V) and measure the voltage at the drain terminals of Q1, Q2 and Q3. These should all be similar.

Alignment may require that coil T1 be adjusted first to some position for the slug, with the remaining coils readjusted for best results.

Problems may occur if a plugpack which uses a centre tapped transformer is used. Check that this is not the problem by powering temporarily using a 9V or 12V battery or power supply that is not earthed to the negative terminal.

Improving precision rectifier response

I am wishing to use the precision rectifier circuit from the Sound Level Meter (SILICON CHIP, December 1996) in another project and I am hoping you may be able to advise me as follows (I also noted the precision rectifier circuit used in the AC Millivoltmeter project from the October 1998 issue). For my application, the signal level feeding the rectifier will be anywhere from zero up to a maximum of 150mV RMS, within a frequency range of 10Hz to 100kHz.

I have bread-boarded the December 1996 precision rectifier circuit but found that it doesn't operate with a ruler flat response to 100kHz (I expect it was never designed for that!).

Are there changes I could make to the December 1996 precision rectifier circuit to optimise its operation for my specific AC voltage levels? Can I obtain a positive output from the (two-stage) rectifier instead of a negative output? If so, what changes should I make to achieve this?

(G. D., via email).

The precision rectifier will work best at 100kHz if the resistor values are reduced. Use 1kΩ instead of the 10kΩ and 2kΩ instead of the 20kΩ resistors. The arrangement is an inverting style.

To obtain a positive output, add another op amp inverter like the second stage with two 1kΩ resistors - one between the inverting input and the output, and the signal applied to the other 1kΩ resistor which is also connected to the inverting input.

Moving message LED displays

Could you please tell me the back issues where there were articles on how to build alphanumeric LED displays and the techniques of scrolling text from right to left without any tailing effect in the moving text.

(D. T., via email).

We have published three moving LED message displays, in July 2000, February 1997 and March to June 1989. If you want to understand the circuit techniques, have a look at the articles in February 1997 and March and April 1989. We can supply copies of these articles for $8.80 each, including postage.

Dimming halogen lights

I have used a touch/dimmer switch with the intention of controlling a 105W halogen light transformer (Jaycar Cat. MP-3054) and I am getting a flickering light. Is there anything I can do to rectify this problem? The touch/dimmer switch works fine on other incandescent lights.

(L. C., via email).

Generally, light dimmers require a snubber across the Triac A1 to A2 terminals to prevent flickering when driving an inductive load such as a transformer. You could try using a 1kΩ 1W resistor in series with a 100nF 250VAC (class X2) capacitor across the Triac terminals.

However, we note that the 105W unit you are using is a switchmode device which may cause problems with a dimmer (even though it is stated as being suitable for dimming).

Induction motor controller wanted

Back in November 1997, SILICON CHIP put out a Universal Speed Controller. However, I need to control a bench grinder with a 2-pole 2850 RPM induction motor. I was hoping you could help by doing a similar unit. I use the grinder to polish polypropylene at low and high speed, so I need good speed regulation under load. Have you considered such a project or would you consider one?

(M. P., South Caulfield, Vic).

We have no plans to produce an induction motor controller. The design is much more complex as the circuit needs to control both the frequency and the voltage. It also needs to cope with high start-up currents. Sorry.

Large seconds for Big Digit Clock

Is it possible to modify the Big Digit Clock circuit (SILICON CHIP, March 2001) so that the seconds display uses the large LED displays (ignoring the changes to the PC board). Would it simply be a matter of changing the values of the 220Ω resistors?

(M. H., via email).

Large seconds displays can be driven in the same way as the other large displays. Just change the resistors as you suggest.

Increasing the rating of the Battery Guardian

My application for the Battery Guardian (SILICON CHIP, May 2002 is to look after the auxiliary battery in my caravan when running an Electrolux RM2510 refrigerator. This can run on 12V, 240VAC and gas and is operated from 12V while towing. When we are camped, the fridge is either on 240VAC from a caravan park supply or on gas from our bottles.

I have discovered that the RM2510's 12V operating current is 14.6A. As it is an absorption unit (common to all caravan fridges), this current does not change between start-up and normal running. However, the Battery Guardian's specified maximum current is 10A. By the same token, I have noted that the specified Mosfet (Q1) seems to be a 60A unit.

What do I need to do to be able to use the Guardian with my fridge in 12V mode when towing?

(G. B., via email).

The answer is to use a bigger fuse. You will need to bypass the existing fuseholder on the PC board and use an in-line fuseholder with a 20A fuse. This will need to be a 5AG or blade fuse type (available from Jaycar).

Compensating for industrial deafness

Along with many others, I suffer from industrial deafness; not too bad but enough to be a nuisance to me and my family. The usual characteristic is a loss of hearing at 2kHz and 4kHz - the so-called "2k and 4k notch". The 2kHz notch is unfortunately right in the range of sounds that characterise so much speech and so make it difficult to pick out words.

What is required is an amplifier which can be attached to the TV set speaker wires to correct for these frequencies and then feed it to the speakers. I am told by an audiologist that it is unlikely that other listeners would notice it much and it would mean that we can run the TV at lower volume.

Another option would be an appropriate amplifier to feed signals to an FM transmitting headphone set. I have a set of these and they save the volume problem for everyone else but are a nuisance.

(R. C., Parkdale, Vic).

Your audiologist is wrong. Boosting the midrange sufficiently for you to hear it will make the sound quite unpleasant for others. Your FM headset is the better solution.

Li'l Pulser Train Controller.

I have just completed the Li'l Pulser Train Controller and find that the output drive starts at about half scale. When the speed control is set down low, there are "bursts" of output. What could be the problem?

(B. S., Conder, ACT).

The controller requires a motor to be used as a load so that the speed can be controlled. If you are using a different load, it will cause problems.

Check the orientation of diodes D4 & D5 and the components around IC1a and IC1b. Make sure that the trimmer potentiometers are installed in their correct place and that the 12V is going to IC1 and IC2.

Using the Mixture Meter with old engines

I have built the Fuel Mixture Meter from the September 2000 issue but I was going to use it on an older pre-unleaded engine (1972 vintage car). I realise that the EGO sensor would be adversely effected by the tetraethyl lead additives that were used but what about the newer fuels; ie, the super lead-substitute fuels that do not necessarily use lead additives?

Can these newer fuels, that are specifically designed for pre-unleaded vehicle engines, be used longer term with the zirconia sensors or is it only short term as described?

(R. Z., via email).

Just use LRP (lead replacement petrol) and it will work fine.

Preamp for neon tube modulator

I was wondering how I could incorporate a microphone input into the Neon Tube Sound Display project (SILICON CHIP, November 2001)? Would I need to add an amplifier, preamp or anything like that?

The type of microphone would probably be a standard electret microphone.

(A. H., via email).

Have a look at the PreChamp project, described in the July 1994 issue.

You can buy products mentioned in this article here : KC5295 : THEREMIN SYNTHESISER KIT