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Cheap 12V power supply wanted

I want a power supply to convert 240VAC from the power point to 12V DC at 3A. I am currently using a car battery but that is inconvenient.

(D. F., via email).

The cheapest way to get a high current 12V power supply is to obtain a computer power supply and tie all the +12V output leads together and all the 0V leads together (ie, connect the 12V outputs in parallel). We have not done a project along these lines.

Problems with LP Doctor kit

I recently completed construction of the LP Doctor (SILICON CHIP, January 2001) kit but now I find several problems that defy repair. In the "bypass" switch position, the straight through phono preamp and output level control work perfectly, however in any of the "processed" switch positions, there is no audio output.

I also notice that the left/right LEDs appear to function normally, although they flash regardless of the selector switch position. Is this normal? I have thoroughly checked my work and can see no apparent error. I have also successfully completed the voltage adjustment as outlined in the construction manual. Can you offer any advice?

(F. S., Ingham, Qld).

It is normal for the click detection LEDs to operate independently to the selector switch. The signal is somehow being lost through IC3, IC7 IC4, IC8, etc. Check the supply to each of the IC3 and IC7 delays and op amps IC5a and IC7a. Also check that there is -7.5V at pin 2 of IC4 and IC8.

Alternatively, the wiring to switch S2 is incorrect. Check that there is audio signal directly at the outputs of IC5a and IC7a.

10-channel remote receiver queries

The following queries relate to the 10-Channel IR Remote Receiver published in the February 2002 issue. Firstly, in forcing IC1 to display its remote control status by connecting pin 1 to the 5V supply (TP2) via a 220Ω test resistor at power up, can the same be achieved by connecting the two stakes (TP1 & TP2) on the completed board? Or should another resistor be employed, so that LED11 does not blow (ie, does the 100kΩ resistor take enough of the current)? Is this resistor best connected from TP2 straight into the terminal contacts of pin 1 with IC1 in place.

Secondly, is the decoded signal at TP2 for all signals received or only those matching Sat1/Sat2 codes? Thirdly, if the use for some outputs is currently unknown, is it OK to just leave out the 1.8kΩ resistor, which selects toggle or momentary, or do some other parts need to be left out too? (eg, respective transistor).

Fourthly, is there a part that comprises a variable resistor on a reversible motor for volume control? What would happen if power is supplied to both directions at once; ie, both buttons are pressed down by the remote user or would only signal for one button be sent from the remote?

(D. K., via email).

TP1 cannot be directly connected to TP2 or the LED (LED1) will have 5V across it with no current limiting and consequently it will be destroyed. This is why we use a 220Ω resistor at pin 1 to TP2 instead. Connecting a 220Ω resistor between TP1 and TP2 will not bring pin 1 of IC1 high enough as it will be at about 1.8-2V as set by the LED voltage.

The decoded signal at TP1 will reflect the incoming code sent by the IR transmitter. The code does not have to be SAT1 or SAT2 but the 10 outputs will not respond unless the code is correct.

The 1.8kΩ resistors on the outputs need to be connected to set the momentary or toggle option. If these are left out of circuit, the output could be either option, depending on what level the pins float at when powered up. Generally, the RA outputs will be set at momentary and the RB outputs for Toggle if no resistor is connected.

The decoder can only respond to one code at a time and so driving the motorised potentiometer using the two outputs will not cause any problems. And no problems occur if outputs are in any state as the motor will only have power (with either polarity) or have no power with the motor terminals both at positive or both at ground.

SuperCharger vs Multi-Purpose Fast Charger I have read the SuperCharger articles with interest and I am keen to build one, if you can first answer a couple of questions. I have been using your Multi-Purpose Fast Charger II for quite some time now. It has charged quite a few of the new high capacity batteries (AA 1800mAh) and seems to do it well. What is the difference between the two chargers in their ability to charge the new high capacity batteries and am I doing my batteries harm by using the Fast Charger? I always refresh before charge. The standard number of AA cells I charge is four; ie, 4.8V. I always use the thermistor when charging and have never had any problems with overcharged or overheated batteries. One advantage of the new charger is it could be used away from a 240VAC power source which is attractive when using a digital camera on motoring or camping holidays. (C. N., via email). There are a few potential problems charging small (AA/AAA) cells with the Multipurpose Fast Charger II. Of most concern is its high fixed charge rate. You may find that you can charge Nicads at this rate (using the thermistor for temperature sensing) but it will depend greatly on the type of battery. Look specifically for "high discharge rate" types, intended for R/C applications and the like. For NiMH-chemistry types, we're not aware of any small (AA/AAA) types that can be safely charged at 6A. You mention that you are currently doing this without problems. While it's true that thermistor temperature sensing should prevent catastrophic cell failure, we doubt that the cells will be fully-charged before the charge terminates. You may like to verify this by doing a cell capacity test. We may publish a simple "add-on" circuit for the Multipurpose Fast Charger that allows you to reduce the charge current at the flick of a switch. No promises though!

Delayed audio on parliament broadcast

I am interested in politics and enjoy watching the daily broadcast, when parliament is sitting, of Question Time on ABC-TV between 2:00pm & 3:00pm. Normally, when ABC-TV is broadcasting the House of Representatives, ABC NewsRadio broadcasts the entire day's proceeding from the Senate and vice versa.

However, on some occasions, both the TV and radio broadcasts were from the same House (in this case, the House of Representatives). On such occasions, when I have the TV on and also try to listen to the proceedings of the same House on the radio, I notice that there is a delay of a few seconds in the audio between the TV & radio broadcast.

Can you explain the reason or reasons for this delay? I can only assume that it may have something to do with the physics of the propagation of radio waves as, I suspect, both transmissions here in Canberra come from local transmitters. I suppose another question might be should there be a delay in the reception of the audio for a simulcast program received on both TV and radio?

(P. M., Queanbeyan, NSW).

The delay has nothing to do with the propagation of radio waves and everything to do with the way the ABC routes signals between its various studios and transmitters.

We would guess that the TV signals come direct from Canberra to the relevant station transmitter while the audio signal may go via the ABC radio studios in Adelaide (or wherever). There may even be a satellite link involved.

Mic for measuring frequency response

I would like to know if there is a cheap way of making a microphone and (preamp) with a known frequency response, to use in conjunction with the RTA spectrum analyser shown in the August 2002 issue. If not, I believe it would make a great project because being able to see the problem areas in the frequency response of a sound system setup would be very handy.

I know you can buy calibrated microphones but these all seem to be quite expensive and for the small amount of use they might get places them out of reach of most people.

Could SILICON CHIP do a project using an easy to get and cheap electret microphone and plot its response, etc, to use with RTA software?

(B. B., Runaway Bay, Qld).

Producing a calibrated microphone is not a simple task - we have looked at it as a project but cannot see an easy way of doing it.

Faulty speed control blows FETs

I have built two 20A Speed Control kits from the June 1997 issue. One works OK while the other worked for one week and then the battery was put in reverse polarity. It blew one FET.

I replaced the faulty parts and it worked for one week and then both FETs blew. I replaced both FETs but now I can't vary the speed - always near flat out.

(S. G., via email).

Strictly speaking, the circuit should not have been damaged by battery reversal. The fuse should have blown with diode D1 forward biased.

However, since the FETs were destroyed, it may be that the transistors and IC were also damaged. This would explain the full-speed only operation. It may be wise to replace the FETs, transistors, IC1 and the regulator, plus the diodes at the battery supply and across the motor.

ETI pH meter wanted

In the December 1980 issue of ETI a digital pH meter project was featured. This used an Intersil 7106 driver and a 3.5-digit LAD204 display. Do you know if this display is still available? The driver and display were sold by Intersil as part of an evaluation kit. Perhaps an updated version of the pH meter could be considered as a SILICON CHIP project?

(T. S., via email).

The ETI pH meter is no longer available. SILICON CHIP described a pH meter based on an LM324 quad op amp and an analog meter, in the April 1988 issue. It would probably be much easier to source the parts for it but you would need to make sure you could get pH electrode before proceeding further.

We can supply a photostat copy of the April 1988 article for $10, including airmail postage.

Hum in Plastic Power amplifier I have built the Plastic Power amplifier module described in the April 1996 issue of SILICON CHIP and it passed all the tests. However, I am getting hum. When I turn the power supply off, the hum stops and the amplifier continues to function for about 10 seconds or so. Do you have any suggestions to get rid of the hum or would bigger filter capacitors do the job? (S. G., via email). The hum could be due to the filter capacitor value not being sufficiently large but this is unlikely. Instead, hum at low levels is more likely due to a hum loop, due to the ground connections not being connected to the one point. To minimise hum, the ground connections for the capacitors should all connect to a single point with the 0V wiring from the transformer also connecting to this point. The amplifier output (speaker) ground then connects to this common ground. The input ground of the amplifier is not connected to the common ground as this is connected to the speaker ground on the PC board. In addition, the rectifier (+) and (-) outputs should connect to the capacitors first, with the amplifier supply rails connecting to the capacitor leads rather than the rectifier terminals. Use heavy gauge wiring for these power supply connections.

Splitting a line level signal

Could you please advise me, if I split a line level stereo output signal (left & right channel) using two "RCA Y adaptors", will it affect the level/quality of the input signal at either of the destination components (power amplifiers in this case)?

(G. M., via email).

That's not an easy question to answer. Superficially, it would seem that splitting line level signals would not cause a problem but there are a number of factors to consider.

Factor one is the input impedance of the amplifiers, etc. Normally, this will be high and should not cause a problem but if it is low (say below 5kΩ), it could reduce the signal level, as well as degrade the frequency response and distortion performance.

Two, if one of the amplifiers is driven into overload, the resulting distortion artefacts may be superimposed on the line signal to the other destination device.

Three, if one of the destination devices and the signal source is earthed via the mains (rather than double-insulated) an earth loop will result which again could degrade the performance.

From this, you will see that we are not keen on Y-connectors. Provided the input impedances of your two destination devices are high, a better way is to make up a splitter box with RCA connectors and feed the signal to the two devices via isolating resistors of, say, 4.7kΩ. That way, if something naughty happens in one destination device (say overload), it won't degrade the rest of the signal chain.

Generating different audio tones

I am about to start a project that requires two different audio tones, one having a positive sound like a "ping", the other having a negative sound like "baamp". If you imagine, like a board game that has "yes" and "no" answers.

I would like to know what I need for creating these two tones. I want to be able to trigger each tone with a microcontroller.

(B. C., via email).

We suggest using Twin-T oscillators set to different frequencies and also set just below the point of oscillation; they can then be shocked into oscillation by a short pulse.

SILICON CHIP has not published anything on this subject but there were two relevant articles in EA: "Electronic Bongos" in December 1979 and "Twin-T Oscillators" in May 1976. We can supply photostats of these articles for $8.80 each, including postage.

How to suppress contact arcing

I have a question about how to reduce the arcing across relay contacts. I'm switching a 240VAC 1A solenoid with a 24V DC relay. What is more reliable, using a Triac or relay?

(Y. G., via email).

The normal way to suppress contact arcing when switching DC is to use a reverse-biased diode with a current rating of more the than load current being switched. This should suppress any contact arcing across the relay contacts.

When switching 240VAC with a solenoid, it is usual to connect a 250VAC-rated capacitor across the (solenoid) contacts. Try a 47nF (.047μF) 250VAC capacitor across the contacts.

High input impedance amplifier wanted

I would like a suggestion for a high input impedance amplifier circuit. What I need is to be able to intercept a line level signal and basically split it in to two line level signals. I need it high impedance so that it doesn't load the line too much.

I want to build five of these amplifiers and then use them to provide line-level outputs from each channel of my Boston Acoustics BA7500G Speaker system so I can feed external amps, without disturbing the internal amplifier. Any help would be greatly appreciated.

(M. R., via email).

You really don't need a high input impedance. What you need is a buffer amplifier that can drive a series of low impedance outputs. We have already published a suitable circuit - the audio portion of the Audio/Video Distribution Amplifier in November 2001. This circuit is based on an LM833 dual op amp.

If you decide to use it, make sure you install 1kΩ resistors in the outputs instead of the 47kΩ resistors mistakenly specified in the original article.

Speed control in cordless drills

I recently bought a variable speed cordless drill/screwdriver (Black & Decker) and would like to know what method is used to provide the variable speed feature. I guess I could open the drill and look inside but that will void my warranty.

(H. P., via email).

Cordless drills are usually permanent magnet motors with a Mosfet switchmode power control, usually running at about 1kHz or so. In fact, you can usually hear the 1kHz tone at very low speed settings.

Temperature controlled fan

I want a circuit for a temperature-controlled fan for the heatsinks of an amplifier I'm working on. I am positive SILICON CHIP published one in the not-too-distant past but for the life of me I can't find it in my back issues, nor can I locate it on your website. Can you recall where the circuit appeared please?

(G. B., Scotland Island, NSW).

The circuit was associated with the Ultra-LD Amplifier in the August 2000 issue. There were two options: a thermistor or thermal cutout.

Adjusting mixture on a VR Commodore I have just purchased a Fuel Mixture Display kit. If the vehicle is running rich, is it possible to adjust the vehicle's computer to make it run in the correct mixture range? For a VR Commodore, is there room for adjustment to make it run lean? (D. G., via email). Although it's possible to change the "chip" in the engine management computer (eg, to alter the timing curve and/or to change mixture settings), this really is a job for the specialist. And generally speaking, it's difficult to do better than the manufacturer's original chip. If the fuel mixture display indicates that the engine in your VR Commodore is running rich, then there are two possibilities: (1) either the fuel mixture display is incorrectly calibrated; or (2) there is a fault in your vehicle's fuel system which should be corrected by a specialist service outlet. Note that there are times when the engine should run rich - eg, under acceleration. If your vehicle is performing normally, then it's likely that the fuel system is operating correctly. Note also that the VR Commodore also runs quite lean when at speed under light throttle settings. It can return very good fuel economy on a trip.