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.

Triac tester wanted

Have you ever published a circuit to test Triacs? If not, what is the easy way to test a Triac?

(Y. G., via email).

We have not published a Triac tester and nor is there any easy way to test them for parameters such as blocking voltage, holding current, gate sensitivity etc.

However, if you want a simple safe check, use a 12VAC supply, a 12V 20W halogen lamp and a 470W resistor to connect the gate to A2. Connect the lamp in series with A2 and the +12V supply. Connecting the gate to A2 (via the 470W resistor) should turn on the lamp.

Curtain motor control wanted

I am searching for information on the control of an electric motor for the opening and closing of window curtains. The device would detect a voltage drop or a current increase when the curtain carrier comes to the fully opened or closed position. The operating voltage is 12V at less than 1.5A.

(H.B., via email).

Have a look at the circuit for a remote volume control in the June 2002 issue. It was a low power 5V circuit but could be upgraded to higher current by changing the transistors in the H-pack. The BC328s could be changed to Darlington BD682s and the BC338s to BD681s. The 10W current sensing resistor would have to reduced to around 0.5W or less.

Speed Alarm for FWD cars I recently purchased the Speed Alarm kit (SILICON CHIP, November & December 1999) only to find that it does not work for a front-wheel drive car (Mazda 626) as you do not have access to the drive-shaft, only the wheel shafts which rotate too slowly, hence generating too few pulses. My question is, has anyone produced a modification to the circuit or to the PIC program to overcome this problem? (P. H., via email). You can use the Speed Alarm on a front-wheel drive car. Just use more magnets on the wheel drive shaft. Eight should be enough, evenly distributed around the shaft. In fact, it is not so much that the Speed Alarm does not work for a front-wheel drive but that the speedometer update time is dependent on the number of pulses provided for a given speed from the pickup sensor. So more magnets on the shaft will increase the number of pulses and provide a faster speedometer update. However, there is another way and this has been mentioned a number of times in past issues: don't bother with magnets and the pickup coil; just use the speed signal from your car's engine management system. The pulsed speed signal connects to the coil input terminal on the PC board, via shielded cable. The shield connection is left open, ie, no need to connect the shield at the pickup point for the engine management speed signal.

Flexitimer does not cycle

I have built the Flexitimer published in the August & September 1995 issues of "Electronics Australia". I find that once the 9V mains adaptor is turned on, the solenoid is energised after about 7s (normal) but will not disengage after a further 7s.

Shouldn't it change output poles on the relay every 7s or is the timer only a straight timer that turns on or off once until the mains is turned off? I have used the Q4 jumper to give the minimum time.

(A. S., via email).

If you want the relay to cycle on and off, you must cut the track between the collector of Q1 and pin 4 of IC1, then connect pin 4 to pin 8 of IC1.

Motor speed controller reversing

I have a 10A Motor Speed Controller kit (June 1997) which I intend to use to drive a 12V DC motor. However, I want to run it in forward and reverse direction.

I am thinking of using a relay to switch polarities but I realise this will cause a short circuit across protection diode D2. I would appreciate any ideas to solve this problem.

(D. C., via email).

The simple way to solve your problem is to use a 2-pole changeover relay. We showed how to use a relay in this way in the L'il Pulser Train controller published in the February 2001 issue. We can supply this issue for $8.80 including postage.

Speed control for distributor tester I am in the process of building a car distributor tester using a 12V heater fan motor to drive the distributor. I need to vary the speed of the motor to test the centrifugal advance curve of the distributor. The motor draws in excess of 15A on startup and drops to 6A at full speed. Could you please advise if the high-current speed controller for 12V and 24V DC motors featured in the June 1997 issue would be suitable to vary the speed of this motor? (K. W., via email). The June 1997 design would be suitable for handling your load, although we are surprised that the fan motor draws such a high current (both at start-up and when running). We assume that this is without the fan connected and it suggests that there is something wrong with the motor. As an alternative, you could use a 240V sewing machine motor and control it with the speed control featured in the October 2002 issue. We can supply this issue for $8.80 including postage.

Bridge amplifier for subwoofers

I am building a subwoofer amplifier using the 50W LM3876T chip from your March 1994 amplifier module. My preamp has the extra output to run two modules in bridge mode, so I decided to build two up for extra power.

The first module works perfectly, while the second module is the same as the first except for the output chip. It was only after I had finished, that I noticed the second chip was an LM3886 (they look identical). The amplifier seems to be working OK except that I noticed that the LM3886 chip was running hotter than the LM3876 chip, so I have switched off for now. The modules are in bridge mode.

My question is, what is the difference between the two? Can the LM3886 be used in the March 1994 project without modification? If not, what modifications are needed? Which chip is best for bridge mode operation when using two?

My sub-woofer has two separate drivers running in a three-chambered bandpass enclosure, so is it possible to use one module for each speaker. Would this be better? I strive for perfection, so I hope you don't mind all the questions.

The transformer I am using is a 200VA type with two secondary windings. These are 25V-0-25V and 22V-0-22V, with a switch to select the one you want. Would either of the chips produce more power into 4W or even 2W in bridge mode if the lower transformer voltage was used? The DC voltage was 33V compared to the normal 37.5V DC for this project.

I have built up a 5-channel power amplifier using the LM3876T chips for each channel. A 500VA toroidal transformer and 40,000μF of filter capacitors are used to power them. This amplifier has fantastic quality for music and home DVD movies.

I use this 5-channel amplifier with a DVD player with an inbuilt 5.1 AC3, DTS decoder and the new 6-channel IR Remote Volume Control project (SILICON CHIP, March 2002). This setup is much cheaper than buying an all-in-one home theatre amplifier and you get to use amplifiers you have built up in the past.

(K. S., Morphett Vale, Vic).

We would not use the LM3876 and the LM3886 in bridge mode; use either LM3876 or LM3886 - do not mix them. The LM3886 is a higher rated version of the LM3876.

If your subwoofer has separate drivers, it would be much better to drive them from individual amplifier modules rather than in bridge mode. In fact, both the LM3886 and LM3876 are not really all that good in bridge mode because their power output into 4W is only slightly more than that into 8W.

If you drive the woofers separately with the modules, you can use both the LM3876 and LM3886.

Use 25V + 25V from your transformer. You will not get much more (if any) more power from these chips by changing the supply rails because the chip has internal power limiting (check the article in March 1994, page 80). They will not drive 2W loads.

Questions on sound level meter I'm interested in building the Sound Level Meter from the December 1996 issue. It is to be used to help set up speaker levels in a 5.1 home theatre system but as I'm fairly new to all this I have a few questions: An ECM-60P type A electret microphone is called for in the design, however I can't find one. Can you suggest a suitable alternative? Should the 10kΩ bias resistor and 68kΩ and 10kΩ feedback resistors be changed to match the substitute microphone? I have found a software-based pink noise generator at: www.nch.com.au. Can this be used in place of the pink noise kit? Also I'm not clear on the calibration procedure. It says to connect the pink noise source to the mic input (I assume with the mic disconnected?). Should this be the line level output from the pink noise source or the speaker level output; ie, should I use the line level output from the sound card or the wires that connect to the internal PC speaker? As the software will only go to -30dB, can I use 700mV as the set point; ie, a 300mV change between 0dB and -30dB? (V. S., via email). The Jaycar Cat. AM-4011 microphone would be suitable. No resistor changes are necessary. The pink noise generator signal from the computer sound card would be suitable. However, the 60dB attenuator circuit as used in the SILICON CHIP Pink Noise generator (January 1997 issue) would need to be used to calibrate the sound level meter. This resistive divider can be added to the output of the sound card.

Does auto lock-up confuse Gear Indicator?

I am interested in the Gear Indicator project featured in the January 2003 issue. I have a VR Commodore with a 4-speed auto and lock-up converter. What do you set the number of gears to? Four or five?

As far as I'm aware, the converter locks up in third under certain conditions, as well as locking up in fourth. This being the case, what would the display indicate, because if you set it for five gears - ie, four plus one for lock-up - and the transmission locked up in third, would it confuse the display by showing the wrong gear?

(P. B., via email).

The Gear Indicator should indicate the correct gear irrespective of lockup in the torque converter. This is because the unit is calibrated when driving on a flat road at a steady speed and so the torque converter should have minimum slippage anyway. However, calibration in fourth gear may need to be done with the converter locked.

If the unit is calibrated in fourth gear when there is slight acceleration and hence slippage in the torque converter, it may be possible to calibrate this as gear 4, with gear 5 when the converter is locked. That's if that is what you want.

Alternatively, you may be able to calibrate for lock-up in gear 3 (call it gear 4), if gear 3 is calibrated with the converter slipping. Then you could use the gear 5 and 6 numbers for fourth gear and fourth gear with lock-up.

If we were doing it though, we would set it up just to indicate four gears and not worry about the lock-up condition.

Bigger displays for PIC TOC I would like to substitute the existing LEDs in the PIC TOC Alarm Clock (SILICON CHIP, July 2001) with the super large 7-segment displays which are available from Jaycar (Cat. ZD-1850) for $10.95 each for the common cathode type. What would I have to alter on the circuit for this to be done? Secondly, I have in mind to build the Peltier cooler Esky/fridge from your September 1999 issue and I wish to power it from 240VAC, 12V DC and from solar cells. Can you advise what the solar panel and battery requirements would need to be. (M. S., via email). Unfortunately, neither of your project suggestions is practical or feasible. Each digit of the giant LED displays has two LEDs and so it is not feasible to drive them in the PIC alarm clock. You would need the drive circuit used in the Large Digit PIC clock featured in the March 2001 issue. As far as the Peltier cooler is concerned, you would need a solar panel capable of at least 4A, depending on the cooler used. That would be a very expensive panel.

Extending the 6-channel remote volume control

I have just completed the 6-channel IR Remote Volume Control project and am very pleased with it, particularly the professional appearance of the finished unit. I bought my kit from Altronics.

It would be a lot more useful to me if I was able to control the volume of the individual channels, as well as having overall volume control. I noticed in your article that you said that "each of the three channels in each IC is individually addressable and could theoretically be loaded with any attenuation value".

Can you provide information on how to do this?

(B. M., via email).

While it is possible to individually address the attenuators for each of the 6-channels, the design would be much more complex, with more hardware and software. The remote control would need to individually select each channel to be adjusted and you would need more indicators on the front panel, to show what was happening.

If you want to pursue this further, you can find the codes to address each attenuator in the LM1973 data on the National Semiconductor website. An alternative approach would be add a trimpot attenuator at the input of each channel to set up the individual volume levels.

Boosting the 5A speed control

I recently completed the AC Motor Speed Controller from the October 2002 issue. However, I need to use it with an 1850W router. Can the circuit be modified to handle this amount of power?

(A. H., via email).

The two main factors setting the maximum current are the current ratings for diode D3 and the speed switch. As far as the switch is concerned, you could either leave it out altogether or substitute a bigger switch, such as the 10A 240VAC DPDT toggle switch from Jaycar (Cat ST-0575).

To get a higher rated diode, you will need to go to a TO-220 package type such as the MUR1560 rated at 15A, 600V (Jaycar Cat ZR-1030). Make sure these components fit comfortably inside the case.

Note, however, that these modifications will not let you run appliances with 10A ratings on their nameplates. To do that, you would need to use a larger diecast case or otherwise improve the heatsinking of the Triac.

You can buy products mentioned in this article here : AM4011 : MIC INSERT PCB MTG U62 ZD1850 : SUPER LARGE 7 SEG DISP 57MM ST0575 : DPDT 10A 250V AC SWITCH

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