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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

Train controller transformer

Can you please assist me with the Train Controller featured in the April 1997 issue? I purchased a kit from Dick Smith Electronics (K-3029) and I have assembled it but I cannot work out how to connect the 15V AC transformer I swiped out of an old controller.

Your circuit shows three connections for a centre-tapped 9V per side transformer to the board but seeing as I only have two from my transformer (0V and 15VAC) I cannot figure out how to connect it. Can you help? (B. L., via email)

The circuit on page 67 of the April 1997 issue shows how a single winding 12VAC transformer can be connected. Simply connect the 0V terminal of the transformer to ground and the 15V terminal to one side of the bridge rectifier BR1. On the PC board you connect one transformer wire to the centre of the three input terminals and the other transformer wire to one of the other terminals to the left or right. The third terminal is left unconnected.

In effect, you are connecting the transformer so that it drives just two diodes in the bridge rectifier; ie, you will have two half-wave rectifiers feeding the 4700μF capacitors.

However, there is a problem in that 15VAC is a little too high for the application. You will probably find that the rectified DC voltage will be well over 20V and if you apply this voltage to your locos they will scream around the track. The DC voltage may also be a little high for the capacitors and it may be wise to replace the 4700μF 25VW types with a higher voltage rating, say 35VW.

By the way, we published Notes & Errata on this circuit in the August 1999 issue on how to avoid a brief backward lurch from the loco when power is first applied.

Clifford the cricket is comatose

I recently purchased Clifford the electronic cricket, as described the December 1994 issue of SILICON CHIP. I’m having some trouble getting him to work. When I connect the battery, the LEDs light and then when I block out the light nothing happens and the LEDs stay on. Do you need a multimeter? Can you help me? (P. H., via email).

It should be possible to get Clifford going without a multimeter. It is possible that you have transposed the two transistors, Q1 and Q2, on the PC board. Check that Q1 is a BC548 or BC547 and that Q2 is a BC557 or BC558. Also check the diode orientation for D1 and D2. The anode (A) is the end of the diode with the stripe. Both anodes for D1 and D2 should be toward the 3.3kΩ resistor.

The overlay diagram for this is a little tricky but if you place diode D1 with the stripe up and D2 with the stripe down and with the wire lead and body placed as shown, the orientation will be correct.

If Clifford still does not operate, check the placement of the resistors using the published colour code as a guide to reading their values.

4Ω version of foldback loudspeaker

I refer to your excellent foldback loudspeaker presented in the November 1999 issue. As I already have a suitable power amplifier which delivers 200W into 4Ω but only around 80W into 16Ω, I would like to modify the design so it has a nominal impedance of 4Ω. I assume the two drivers would be connected in parallel but how is the crossover modified? (G. D., via email).

The loudspeakers can be connected in parallel to provide a nominal 4Ω impedance. The inductor should be reduced in value to 450μH.

Since the woofer sensitivity will increase by 6dB it is necessary to remove the 0.33μF capacitor in series with the tweeter and replace it with a short circuit. Make sure the phasing of woofers and tweeter is as shown on the diagram on page 73 of the November issue. Also both woofers should be connected with the plus terminals tied together. The plus side then connects to L1 and the two minus terminals to the common ground.

Audio signal generator switch confusion

I am trying to assemble the Audio Signal Generator described in the February & March 1999 issues. It has been about 15 years since I did any circuit board work and I must confess that I am puzzled with your rotary switch (S2) alignment instructions (page 66, March 1999).

How do I know when I have the switch properly located over the drilled holes? You see, the holes seem to align fairly closely in three different positions. When I use the position which puts the white line or knob marker closest to the start of the dial selections on the panel, the first selection is not aligned to the first position of the knob by one click. What do I do? (S. D., via email).

The switch can be placed in any of the three possible orientations. This is because the poles of the switches can be interchanged and used for any of the switching functions. The type of knob must be able to be adjusted to align correctly with the panel markings. Some plastic knobs can be adjusted by removing the front pointer plate and repositioning correctly in place in line with the panel markings.

Speed Alert for racing car

I read with interest John Clarke’s Speed Alert project in the November 1999 issue. Is there any way of modifying this kit to indicate a higher speed? I noted that John mentioned that this kit was not suitable for racing cars but I do need an electronic speedo for my racing car. (P. M., via email).

The software could be changed to accommodate a higher reading of speed up to 255km/h but not without some major changes. If other readers are interested, we would consider producing a modified version however it seems likely that 255km/h would not be sufficiently high for a racing application.

Pulse counting with the Stress-O-Meter

I have made the pulse counting section of the Stress-O-Meter project in the October 1998 issue of SILICON CHIP, with both the connections to it from the games and printer port. It should work since I have checked all the components. The problem is that I can’t use it with the software on your website because you include the GSR section which I don’t want.

How do I change the Q-Basic software so that it will display my average pulse rate on the screen without needing a GSR input? Can this be done easily or do I need the other section of the project? (D. T., via email).

All you should have to do is to delete lines 80, 100, 110 and 120. You will have to use CTRL+PAUSE to exit the program. Deleting these lines will allow the program to continually loop to subroutine 3030.

Sensitivity of carbon monoxide alarm

In the May 1999 issue, you published a circuit for a carbon monoxide sensor using a Nemoto sensing element. You incorporated the sensor into a voltage divider to obtain the right sort of levels to feed to a comparator. The problem is, from the manufacturer’s chart that I have, the sensor should have about 27kΩ resistance at 200 ppm CO but your design seems to use a value closer to around 20kΩ (it’s difficult to tell as you used a trimpot in the divider).

Can you tell me how you arrived at your figures? (N. S., via email).

The CO Alarm is an uncalibrated unit and is set so that the alarm does not sound under normal driving circumstances. Trimpot VR2 does not allow the sensitivity to be set as low as 200 ppm since this would be too sensitive in our application. You could substitute a 10kΩ trimpot for VR2 to allow adjustment down 200 ppm.

Note that the 27kΩ resistance of the CO sensor at 200 ppm is a typical value only and does not describe a precise calibration.

100W PA amplifier wanted

I am contemplating constructing a PA amplifier with six inputs; five microphone and one auxiliary. I have searched through my older magazines but cannot find anything that meets my needs. I require 100 watts RMS into a suitable matching transformer for 100V line use. Have you at any time produced such a design?

There are a number of good kits available for a main amplifier but none have a suitable preamplifier to match into. Have you any suggestions? (E. C., Bundaberg, Qld).

We published a 120W PA amp with 100V line transformer in the December 1988 and January 1989 issues. We can supply photocopies of these articles at $7 each including postage.

No picture from PC monitor checker

I constructed the PC Monitor Checker project published in the August 1999 issue from a Jaycar kit. The only problem I found was that a .01μF ceramic capacitor was supplied but both the layout and circuit call for a 100pF ceramic capacitor (I used a 100pF capacitor, as specified). My problem is that I am unable to get a picture in sync on a known working monitor. Your help would be appreciated. (D. B., via email).

 The capacitor associated with pin 3 of IC3 is 100pF as shown. If you are not getting a picture there could be any number of faults which could stop the signal getting through. You really need to check for the existence of video signals at the emitters of the five transistors, using an oscilloscope. If you don’t have one, try setting the unit for 15kHz operation and then connect the composite video output to drive a normal video monitor.

Longer ticking egg timer wanted

I want to build the ticking egg timer described in the November 1990 issue but I want to have a longer time. How do I do it? (S. W., via email).

The way to increase the time is to increase the time constant of oscillator IC1a. This can be done by using a larger pot for VR1 (say 1MW) or a larger capacitor instead of the 0.1μF.

Digital tacho adjustment procedure

I purchased and assembled a digital tachometer from the February 1994 issue but I have now lost the calibration instructions. I need to know which resistor I have to adjust so that it can work on 4, 6 and 8-cylinder engines. If possible, could you tell me the resistances that are required and which resistor it is? (D. B., via email).

There was no tacho project in February 1994 so we assume you mean the circuit in August 1991. The resistor marked Rx, in series with trimpot VR1, should be 82kΩ for 4-cylinder engines, 56kΩ for 6-cylinders and 47kΩ for V8s. We can still supply a photostat copy of the article if you wish, for $7 including postage.

Multi-Spark CDI timing queries

I recently purchased the CDI kitset as featured in the September 1997 issue of SILICON CHIP and have some questions about the unit to help me understand how it works.

What was the formula you employed to get the values for sparks presented as the product’s specifications? I am unable to see how the above information is related to Table 1 (RPM vs. Spark No. & Duration) on page 23. For example, please provide a mathematical formula on how to get:

(a) four sparks covering 37Deg of crankshaft rotation at 4500 RPM for a 4-cylinder engine?

(b) eight sparks covering 20Deg of crankshaft rotation at 1400 RPM for an 8-cylinder engine? (M. R., via email).

The spark duration table is based on the fact that the there are always multiples of two sparks produced. The duration and spacing was measured with an oscilloscope when driving a standard ignition coil. The times may vary with different coils and the calculations will vary slightly from the measured values due to differences in charging the timing capacitors.

The crankshaft angle is derived by calculating the rotation of the shaft over the total spark duration.

For example, the four sparks at 4500 RPM for a 4-cylinder engine gives 37 Deg . At 4500 RPM, the frequency of rotation is 4500/60 or 75Hz. This is a duration of 13.3ms. The duration of four sparks is about 1.3ms and so we have 1.3/13.3 x 360 Deg = 35 Deg .

The result for eight sparks at 1500 RPM, giving 20 Deg of crankshaft rotation, is calculated similarly: 1400 RPM is 23Hz or 43ms; eight sparks is 3.1ms and 3.1/43 x 360 Deg is 25 Deg .

As you can see, the calculations do fit the measured values reasonably closely.

Signal loading problem in rev limiter

I am building the Rev Limiter kit described in April 1999 to show three shift lights. I have an imported Nissan Skyline which is already factory rev-limited so I don’t need the Ignition Switcher as well. I have a second "interceptor" computer which controls fuel for different revs. It is hooked to the factory computer for positive, earth, revs, air flow, throttle position and fuel.

I hooked the Rev Limiter up to my factory computer for positive, earth and rev wires, the same as the second computer. When powered up the Limiter circuit seems to sense the different speeds and the lights come on OK and the second computer continues to show correct revs. However, my in-dash tacho stops working. It starts working again if I cut the earth return to the Rev Limiter kit.

I then changed to the Hall Effect/Points/Distributor input which gives me back the tacho but doesn’t seem to detect revs for the shift lights.

Do I need to isolate the Rev Limiter kit from the computer by putting in a diode on the input to allow one-way signals into it? Can you possibly shed some light on what I need to do to get it working? (G. J., via email).

It is possible that the loading from the low voltage input is upsetting the tachometer. The ignition coil input is not sensitive enough to detect the low voltage signal. You could try a diode connected in series with the 1kΩ low voltage input to isolate the signal and connect it with the anode to the low voltage input and the cathode to the 1kΩ resistor.

Alternatively, you could connect to the ignition coil input if the 22kΩ resistor is reduced to 1kΩ.

Class A amplifier questions

I would like to ask several questions pertaining to the class A amplifier published in July 1998. (1) Apart from Altronics, are there any other places where kits can be obtained? (2) What is the open-loop bandwidth of the amplifier? (3) How much could the supply rails be increased before the circuit would need altering (neglecting the need for increased heatsinking)? (A. B., via email).

Altronics is the only source for this kit. We have not measured the open-loop bandwidth but it should be in excess of 20kHz.

The main limitations on the supply rails are the ratings of Q11 and Q13, the small signal transistors used as drivers. If you wanted to run at high current and voltage, you would have to substitute much higher-rated transistors for Q11 & Q13. Trouble is, virtually any bigger transistor you find will have poorer frequency-gain product and so the performance will suffer.

Notes & Errata

Switching Temperature Controller, August 1999: two capacitors are marked C6 on the circuit. The 100μF capacitor associated with diode D1 should be C2. Also the text in the last paragraph on page 58 is wrong. It should read: the BUK453 is for cooling, the IRF9530 is for heating.

Refinements To PC Monitor Checker, Circuit Notebook, November 1999: the rotary switch is incorrectly labelled 12345 in an anticlockwise direction from top to bottom. The correct labelling sequence is 43215.


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.

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