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Ask Silicon Chip

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.

High energy ignition for a Citroen 2CV

I built the High Energy Ignition system from the June 1998 issue and successfully installed it in my Citroen 2CV (2-cylinder 600cc air-cooled engine) using its original contact points for triggering. It worked first time and worked well for about two weeks, starting first time, every time and running very well with noticeably better acceleration.

Then I noticed that its performance degraded rapidly over a period of a week or so. It would tend to stall at traffic lights and would be difficult to start, with the engine missing a beat. As I use the car daily, I switched the car circuit back to its normal direct point switching.

Would you know what is wrong with the circuit? Is the output transistor at fault?

(M. A.,
St Marys, SA).

We suspect that the problem is that the points are oiling up from fumes in the engine. Periodic cleaning of the points will help but you may need to increase the points wetting current by using a second 5W resistor in parallel with the original.

Li'L Snooper camera switcher

I am interested in building your Li'L Snooper camera switcher from the June 2001 issue. Will this circuit be suitable for colour cameras; one is CCD, the other is CMOS? If not, can you recommend any mods to make it suitable?

(C. L.,
via email).

The Snooper will work with any camera that puts out a 1V composite video signal.

Volume control for amplifier module

I recently bought a 50W stereo amplifier module kit from Altronics. It was published in the February 1995 issue of SILICON CHIP. I have reviewed the design and the only thing that I could think of putting in the circuit was a volume control. How do I do that?

(T. H.,
via email).

The answer is quite simple, use a 10kΩ dual gang logarithmic pot. The pot wiper goes to the amplifier module in each channel.

Raucous alarm not loud enough

I have just built the "Raucous Alarm" from the January 2002 issue and it works just fine. Is there a simple, easy way to increase the output to say 110dB or thereabouts, as I would like to use it in a large hall, full of noisy people?

(A. W.,
Launceston, Tas).

The answer is to use the most efficient piezoelectric tweeter you can buy and use a 15V supply. We suggest the CTS KSN 1177A from Altronics (Cat C-6170). It has an SPL rating of 99dB at 2.83V/1m.

Noisy transformer in battery charger

I recently purchased the Multi-Purpose Fast Battery Charger MkII from Jaycar Electronics (featured in the June & July 2001 issues) and I am getting serious buzz or hum from the inductor. The severity of noise depends on the battery being charged. A single NiMH cell makes more noise than a pack of 4 or 8. I wound the inductor with even pressure but the windings are not settling. Is this cause for concern? Would dipping the inductor in epoxy or "Liquid Tape" assist in the reduction of high frequency noise? Or do I need to wind the inductor again?

I was curious about the gap in the ferrite core and do not understand the concept behind this, especially since the centre core (E core) has an air gap. Does this need a spacer as well?

Also, when the batteries are discharged I am surprised to find that a (cycled) 4.8V 1500mAh NiMH pack lasts only 2-3 minutes. With the constant discharge of 2A (Refresh Rate) shouldn't they last longer? Around 30-45 minutes?

(B. C.,
via email).

Noise from the transformer is not a cause for concern, apart from the irritation, of course! Usually it is caused by the ends of the core pieces vibrating against each other and the surrounding former (magneto-striction) or is caused by loose turns in the windings. The solution is often to glue the two halves together or, as you suggest, to impregnate the core with varnish. You could also dip it in thin epoxy but make sure it is thin, as heat must be able to escape from the core and windings.

The gap between the core's centre legs is created to prevent the core from saturating at high power levels. When the core of an inductor saturates, it acts more like a resistor than an inductor, with current flow through it limited only by the resistance of the windings. When that happens, power dissipation is usually very high and efficiency drops.

The size of the gap is very important and is calculated according to core characteristics and peak current - be sure to follow the construction details exactly.

In our opinion, the Multipurpose Fast Charger Mk II is not suitable for charging small cells, and in particular NiMH-chemistry types. Manufacturers recommend that small NiMH cells (AA, AAA, etc) should NOT be fast-charged at greater than 1C, which equates to 1.5A for your particular application.

Having said that, you will probably be able to successfully charge a NiCd pack this small using the temperature sensing option, but make sure that the thermistor is in close contact with the pack (or one of the cells).

The ideal answer though is the SuperCharger design featured elsewhere in this issue.

Full range equaliser

I was hoping you may be of assistance with a project featured in the July 1996 issue, namely a Parametric Equaliser. What modifications would need to be made to make each band full range instead of the three separate bands (low, mid & high)?

(A. H.,
via email).

Although we have not tested this idea, the circuit could be made adjustable over the entire audio spectrum if the capacitor and resistor values are used for the treble section but with the frequency adjust potentiometer (VR9a and VR9b) changed from 25kΩ to 200kΩ. The change in frequency with respect to pot movement would be rather coarse, however.

Video enhancer adds noise

I have purchased Jaycar's AR-1820 Video Enhancer. I realise it's a simple device but I'm wondering if you have any further user tips for this item.

For example, I notice that when applying the "Sharpen" function to a DVD dub, ghosting occurs. I find that DVD dubs are best without this function enabled. Is this correct?

(R. E.,
via email).

Use of the sharpen function will tend to emphasise any noise in the video signal so ghosting is also a possibility. With a DVD the signal should optimum anyway, so don't use the sharpen function.

Needs 12AX7 preamplifier kit

I've been searching in vain for a simple kit to plug my bass guitar into. I currently run it into a 4-channel mixer, then to a 50W SILICON CHIP amplifier (February 1995). Anyway, the bass misses the "valve sound" and I was wondering if you have a kit, or know of a kit that is a basic mono preamp, with something like a single 12AX7?

(B. D.,
via email).

"Electronics Australia" described guitar amplifiers with a 12AX7 input in October 1962 and in June 1967. They are quite different. We can supply photocopies of the articles for $8.80 including postage.

Universal stereo preamp level control

I have purchased and built a Universal Stereo Preamplifier from Jaycar Electronics (SILICON CHIP, April 1994). I have built it as a phono configuration to connect to a computer sound card for recording LPs. The unit seems to work well, however I found that the output level is a little high causing a small amount but annoying distortion.

As I am new to electronics, I would like to know if it's possible to change the value of one or some of the components to reduce the output level or better still put in a potentiometer or similar.

(S. A.,
via email).

The output from the preamplifier can be attenuated using a logarithmic potentiometer. Connect the preamplifier output to one side of the potentiometer. The other side of the potentiometer connects to the ground while the wiper or centre connection connects to the PC's sound card input. The potentiometer should increase the level of signal when wound clockwise and decrease the level when wound anticlockwise. A 10kΩ value would be suitable.

If the pot works the other way around, with decreasing signal when turned clockwise, reverse the outside connections. A dual-ganged pot would be more suitable as it can adjust both left and right channels together.

Tape cassette record circuit

I was wondering if you have a circuit, PC board layout and construction method for a cassette tape record/playback device, with AC bias. I need to construct about 20 of these units for a 4-track tape loop machine I have built. I have the mechanics done and now need to put in the electronics.

Because I need so many circuits ( 5 heads, 4 tracks = 20 circuits), I did not want to try wrecking old cassettes, as it would get too messy to put it all together. Ideally, I would like a circuit which uses the same head for record and playback, so I can record and playback using any of the five heads.

(M. B.,
via email).

"Electronics Australia" described a stereo cassette deck in August & October 1974 which had AC bias. We can supply these articles for $8.80 each including postage.

Jammed slugs must be fixed

I have been given the task to assemble the MiniMitter kit featured in the April 2001 issue. I have completed the assembly of the kit, including changing the mike plug to an RCA type. On test, the output was very low and unstable. I have installed new batteries.

Being an amateur radio operator, I found this kit good to assemble but found the slugs jammed in the coil formers; once they were screwed in, that's were they stayed. What next?

(F. M.,
Medlands Beach, NZ).

The jammed slugs in the formers must be fixed as they are the means to aligning the transmitter to a particular frequency. If you need to purchase new slugs, use F29 types.

Use an aligning tool to adjust the slugs to prevent them cracking. Adjust both cores carefully to obtain the correct tuning in stereo for the particular station frequency you are receiving on.

What is a dummy battery?

Call me a dummy if you like but the Circuit Notebook item in the December 2001 has caused confusion. It was for measuring current of a DC-DC converter. Now I've used dummy loads but what is exactly a dummy battery. Please explain in more detail please.

(G. M.,
South Morang, Vic).

The expression "dummy battery" is explained in the fourth paragraph of the article on this circuit. As it says, the dummy battery replaces all the cells in the device under test and a variable voltage supply provides the power for the device. In the simplest application, the dummy battery could be a 9V battery snap connector.

Sound meter wanted for PA installations

You may care to consider doing a project which is able to measure in a dynamic fashion the sound level in a PA installation. I help out at our local church with operating their PA system and with the varied number of people using the PA system, it can become awkward to ensure that the level of sound heard by the audience is constant.

In most fixed PA installations, unless the controller is located within the listening area, it can become tedious and inconvenient to have others relay messages to the controller on whether the volume levels are set correctly. Even if you can see the audience and the speaker, often the sound heard by the operator is different since he is a little remote from the audience.

What would be useful is a sound level meter which could be mounted in the control room with a microphone located in the listening area. The meter could be switchable between RMS and peak levels to determine the optimum volume level.

While there are a number of sound level units available from many electronics outlets, these are typically portable units which require the operator to be in the listening area and do not generally lend themselves to being run remotely. In any event, the displays are typically quite small.

What would be more useful is a jumbo size display comprising LEDs which could be mounted on a wall in the control room to allow easy viewing by the operator.

(N. A.,
Lyneham, ACT).

You may want to consider our Sound Level Meter Adaptor for DMMs published in the December 1996 issue. You could install several of these around the auditorium and then switch the DC output signals to a DMM with large display at the control desk. We can supply the December 1996 issue for $7.70 including postage.

Digital thermostat needed for a PC

I am currently studying electronics at school for my Tasmanian Certificate of Education. One of the assessment things I need to do is a project by the end of the year. I have been looking into building a digital thermostat to be placed inside the PC case. I would like this to control a fan so if the temperature exceeds a particular level, the fan switches on. I would also like it to run off the PC power supply.

(C. A.,
Devonport, Tas).

We published a thermostat fan control as part of a speaker protection circuit for the Ultra-LD amplifier, in the August 2000 issue. You could just build that part of the circuit.

Checking Dr Video for damage

I recently assembled a Dr Video kit, as published in the April 2001 issue. Upon running power through it I found I had made a couple of errors. Could you please advise me whether this may have damaged some of the circuitry?

My first problem was that I accidentally bridged solder across two of the terminals on IC2. Looking at the PC board overlay, the bridge occurred between the lower 5th terminal and the solder imprint running to the terminal on the opposite side.

The other problem was that I inserted three of the chips upside down. This was because I determined their direction by following the lettering as in the circuit diagram, as opposed to paying attention to the direction of the cutout markers. Having discovered and rectified these faults, the unit is working after a fashion.

There is a television picture when the unit is on and no picture when the power is turned off. The problem is that white horizontal lines flow down the picture and make it unwatchable. This occurs both when the sharpen button is depressed and when it is not. Is this horizontal jitter that may be rectified as per your instructions or am I likely to have some other problems?

(D. K.,
via email).

Have you replaced the chips which were inserted wrongly and have you checked the voltages, particularly the -5V rail. It is highly likely that IC4 was damaged and we would be surprised if IC1 and IC7 were not also damaged. IC2 is unlikely to have been damaged.

Interference problem with volume control

I have built the Remote Volume Control featured in the June 2002 issue of SILICON CHIP but it has a bug. At switch-on, (after initial checks) the ACK LED is permanently on - flashing only when a remote button is pushed. Also when Mute is pushed, the pot doesn't rotate the full distance, regardless of where VR1 is positioned.

I've replaced all the transistors but since then the mute hardly works at all. I did notice that when placing a finger near or on the terminals of the 100μF capacitor on terminal 3 of IRD1, the ACK LED seemed to behave properly. Replacing the capacitor did not rectify the problem. I suspect IRD1 but would like your thoughts first.

I only tacked IRD1 in by the tips of the leads (for testing purposes) so I doubt that it's heat damage. Also IC1 and IC2 are in sockets, inserted with a proper insertion tool to prevent static damage, so they should be OK. Could IRD1 be static-damaged or crook from day one?

(J. I.,
via email).

It sounds like IRD1 is playing up. It probably is not faulty but is picking up electromagnetic interference or even infrared interference, causing it to receive and deliver a signal all the time.

To solve this, try using a larger capacitor (than the 100μF) across the supply to IRD1 - 470μF may be sufficient. Also IRD1 may need shielding. Try covering it with a metal shield, using aluminium or tinplate (cut from tin can). The metal shield needs to be earthed to the 0V supply of the IRD1. Also a hole is required for the lens.

Calibrating the Lil Powerhouse supply

I've built the Li'l Powerhouse kit from the June & July 2000 issues and I am having some problems with it.

First, the maximum output voltage is only 38V not 40V. I cannot calibrate the panel meter to the voltage; ie, I calibrate it to 0V but when I increase the voltage, the panel meter doesn't match what the DMM reports. I'd be grateful for any tips.

(F. W.,
via email).

The answers to your questions are as follows:

(1) The lower output voltage is probably due to either a low mains supply or a transformer that has a little less output than the unit we used in our prototype.

(2) You don't mention the actual difference between your multimeter and the panel meter. A small difference is OK, as the panel meter (and perhaps your multimeter) are not precision devices.

However, if the difference is large, then it is almost certainly due to either: (a) incorrect full-scale calibration of the panel meter; or (b) a problem with the +5V or -5V rails.

When performing the voltage calibration, be sure to follow these steps exactly (there should be no load connected to the output terminals):

(1) Disconnect the wire from the pole of S4b. This ensures that the non-inverting input of IC4 (pin 3) is at 0V. Adjust VR5 for a reading of 000 on the panel meter.

(2) Reconnect the wire to the pole of S4b. Set the meter switch (S3) to the "Volts" position and connect your DMM to the output terminals. Set the load switch (S2) on and using the "Voltage Adjust" pot (VR1), adjust the output voltage to get 37.00V on your DMM.

(3) Now connect your DMM between pin 6 of IC4 (marked TP1 on Fig.6 of the overlay diagram) and 0V (marked GND) and set it to read millivolts. Adjust VR4 for a reading of 370mV on your DMM.

(4) The panel meter should read 37.0V. If it does not, then adjust the trimpot built in to the panel meter (accessible through a small hole at the rear) to get the correct reading.

By the way, you should refer to Notes and Errata from the August 2001 issue for additional information about calibrating the current reading.

Better reception from two antennas?

I have a 4X4 and travel to many places in the Outback. I like to listen to the car radio and have noticed that with an aerial mounted on the right rear quarter of the vehicle, the radio has a much better reception if the left front of the vehicle is aimed at the transmitter. This presents a problem because roads are not always correctly orientated.

Is it possible to use two antennas and couple them via matching stubs of coax or a resistor network? The plan is to use two equal antennas, vertical, on the mudguards, just forward of the windscreen to get a more circular reception pattern. Do you know of the correct way to couple antennas to work on both FM and AM?

(B. W.,
Curtin, ACT).

It is theoretically true that two antennas can be connected together with a suitable phasing system to improve reception - for example, phased TV antennas are often used in low signal areas. And for years, truckies have used twin CB antennas mounted on their mirrors, again connected via a phasing harness.

However, because of the differences in mounting positions and vehicle types, it is rather difficult to forecast the directional pattern you will achieve using two antennas. The best receiving antenna (as far as uniformity of direction is concerned) would be mounted right in the middle of the vehicle roof.


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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