Fence tester does not flash

I was wondering if there was a modification to the Maxi Electric Fence Tester described in the May 1999 issue. I purchased it to monitor the performance of a mini electric fence module. We are failing to keep the cats off a balcony and have been losing fish.

The electric fence tester is not flashing. I can measure 500V+ from the anode to the cathode, which I suspect should just about fire the tube without any triggering pulses. I don’t have a means of measuring a trigger pulse. The 220kΩ resistor reads correctly. Any suggestions or advice would be appreciated.

(J.G., via email).

It seems possible that the electric fence tester is not delivering a suitable pulse to fire the Xenon tube even though it has enough voltage across it to fire. Try connecting the tester with the opposite polarity; ie, connect the HT end to the ground stake and the ground end to the HT of the fence. This will reverse the trigger pulse polarity and may be all that is needed to fire the Xenon tube. It will not reverse the voltage across the Xenon tube as this is rectified using diodes D1-D4.

If this does not work, try connecting the 220kΩ resistor to a point further up the 820Ω resistor string; ie, connect it a few resistors further toward the fence high-tension end of the string. This should give more trigger voltage to the Xenon tube. Again, try both fence connection polarities.

Thermostat not needed for Turbo Timer

I’ve have just recently bought the Turbo Timer kit (described in November 1998) from Jaycar Electronics. While putting it together, I decided that I would not use the thermostat as it is too much trouble to install it. So what needs to be altered in order to bypass the thermostat in order for the Turbo Timer to always be activated when the car keys are taken out? Is there even a need to alter the thermostat?

(L. T., via email).

If you do not wish to use the thermostat, then simply leave this connection open-circuit. In other words, do not make a connection to the thermostat terminal on the PC board.

Charging smaller Nicads I have a query about the Multipurpose Fast Battery Charger as described in your June 2001 issue. The charge and discharge rates for Nicads seem more suited to larger capacity cells. I use a lot of AA Nicads, most of which are 500mA.h. Is it possible to provide lower charge and discharge rates for these smaller cells? (J. B., via email). The design is not easily changed to provide lower charging and discharging rates. These changes would be best done by redesigning the project with smaller and cheaper mains and switching transformers. However, without changing the design too much, the charge and discharge currents can be effectively halved by using only one instead of two 0.1Ω 5W resistors for current sensing. This will mean that the setting for 1.2Ah cells will now be 600mA.h. Switching could be incorporated to select a charge current of 6A or 3A. Keep the leads to the switch short. The best position for the switch would be directly on the PC board, adjacent to the 0.1Ω resistor.

Query on transformer VA ratings

In the 1978 edition of the ARRL handbook it says that for both a full-wave bridge (four diode) and a two-diode centre-tapped full-wave rectifier, the diodes can to be rated at half the DC current being drawn by the load. It also says that for a full-wave bridge, the transformer VA needs to be calculated based on the DC current being drawn but for a two-diode centre-tapped arrangement, the VA needs to be calculated based on 0.7 (ie, root 2) of the DC current. For a non-filtered load is this correct? It would mean that the centre-tapped transformer would have to be 1.4 times the VA of one that used a bridge diode setup.

In practice, you would take a much more conservative approach in the ratings of the diodes, allowing for transient voltages and initial surge currents. Either way, the VA rating on the transformer would be the same since the total power supplied would be the same.

Charger for AAA cells wanted

I have a Kodak MC3 digital camera and MP3 player and it uses three AAA batteries. Buying new disposable batteries all the time is impractical and expensive. I need a charger that can fast-charge two sets of three AAA batteries at a time. Could you suggest a kit or a product I could buy?

(A. T., Wagga Wagga, NSW).

Our Fast Universal Charger published in the June & July 2001 issues will charge 3.6V Nicads or NiMH batteries (ie, it can do 3 AAA nicads in series) but it could not do two sets at one time because it could not properly monitor the "end-point" voltage of both sets.

Computer video signals are different to PAL

I’d like some advice on combining the y/c signals from a computer video card so that the signal can be recorded on a VCR with composite video input terminals.

Can this be done as simply as audio signals can be combined with a virtual earth mixer or is it more complex than that?

(S. B., via email).

It is not possible to record the video signals for a computer monitor on a VCR because the video standards are quite different. The horizontal and vertical sweep frequencies are faster than for PAL or NTSC and there is no colour burst signal. While there are VGA to PAL adaptors, they are quite complex.

Fuel mixture display shows lean I bought the Jaycar fuel mixture display kit and hooked it to my 1994 Holden Barina GSi but the only LEDs that come on are the red ones. When I start the engine the last LED comes on but as I turn the trimpot fully to the left, only the second last LED lights. As the car warms up, the last LED comes on and it stays that way. Is the Barina designed differently and do I need to replace the resistor with ones with different values. There are no solder bridges and all the LEDs are correctly positioned. What can I do? (G. E., via email). We are not sure what you mean by "only the red LEDs come on" as all the LEDs are red. We assume you mean that the display always shows lean mixture with the lower two LEDs lighting. Possibly the problem is with the adjustments of both VR2 and VR3. You need to set them as described in the adjustment section of the article. Also check that the connection to the EGO has a good ground. Differences between the ground connection for the power supply on the fuel mixture display unit and the sensor ground can cause the display to show lean.

Coil for ignition system

What sort of coil do you have to use with the High Energy Ignition kit? Do you use a coil that needs a ballast resistor or one without, eg, Bosch GT40R?

(M. K., via email).

Definitely do not use a high-current coil like the Bosch GT40/r series. Use the standard coil intended for your car. Failing that, use a 12V coil.

Setting up a CCTV system

I have been wondering for some time now how I may be able to install a home closed circuit video security system that is simple and avoids the high expense associated with the professionally installed systems. I do not want the computer-based models as the computer is often the first item stolen or damaged and besides, I do not want one running perpetually while I am away. A 12V supply for cameras etc I can handle.

All of the equipment I have seen/read about to date requires a time-lapse video recorder. These are expensive and no doubt have been developed for shopping centres and banks.

I can buy three or four normal VCRs for the price of a time-lapse model and all I need it to do is to switch it on in record mode. I want this mode so that I can operate the VCR from a PIR detector when the house is unoccupied. PIR on, the VCR records, PIR off, the VCR stops – simple?

My enquiries to service people would seem as if I was asking for a trip to the moon! Surely someone can develop a circuit/device that would allow a home VCR to be so modified, even if the VCR has to be taken to a technician to have the work done. What do you think?

(G. E., via email).

We have done exactly that project (camera, PIR, VCR etc) in the September 1997 issue.

Tacho display too bright

I recently made the Digital Tacho from the August 1991 issue. Everything went well and it worked as it should. In the daytime you can see the numbers well but at night-time, boy are the numbers bright; they are so bright that I have to pull the plug on it. Is there a way to dim the numbers to a readable brightness, which would not require a great deal of change to the circuit, using an LDR or something like that?

(D. C., via email).

The only way to reduce the brightness of the displays would be to reduce the 9V rail by replacing the 150Ω resistor in the GND pin of the 7805 with a 200Ω wirewound pot. These are available from DSE, Cat R-6911. However, if you reduce the supply much below 6V it is likely that the circuit itself may malfunction.

Hall Effect sensor for ignition system

Could you please advise me if I can use the UGN3503u Hall Effect sensor instead of the HKZ101 Hall sensor with: (1) a distributor ring magnet assembly, similar to the Sparkrite ring magnet sold by Jaycar some years ago; (2) a crankshaft-mounted notched trigger wheel or (3) a crankshaft-mounted trigger wheel with magnets?

(K. W., via email).

You can use a magnet assembly to drive the UGN3503 Hall effect unit. It comprises a Hall effect unit biased to produce a 2.5V output with a 5V supply. The voltage swings above or below 2.5V, depending on the polarity of the magnet.

You will need to capacitively couple the Hall effect output to the reluctor input circuit of the High Energy Ignition. Use a large bipolar electrolytic such as a 47μF for coupling. Also note that the UGN3503 needs a 5V supply (see the Dick Smith Electronics catalog data section). A 5.1V zener can be used in conjunction with the 12V supply and a 330Ω series resistor to provide this. Decouple the 5.1V supply with a 100μF electrolytic capacitor.

A toothed crankshaft wheel will work if it has magnets attached.

Li’L Pulser Train Controller is full on My son and I have enjoyed putting the kit for the Li’L Pulser Train Controller together but there are a couple of problems we have been unable to resolve. First, the speed adjustment doesn’t work; ie, the train either goes forward at full speed or back at full speed. The Track LED (#3) shows green when switched to reverse and red when switched to forward, and its intensity doesn’t vary with the position of the potentiometer (VR1). It does not appear that the LED was fitted the wrong way round, as the flattened (K) side of the light base is in the correct orientation (the ends, of course, have been trimmed). It is possible that these problems are related. I have been through the suggested checks: the voltages across the ICs are correct but I have not adjusted the trimpots yet. The shorting siren works and potentiometer (VR1) seems to work. I am assuming that there is a problem with the delivery of the sawtooth signal to IC2 but really I have no idea where the problem lies. (J. W., Nedlands, WA). The circuit appears to have a problem driving the Mosfet with a switchmode signal. This could be due to any number of causes including: the pin 7 output of IC2a is tied to pin 8 via a solder bridge; the 6.8kΩ resistor at VR1 is either the wrong value or it is not connecting to ground; IC1b is not producing a triangle waveform which should show about 6V at pin 1 when measuring DC volts; or error amplifier IC1a has pin 7 set low because of incorrect setting of VR2. The colour of the LED is not important. Some bi-colour LEDs have the anode terminal for a red LED while other LEDs may have the anode for a green LED. To change the colour for forward or reverse simply swap the leads.

Keyboard amplifier wanted

I propose to use your 100W class AB amplifier – ie, the one using the Motorola plastic power devices – as a keyboard performance amplifier. I’d prefer to dispense with the regulated rails and just use one unregulated plus & minus supply for the sake of simplicity. Obviously, in this application, I’m prepared to sacrifice a degree of fidelity. Would you mind giving me your opinion on whether this would be OK?

(B. V., via email).

Yes, you can do that but you are wasting the design. You would be better off building the much cheaper (and more powerful) Plastic Power amplifier module described in the April 1996 issue.

New remote control for alarm

I have a Repeller alarm in my car. Is it possible to get another remote (preferably smaller) for it?

(A. S., via email).

There are couple of problems here. You cannot just replace the remote handpiece because the new transmitter must have exactly the same coding as the original. Unless the substitute transmitter uses the same encoder chip as the original, then the chances of matching codes are zero.

Another approach is that you may be able to replace the transmitter and the receiver/decoder in the alarm itself. If this is possible, you could look at one of the remote controls featured in our February 1996 issue.

Alternatively, you could purchase a learning UHF remote such as the Cat LA-8990 from Jaycar Electronics.

CD burners with 200 MMX Pentiums I loved the September issue, especially the article that allows me to turn my old computer into an MP3 music machine. Unfortunately, I am stuck with a 200MHz MMX Pentium VX motherboard for the present. I recently upgraded to a 40GB Seagate 7200 RPM hard-drive (only 8GB is formattable), with Windows 98SE as my operating system. This allows me to do many things but best of all I can connect USB items. I want to connect a CD burner but get conflicting information from salesmen. None of the burnproof units seem to be compatible with a 200MMX Pentium. Can you tell me why these units will not work with my system? (M. T., Donvale, Vic). Manufacturers seem to allow quite a big performance margin in their spec list. There is every chance that these drives will indeed run on your machine. Our guess is that you’ll need at least 48MB of memory and you may not be able to run other applications while burning CDs (this can be risky even on fast machines!). The specs are pushed up somewhat in order to include support for on-the-fly CD audio track ripping and encoding. Even if your machine isn’t fast enough to do this on-the-fly, it’s not a major problem; just save to hard disk first in .WAV format and then convert to MP3. It will be necessary to upgrade the motherboard BIOS to format all 40GB of your new hard disk drive. Alternatively, you can use Seagate’s Disk Manager utility. You’ll find it at: www.seagate.com

Using 4-ohm loads with the class-A amplifier

I have built the class-A amplifier from the July & August 1998 issues and am pleased with its performance. The specs state that it is rated at 15W into 8Ω. Can you tell me what it is likely to achieve into 4Ω? Also, is it possible to increase the output by paralleling the output transistors and the power supply?

(S. F., Lake District, UK).

You can operate the class-A amplifier with 4-ohm loads and it will probably deliver about 25 to 30 watts maximum. The only problem is that once the load current exceeds about 1.9A peak, the amplifier will no longer be in class A. Instead, it will be in class AB (albeit with exceedingly low crossover distortion artefacts) for power levels above 7.5 watts.

While we have not looked closely at the biasing, if you wanted to run in class-A up to full power for 4-ohm loads, you would need to double the quiescent current. You would not need to double up on the output transistors but you would need a fan-cooled heatsink to cope with the 80W dissipation and you would need a bigger power supply.

Boosting the remote control extender

Is it possible to run more than one IRLED on the remote control extender (July 1996)? If so, what do I have to do so it still works because I have two units in different places?

(R. S., via email).

You can run another IRLED via a separate 220Ω resistor connected to transistor Q1.

WARNING 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. Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Trade Practices Act 1974 or as subsequently amended and to any governmental regulations which are applicable.