Tacho connection for a V8 Falcon

I am just wondering how does a tachometer for an XC Falcon 4.9l V8 Cleveland work. I am trying to hook the tacho up from the +ve coil and the -ve battery and it doesn’t seem to be working. Any hints? (A. H., Kingaroy, Qld).

If the tacho requires an ignition coil signal, the connection would be from coil (-) and ground (chassis) rather than coil (+) and ground. This is because the primary coil voltage is developed at the switched end – ie, the coil connection to the points or switching transistor.

FM transmitter does not work in stereo

I have recently built the Micromitter stereo FM transmitter from the December 2002 issue. It works but not in stereo. Turning VR3 clockwise or anti-clockwise adjusts the output volume but I still can’t adjust it for stereo. Do you know how to get it working in stereo? (M. C. Kilsyth, Vic).

Make sure the frequency set by the four switches (S1-S4) matches that on your receiver. You must accurately tune the receiver to the transmitted frequency.

Check that the phase lock loop is operating by measuring the voltage at TP1. Does this stay at a fixed voltage of around 2V that changes if the frequency is adjusted with the switches? If not, check the L1 coil winding. The direction of the winding and height above the PC board must be the same as shown in the photo.

Energy meter offset quandary

I have recently constructed the Energy Meter (SILICON CHIP, July & August 2004), the kit being purchased from Dick Smith Electronics. I was rewarded with immediate success but noted an interesting effect with the offset calibration adjustment.

Firstly, with a charged 9V backup battery installed and with mains power and load disconnected, I adjusted the offset setting (+7, I think) so the display indicated 0.00W. However, when mains power was applied and still with no load connected, the display read +0.21W.

Secondly, again with no load connected, I tried zeroing the offset setting (-15) while mains power was applied. This time when mains power was disconnected, the display read -0.21W. I am assuming there must be a small amount of AC leakage when mains power is applied. Is a relatively small no load offset of 0.21W within the expected limits for this circuit? (B. B., via email).

Having AC mains connected without a load and having no mains connected will give different offset requirements. This is because the mains voltage introduces signal into the IC via the voltage input and current inputs.

This was minimised as much as possible with the balanced inputs. However, there will always be some difference. Typically, the Energy Meter will be operated with power applied and the battery backup is only used with blackouts where this is expected to be short compared to when power is on.

Speed Controller For A Golf Buggy I’ve just built the 20A version of the DC Speed Controller, as described in your June 1997 issue. This was to replace an electric golf buggy controller which had ceased to operate. The unit works well but I can’t get the motor to stop completely. The trimpot supplied varies the speed from close to zero to full speed but full speed occurs at just half travel. Can you suggest a way around this? Also the motor emits a high-pitched sound, due (I imagine) to the FETs turning on and off. Is there any way to filter this out or at least minimise it? (D. R., via email). To get a more realistic adjustment range in your application, try inserting a 3.3kΩ resistor in series with the positive side of VR1 – ie, the side that connects to pin 14 of IC1. To get zero speed, try reducing the size of the bottom resistor in the 18kΩ + 4.7kΩ divider network connected to "MOTOR-". We suggest a value of 4.3kΩ instead of 4.7kΩ. You may be able to reduce harmonics in the switching circuit and therefore the noise from the motor by placing a small capacitor in parallel with the 4.3kΩ resistor. Start off with about 68nF and test the operation of the circuit. If speed control becomes erratic, try a smaller value. If the above doesn’t help, try reducing the switching frequency of the TL494 by increasing the value of capacitance on pin 5 from 68nF to 100nF. This may make the noise a little less irritating. It’s unlikely that the noise can be completely eliminated.

Multi-picture TV program monitor

The commercial TV channels are tending to over-run their evening programs. How about a project that could produce two, four or six sub-screens on a standard TV set with selectable sound. This could be accessed by either a spare channel or AV input. You could then watch the end of one show while keeping an eye on the start of the next show.

It could also be used without sound to now and again glance at all channels at once while reading, etc to see if anything worthwhile is on. (J. O., via email).

That’s an intriguing idea but it would basically mean one TV tuner/IF strip/video processor and picture-in-picture chipset for each channel you wanted to watch. In the capital cities, that means at least five channels and many people would want Pay TV as well, making it very complex and expensive.

If you’re that keen to watch TV, maybe you should just get an array of small TV sets, each one permanently tuned to one channel. When one comes up with something interesting, switch to your large screen set.

LED ammeter for battery charger

I am wanting to fit an ammeter to a car battery charger. I am thinking of a 10 LED readout to 5A. Have you published a circuit for this? Could the 25A car LED ammeter (SILICON CHIP, January 1999) be altered to do this? (G. J., Lara, Vic).

The 25A LED meter can be used in your application without any modification. All you have to do is have it monitor a suitably low resistance value in series with the battery charger. You can then calibrate it to display 5A.

Mind you, since it will only ever read positive current (ie, charge), only five LEDs of the display will be involved. If you want to use the 10 LEDs, then you will need to remove the offset trim circuitry involving VR2.

Remote Control Volume Via CD Player I recently purchased a Remote Volume Control kit (SILICON CHIP, June 2002) for my hifi amplifier. Your instructions state that it should work with any Philips-based remote control. I also purchased a suitable multi-item remote, recommended by the kit supplier (Altronics) and the default PC board (with no links) should work with a TV code. The red Ack LED flashes but the potentiometer does not move. Ideally, I want to use the volume buttons on my CD player’s remote control. For this, I need to insert a link between pin 14 and ground (LK2). It is nearly impossible to place a solder bridge across this point, so I decided to fit a small wire link to a point on the ground track. With this link in place, the PC board fails to acknowledge any remote control command, regardless of code. My CD player is a Marantz (formerly part of Philips) so am pretty sure it should work. Can you please suggest anything I could try or may have done wrong? (M. G., via email). First of all, the Remote Volume Control will not work with the Marantz remote – you must use a universal remote, which can also be set up to control your CD player. The linking options on the motorised pot volume control are to select whether you want this item to be controlled by either the TV, CD, SAT1 or SAT2 selection on the remote. The alternatives are there so that there will not be a clash with the codes if, say, you have a TV, CD player or satellite receiver that also operates on one of these codes. The idea is to select either the TV, CD, SAT1 or SAT2 code that does not affect other equipment. So the link installed for your CD player is incorrect. Select either the SAT or TV codes. We did specify the codes that will work using the Altronics handheld unit. These are 651 for CD and 424 and 425 for Sat1 and Sat2. Try to get the pot working with these codes and the correct link. If this does not work, check your soldering for shorts between connections. Also, is there supply between pins 5 and 14 of IC1? Is there 5V between pins 2 & 3 of IRD1?

Eprom programmer doesn’t work

I have assembled the Windows-based Eprom Programmer (SILICON CHIP, November & December 2002, February 2003) and carried out the voltage and frequency checks. I tested the software to toggle the Load PGM* Pulse Duration LED. That works. Everything is OK but I am unable to get a valid EPROM read or write.

I set the read for an ST 27C256 using the preconfigured settings provided. All I get on the read is FF on all addresses. I know for a fact that the chip contains information because it is used to operate a machine. What could be wrong with the programmer? (D. C., Christchurch, NZ).

It sounds as if you may have either a faulty solder joint on the programmer board or a faulty chip. Either that, or your PC’s printer port is not allowing the software to read data correctly.

We suggest that you try checking the clock frequency test points with a scope, to make sure that the clock circuitry is working correctly. You could also try checking at pin 10 of IC18 (74HC02) to make sure that it pulses low during each read operation.

Fuse blowing is a warning

I am having a problem with my car audio connections. For some reason, my power wire keeps burning fuses so I upgraded to a 100A fuse. Now the fuses on my amplifier are blowing. What gives? (M. R., Los Fresnos, California, USA).

Possibly the loudspeaker impedance is too low for your audio amplifier or maybe you have an intermittent short in one of your speaker lines. Always assume that fuse blowing indicates a fault.

PowerUp won’t work with STB

I purchased the Auto PowerUP (SILICON CHIP, July 2003) and finally got it to work. I want to use it to power a 240AC cooling fan when my Foxtel and/or HDTV set-top boxes are turned on. However, I cannot get the adjustment on VR1 right; it is very sensitive!

With careful adjustment, I can get it to switch the slave on (fan) but when the STBs are turned off (standby), the relay cycles continuously at six seconds on and three seconds off.

The HDTV STB is 8W on Standby and 22W on, while the Foxtel PACE box pulls 25W. The unit works well with a heavier current draw but can’t handle the lower power differential. Any suggestions would be much appreciated. (J. D., via email).

For more sensitivity, the 470kΩ resistor should be made larger, however, this is probably not going to help as there will be noise triggering the circuit. Instead, it may be better to increase the number of turns on the former of L1 former. You could place on 80 or more turns to double the sensitivity for detecting low current appliances.

Multiple power supplies in parallel

I was wondering if it was possible to connect multiple power supplies together to create a single high current supply? I have four spare notebook computer switchmode power supplies, all able to supply 20VDC @ 4.5A and was wondering if there was a way of connecting them all together to provide a single output with 18A ability.

Once I get to that stage I would throw in a regulator and filter circuit to make a cheap and very small 13.8V @ 20A power supply to power automotive audio equipment from the mains. (S. R., via email).

It is not really practical because unless they each put out exactly the same DC voltage and have the same output impedance, they will not share the load equally.

75MHz NBFM Receiver For Fire Service I am a volunteer with the local fire department and although I have been issued a handheld radio for communication on the fire service frequencies, I would prefer to have a simple fixed receiver for listening while I am at home when the RT is off (thus saving battery life). FM transmitters are a dime a dozen (though all targeted at the 88MHz-108MHz FM band) but FM receivers are harder to come by. I would very much appreciate it if you could point me in the right direction to build a narrow-band FM receiver for the 75MHz band. I know I could purchase a scanner but I like to build things myself and scanners are still expensive. The requirements are very straightforward, though I suspect, not simple: receive narrow FM on 75.xxxx MHz (the actual frequency depends on which repeater you use) and be stable enough that it doesn’t drift off frequency. The laws in New Zealand allow listening to these frequencies, so there should be no legal problem with this project. (N. Z., via email). We described a narrow-band FM receiver circuit in the February & March 1989 issues. While it was intended for the 2-metre band, it should be easy to change the tuning to cover the band you desire.

Guitar inputs for preamplifier

Regarding the Balanced Microphone Preamp in the August 2004 issue which I want to use as a guitar preamp, could I add a guitar input or change the balanced input? How I could change the circuit to achieve this? (B. G., via email).

The rolloff frequency of the bass control can be lifted to around 230Hz by decreasing the 15nF capacitor across VR2 to 6.8nF. The mid frequency can be shifted from 1kHz to 2.3kHz by decreasing the 2.7nF capacitor across VR3 to 1.2nF and decreasing the 12nF at VR3’s wiper to 5.6nF.

Dummy C-cell battery wanted

In the Circuit Notebook pages for December 2001 you mention a "dummy battery." I have an exercise bike that requires four C-cell batteries but I would like to power it with an AC to DC multi-voltage adapter instead (which is easier and more economical).

However, I can’t find C-cell sized dummy batteries that I could connect to the AC to DC multi-voltage adapter. What might you suggest? (R. S., Rochester, NY, USA).

While we do refer to a dummy battery in the article, in your case the simplest approach would be to just wire your multi-voltage adaptor (we call ’em plugpacks) directly to the appropriate contacts in the battery compartment. Either that or install a suitable socket on the bike battery container so that you easily plug in your adapter.

Incidentally, we haven’t seen a dummy battery recently but you could make your own.

Acoustic feedback with Champ

I recently bought a Pre-Champ pre-amplifier kit (SILICON CHIP, July 1994) from Jaycar to use with an electret microphone and a Champ amplifier (February 1994). However, despite paying attention to the connections, the results are not satisfactory.

If using a 12V source and an electret mic insert at the input, plus a pair of headsets in series at the output, I get good sound. However, when I connect the output of the preamplifier to the input of the Champ amplifier, which is connected to an 8-ohm speaker, the whole system goes into oscillation and produces a high pitch squeal. I tried another amplifier (50W) and got the same result.

Please let me know what I should do to get the system working. (L. F. Auckland, NZ).

It sounds as though you have acoustic feedback from the speaker to the electret microphone – they must be kept well separated, as in any public address system.