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.

Substituting a 3-spring reverb unit

Could you please advise if I could use a "Belton" 3-spring reverb unit in your module described in the January 2000 issue? If so, where would I be able to buy the PC board (014002000) from?

(H. P. via email).

Whether or not you can use your 3-spring unit largely depends on its input and output impedances. As presented, our circuit drives an 8Ω input coil and the output impedance is 800Ω. If your unit broadly matches these figures or has higher impedances, then you can probably make the circuit work.

You can purchase the PC board from RCS Radio: www.cia.com.au/rcsradio

LEDs flashing on mixture meter I have constructed the Fuel Mixture Display kit described in the "EFI Tech Special". The kit is not functioning as it should, with lights buzzing left and right on idle at normal temperature. The red light stays on all the time. If I adjust the trimpot, the yellow light shows. What could be the fault in this situation? (E. B., via email). There isn't too much that can go wrong with this circuit. It seems that the IC is driving the LEDs from one extreme to the other as the yellow (rich) and red (lean) ones light with variation of VR1. Check that there are no shorts between tracks on the PC board, by scraping between tracks with a sharp knife. Also check that there are no solder bridges between pins on IC1 by comparing the published pattern with the underside of your board. It is possible that the input at pin 5 has been damaged. It can be protected by connecting a .01μF capacitor between pin 5 and pin 4 and using a 100kΩ resistor in series with the input from the oxygen sensor.

Altec Lansing re-cone kits available

All those owners of Altec Lansing professional drivers (515s, 421/921, 418/918, 411, etc) who may have been told that genuine re-cone kits were no longer available, now that the Altec Lansing name has been sold to that computer speaker company, might be interested to know that there is a source of GENUINE parts in the USA at what are reasonable prices.

I've recently imported 22 cone and coil sets for various types of Altec Lansing drivers and they are as genuine as you could get; they even smell like the originals! The coils are wound on the original machinery that Altec used by a guy who used to work for them, while the cones are sourced from outside, to the original specifications.

(Brad Sheagold, Collaroy, NSW.
bwscdm@tech2U.com.au)

Transistor-assisted ignition query I have a 1973 Mercedes 280S with carburettor and standard points/coil ignition. Did you ever publish an article on building a transistor-assisted ignition system, where the transistor carried the coil primary current and the points fed only a small current for the base circuit? I have already emailed your office for the two High Energy Ignition System articles described in 1998 and 1999 but the simple system mentioned above is suggested in the Bosch automotive book. (D. J., via email). The High Energy Ignition System is a transistor-assisted system. A high voltage transistor does the switching while a current of about 250mA is switched by the points. The points current cannot be too small otherwise they tend to oil up and stop working.

Hum on FM transmitter

I recently bought the MiniMitter FM transmitter from Jaycar and after assembling and tuning it in, I'm getting a hum on the radio. I don't notice it too much with music playing but I am using it as a baby monitor sometimes and was hoping for better results. I've tried it on two different radios and find the same problem. I've also tried moving the antennas around but that doesn't help either. Do you suppose I'm transmitting too close (7-8m)?

(J. R., Brisbane, Qld).

There should not be any evidence of hum when used with a signal source such as a CD player. The hum could be caused by expecting the transmitter to operate from very low signal level or from a high impedance. You do not say how the baby monitor circuitry is connected or how the microphone is amplified, but using a microphone could be the problem.

Hum can also be evident if the FM tuner or radio is set to receive a subharmonic rather than the main fundamental frequency. Try tuning the receiver to another frequency for best signal. Also you may be getting interference from another off-air station which could cause the hum problem. In this case, retune both transmitter and receiver to a quieter part of the FM band.

Control circuit for thermatic fans I have a Falcon EBII XR6. These models came with a belt-driven fan. I purchased the twin thermatic fans with shroud from an EF Falcon as the radiator is the same. I need to set up a switch somehow. I'm not wanting to use the Davies-Craig unit as I'm trying to get a factory look. I thought I could get something to work off the output of the temperature gauge in the dash but I can't find something that will work. Any help would be gratefully appreciated. (R. W., via email). Have a look at our article entitled "Thermostatic Switch For Car Radiator Fans" in the March 1992 issue. We can supply the issue for $7.70 including postage.

Fuel mixture display for a hotrod

I've built the Fuel Mixture Display kit (September & October 2000) which I brought from Dick Smith Electronics in New Zealand. The kit is going to be used on my hotrod. However, I'm having problems locating the Bosch EGO probe you listed in the kit as being matched to the unit. The local Bosch agent said the part number is incorrect (LSM11 , 0258104002). Is the number correct or was there a mistake?

Even if you can tell me what type of car the above probe is from it would be helpful.

(J. B., Stratford, NZ).

The Bosch type number is correct. It is a sensor generally used for sensing exhaust smoke stacks, not necessarily in the automotive industry.

The sensor can be purchased from Farnell (NZ 649 357 0646) but it is cheaper to get an EGO sensor (eg, as used in Ford and Holden 6-cylinder cars) from a wrecker's yard. The Fuel Mixture Display operates successfully with most automotive sensors.

Balance control for the Ultra-LD amplifier I note with interest that you have not provided for a balance control in the design of the Ultra-LD Amplifier described in the November 2001 to January 2002 issues. This a disappointing omission for me as I suffer from a slight hearing loss in one ear and the balance control, to some degree, compensates for this. I wonder what would have been behind the decision not to include the control? (L. S., via email). Most audiophile stereo amplifiers eliminate the balance control and the tone controls because they do degrade the performance. Balance controls also tend to be hard to obtain. If you wanted to put in a balance control you could do so using the same scheme as we employed in the 50W stereo amplifier published in the March & April 1995 issues. This used a single pole 12-position rotary switch and resistors. We can supply these back issues for $7.70 each including postage.

Using the Sparkrite Hall Effect pickup

I just purchased a high energy ignition kit (June 1998) from Dick Smith Electronics. I am using this kit with a Sparkrite Hall Effect pickup which I purchased secondhand. This Hall Effect pickup is over 10 years old and I know it was used with one of the first versions of the High Energy Ignition over 10 years ago. I would just like to ask if you know how to connect this Hall Effect sensor to the current High Energy Ignition?

The Hall Effect pickup only has two connections, which is a problem as the High Energy Ignition requires three connections to the Hall Effect sensor. I also didn't get the instructions with the Hall Effect sensor so I don't know what's going on. Also I found out that the Sparkrite sensor can't be used with V8s. This is a problem as I bought it to put on my Holden V8. Would it be possible to modify the sensor to fit a Bosch V8 distributor?

I can make up my own mounting plates/brackets and was thinking about setting it up like the Siemens HKZ101 Hall Effect sensor, with a stationary magnet and using a Bosch Vane. Will this work? I would get a Siemens sensor but they are no longer available. Would you know of any other Hall Effect sensors suitable for my application which are currently available?

(S. N., via email).

The Sparkrite Hall sensor is only a one-wire unit with the case connection for earth. The wire output is actually connected directly to the supply for the Hall Effect sensor, with a 330Ω resistor between the supply and output. Connection to the 12V supply requires a resistor in series so that the output terminal will pull the Hall supply lower than normal. The output voltage does not swing fully from 12V to ground and is not suitable for the High Energy Ignition circuit.

You could experiment with the resistors at transistor Q2 so that the Sparkrite sensor will operate the circuit. Using a 330Ω pullup resistor from the Hall Effect sensor terminal to the 12V supply will give an output swing from about 5V down to 3V with magnets passing the sensor. Use a 1kΩ resistor at Q2's base to emitter. This value may need to be determined experimentally using a trimpot.

As you state, the Sparkrite sensor is not suitable for 8-cylinder engines as they did not make a rotating magnet assembly for these distributors.

The HKZ101 Hall sensor is still available from Jaycar (Cat ZD-1900) but the Bosch rotating vane is not available. This would need to be manufactured using a steel cup-shaped vane with eight slots spaced 45° apart.

This vane can be glued to the existing rotor button using high temperature (120°C or more ) epoxy or similar.

Matching amplifiers to speakers I am concerned about possible damage to an amplifier when more 'powerful' speakers are connected. For example, if 150W speakers are connected to a 100W amplifier, is there the possibility of the amplifier being damaged? Thinking along the lines of source/load matching as in antennas it seems to me that, provided the amp/speaker matching is correct, there should be no problem. (N. D., Carine, WA) There is no chance of damage by connecting 150W speakers to a 100W amplifier. Speakers are passive transducers. Provided their impedance is within normal limits, no damage will result.

Speed controller for R/C cars

I would like to suggest a project for an electronic speed controller for radio controlled cars. There are commercial models available but they are in the "cost you an arm and a leg" price range and I am sure such a design would be greatly appreciated by all of us dads who spend half of our lives repairing our son's R/C cars.

(R. H., Yatala Vale, SA).

We described a 50A speed control for R/C cars in the May 2000 issue. We can supply this issue for $7.70 including postage.

Converting a UPS to use as inverter

I have a Chloride 12V computer UPS I want to utilise as an inverter but it needs 240V to be on at the moment of a blackout before the 12V inverter will work. This is of no value to me in the fieldwhen trying to operate a drill or grinder. How can I fix it?

(R. M., via email).

Without knowing anything about the circuit, we cannot suggest a modification. However, it probably has a stepdown transformer and rectifier to produce a DC voltage from the mains. When that disappears, the inverter kicks in. You need to measure that voltage and then alter the monitoring circuit to disable it.

18V setting for Nicad discharger Looking at your Automatic Discharger for nicad battery packs in the September 1994 issue, is it possible to include an 18V range? I have an 18V cordless drill battery pack. What would I need to change or add? (H. P., via email). Yes, you can use the Nicad Discharger to discharge an 18V battery pack. This pack comprises 15 cells and the 1.1V per cell voltage would be 16.5V. To add the necessary 18V switch position, you need to replace the 1.8kΩ resistor at the bottom of the voltage divider with a 600Ω (560Ω plus 39Ω) resistor and a 1.2kΩ resistor, with the 1.2kΩ value going to ground. The junction of the 1.2kΩ resistor and 600Ωresistor would be the new 18V battery position. Note that the LM358 is rated for a supply voltage of 32V and so it can be used here without change. Other components should also cope. The exception is the 27W 5W resistor which will glow red hot under reverse polarity connection with 18V applied. It would be best to replace this with a parallel combination of two 56Ω 5W resistors, one on top of the PC board and the other in parallel, on the underside of the board.

Tachometer for a Go-Kart

I've recently purchased your tacho kit from Jaycar (Cat KC-5290) and would like some advice on modifying it for use on a Go-Kart.

First, as a kart has no power supply, I would like to use a 9V battery and on/off switch rather than a 12V auto supply. Next is the connection to the kart's ignition system, which has three wires to the motor and a fourth brown wire for a cut-out switch. They are as follows: Red - high voltage exciter input; Black - earth; Yellow - trigger.

Could you please advise the correct connection from these to the tacho unit and the necessary component modifications to adapt it to a 9V battery.

(S. B., via email).

You can operate the tachometer on 9V without changes to the circuit. Your Go-Kart appears to have a magneto ignition and would probably not drive the tachometer directly without some changes.

You could try connecting the red wire from the magneto to the "ignition coil -ve" input on the tachometer. If this doesn't work try the yellow trigger wire at the "low input" on the tachometer instead.

Note that the 2.2μF capacitor at the anode of D1 may need to be removed from circuit for successful operation.

You can buy products mentioned in this article here : KJ8114 : #15 MINIMITTER FM TRANS KC5290 : Digital Tachometer Display Kit for Cars