Signal pickup for digital speedo

I have a need for a digital speedo in my 1995 Daihatsu Charade. The analog speedo has a reed relay attachment that produces a 10ms square wave output that feeds into the engine CPU. This square wave has a rate of 40 pulses at 60km/h and so forth.

In "Ask Silicon Chip" for June 2001, page 99, you say that the photo interrupter uses a magnetic PU to detect shaft rotation.

Could I use the speed alarm circuitry to provide a digital speed readout from my car’s speedo square wave? I have not read the November & December 1999 issues about this device. Why did you not use the square wave output from the speedo of most cars with fuel infection systems?

My car computer repairman says that most EFI cars have this feature.

(D. J., Banora Point, NSW).

You can use the speedo signal from your car to drive the Digital Speedometer. Just connect the signal to the pin 2 input of IC2a via the 1kΩ resistor. This terminal is the top of L1, as shown on the circuit.

In our reply in June 2001, we commented that we used a magnetic pickup not a photo interrupter, to allow for relative movement between the suspension and the drive shaft. We could have used the speedo signal available on many cars but we provided the magnetic pickup so that all cars are catered for.

LP Doctor neon problem I have just finished assembling an "LP Doctor" supplied as a Dick Smith Electronics kit. I have not yet got around to testing it because there is one thing puzzling me. The neon in the power switch is on all the time. It does get brighter when the switch is on but it is still clearly visible when the switch is off. Have you any idea why this is so? I am worried that it might indicate a faulty switch and I don’t want to complete the assembly while ever that chance exists. I’ve got around 600 LPs which I enjoy listening to, despite the "clicks and pops". It will be nice to hear them in a quieter mode, when I get the LP Doctor working. (J. L., Geilston Bay, Tas). The neon is on because the .01μF capacitor across the mains switch is letting sufficient voltage through to fire the neon. However, since you need at least 100V across the neon/resistor to fire it, we wonder if you have the mains transformer, power supply and rest of the circuit connected. Or perhaps you have no ICs installed on the PC board?

Earth stake corrosion can cause interference

I read with interest the letter in the June 2001 issue of SILICON CHIP regarding interference on 4QR on 612kHz. I listen to this same station and experience a loud "hum" most of the time. It is especially prominent when using a high-quality FM tuner that has an indifferent AM section. To my surprise, I discovered recently that the "hum" would disappear when I switched on any of the heating elements of the kitchen stove. I have no idea why this occurs – any suggestions?

(D. A., Aspley, Qld).

It could suggest a problem with your mains wiring. Check the main earth stake for your home as the connection may have corroded away. Also, is the oven earthed properly?

(Editor’s note: this correspondent subsequently confirmed that the main earth stake connection had corroded. Renewing the connection fixed the problem).

Thermistor in Nicad battery packs

What would I have to do to the Universal Fast Battery Charger described in the June & July 2001 issues to be able to charge batteries such as 1400 or 1500mA.h AA batteries. Also is the thermistor that you attach to the battery pack when charging Nicad or NiMH cells for safety or is this used to detect the peak charge of the battery pack?

(R. R., via email).

Use the 2A.h setting for 1.4-1.5A.h batteries. The thermistor detects temperature rise in the battery pack and is a backup to the normal end-of-charge detection.

Adjusting VR5 in the DI Box

In your article on the DI Box in the August 2001 issue you refer to the purpose of VR5 (offset adjustment for IC2) but you do not give any instructions on how to set this in the "Testing" section. Neither is VR5 included in the parts list.

(P. S., Glen Innes, NSW).

VR5 should be a 16mm 10kΩ linear potentiometer. To adjust it, connect a DMM across the bass pot (VR2) and set VR5 for 0V DC. This eliminates any DC current through the bass control and stops it from becoming noisy.

Pulse switching makes motor noise

I have just built the 24V motor controller described in the June 1997 issue of SILICON CHIP and it works fine except for one thing. When I reduce the speed, the motor sounds like a rampaging cricket.

I have followed your instructions (fitting the capacitor and diode) and checked the board and components. Is this common and do you know how to stop it?

(J. E., via email).

Unfortunately the noise is a side effect of the pulse width modulation and it is most noticeable when you operate at very slow speeds. You might try reducing the pulse frequency by increasing the .068μF capacitor at pin 5 of the TL494. Try 0.22μF at pin 5 and increase the 10kΩ resistor at pin 6 to 56kΩ. This will reduce the frequency to 200Hz.

Troubles with ignition system I recently purchased the Universal High Energy Ignition (June 1998) kit, along with the Programmable Ignition kit (June & July 1999) to go with it. I built and tested them on the vehicle and they both worked. I then installed them properly and they don’t work. First, I need to know how to change the current limiting part of the circuit as I need it to fire two coils. I know the trigger part of the circuit is working as the tacho feed is working and the Programmable Ignition is also working and sending a signal to the Iigh-Energy Ignition. I need to know how I can test the signal that comes from pin 7 of IC1 and to the transistor as the fault is somewhere around there. The plastic washer for the transistor melted after trying to get the circuit to work. How can I test the transistor? I also read about the Multi-Spark CDI kit and I know the trigger circuits are the same. I am wondering if it is possible to convert the high energy kit to a multi-spark kit. If this is possible how would I go about it? (S. G., via email). The pin 7 output of IC1 is normally high and goes momentarily low when the coil is to be fired. You would not be able to monitor this unless you have an oscilloscope. The output transistor has possibly been destroyed as the washer melted. This would suggest that the transistor was wired directly between the ground at the emitter via the 0.1Ω resistors and the 12V supply rather than to the coil. The best way of increasing current limit is to parallel another 0.1Ω 5W resistor across the other two existing resistors. The capacitor discharge ignition can be used with the programmable ignition but with some modifications to invert the signals. We suggest you get the high energy unit going first. It cannot fire two coils and nor can it be used in multi-spark mode.

Power transistors for 100W amplifier

I’m having trouble finding the power transistors for the 100 watt LD amplifier described in your March & May 2000 issues. Do you know of any distributors which may stock the following: MJL1302A PNP and MJL3281A NPN power transistors?

Also I noticed that in the March issue the mono amplifier clips just after 100 watts while the stereo version in the May 2000 issue clips at 90 watts. Is this due to limitations of the toroidal transformer used or something else?

(B. M., via email).

The stereo version clips at 90W with both channels driven. With one channel driven, the distortion curves would be more or less identical to those shown in the March issue. This is solely due to transformer regulation and is not a problem.

You can purchase the MJL transistors from Farnell Electronics. Phone 1300 361 005.

Connecting the Immobiliser to the HEI

I have just purchased two SILICON CHIP kits from Jaycar, the Engine Immobiliser Mk2 from December 1998 and the Universal High Energy Ignition kit from June 1998. I wish to combine the two kits in the ignition kit case. Can you see any problems in combining the two kits? If the answer to question 1 is OK, do I pull down (earth) the input or the output of the ignition kit with the Immobiliser?

(B. H., via email).

Simple: just connect the Immobiliser output to the coil output of the ignition circuit. In effect, the Immobiliser power transistor is connected in parallel with the power transistor in the HEI circuit. When the Immobiliser transistor is on, the HEI transistor cannot interrupt the coil current and so no spark voltage is delivered.

Minimitter MkII FM transmitter

I’ve constructed a Minimitter MkII FM transmitter as described in the April 2001 issue. However, I had to turn my stereo to the highest volume so that I could just hear the music from my CD player through the transmitter & tuner, with some noise.

(J. T., via email).

It seems that you are not applying sufficient audio signal into the Minimitter. Adjust trimpots VR1 and VR2 for a larger signal level from your FM tuner. Alternatively, you may be tuned into a harmonic of the transmitted frequency rather than the actual signal. Try retuning the transmitter and FM tuner.

Dropout problem with 12V amplifier I have recently constructed the 12V Mini Stereo Amplifier as described in the May 2001 edition of SILICON CHIP magazine. But when connected to a 12V battery (or power supply), a mini disc player and some 8Ω Bose speakers, the sound cuts out with a thump every three seconds or so. The current consumed also increases to about 3A and the voltage correspondingly drops (causing the power amps to drop out). This happens only when an audio signal is present; turning the volume up to maximum with no input does not make it happen. When it does happen, both speakers do it at the same time. I have checked all components, soldering and tracks and they seem fine. I disconnected 10Ω resistor at pin 1(7) of IC1 and injected the audio at that point, both power amps worked OK with no cutouts (but some distortion was evident). Next I put the resistor back and put the audio input at the 10μF NP capacitor on pin 8(14) of IC1 and the problem was back. This led me to believe there was something amiss around the second op amp. Jaycar Electronics supplied a 1μF MKT type capacitor instead of an electrolytic non polarised variety. I replaced this with a different 1μF MKT and the problem was still there. I then tried other capacitors until I used a 0.1μF greencap and the amp worked fine (the audio seemed a little distorted but that is most likely due to the wrong value capacitor). Will this be a problem to use an MKT capacitor instead of a electro? It seems I have a problem around this point. Could it be component tolerances? I have replaced IC1 as it was easy (thanks to the socket – but no effect) and I could replace the other components around IC1b&d as I have them at home but 1μF NP electros are very hard to find here in Singapore. (J. L., via email). Depending on how loud you play it, the 12V stereo amplifier can draw 5A or more and if you are using a small battery or power supply the voltage may drop below 9V or so, and this will be sufficient to mute the TDA1519As in both channels. If your battery or power supply seem beyond reproach, check the voltage at the PC board with an analog meter. If it is fluctuating in accordance with peaks in the music it may just be that the resistance of the connecting wire is too high. If this is the case, use thicker hookup wire such as 4mm auto wire. The 1μF MKT capacitor is fine; better than an electro, in fact.

Turbo timer false triggers

I recently bought and put together one of the Turbo Timer kits described in the November 1998 issue and it works well except for one thing. Instead of activating the relays only when the ignition is turned off, they are activated as soon as the ignition is turned on and then it switches itself off after the set time. This means that to activate the timer after going for a long drive, you have to turn the car off, then turn it back on (timer activates straight away) then take the keys out.

I’ve checked and double-checked the layout. Help.

(J. M., via email).

Some vehicles do not drop the ignition voltage fast enough to trigger the Turbo Timer circuit. You can improve this by increasing the 2.2μF capacitor value connecting to pin 2 of IC1 to a much larger value. A 22μF capacitor should be sufficient.

LM3876 vs LM3886 amplifier chips

I have just built your 50W Audio Amplifier kit for my home stereo. This uses the LM3876 audio amplifier chip from National Semiconductor. However, the data sheets for this chip suggest that it was designed to work with 8-ohm speakers whereas mine are 6-ohm. Can I just substitute the LM3886 chip instead, as it seems to better suited for lower impedance loads?

(P. F., via email).

We would not bother substituting the LM3886. The performance difference will be very slight, certainly not noticeable to the ear. The LM3876 will work quite happily with 6-ohm speakers but in any case, the actual impedance of your speakers will be above 6Ω for most of the audible spectrum. If you really want to improve the performance, you need to go to one of our discrete bipolar transistor amplifiers such as the Plastic Power 125W module described in April 1996.

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.