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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.

DIY 300W car amplifier not practical

A friend showed me a back issue of SILICON CHIP and I was interested in the specs for the 300W amplifier. Is it stable at 2Ω? If not, is it a matter of doubling the output stage to obtain greater stability. I'm interested in using it together with a switchmode power supply in a car.

(A. B., via email).

We have not described a 300W amplifier although we did describe a 350W Mosfet amplifier in 1994 - it was not suitable for 2Ω speakers though. EA described a 300W amplifier back in April/May 1995 but again, not suitable for 2Ω. Finally, a 300W module was published in EA in May, June & July 1980 but it also was not suitable for 2Ω.

In any event, if you are going to install it in a car you need a high-power inverter to produce the DC supply rails of about 80V. By the time you do that it is definitely cheaper to buy rather than build. Have look at the car amps available from Jaycar.

How to increase Multi-Spark voltage

I am interested in building the Multi-Spark CDI project from the September 1997 issue of SILICON CHIP. I was wondering about modifying it slightly. I'm wanting to increase the primary voltage from 300V to around 425V or so. Is this possible? The reason for this is to increase the output energy from 45mJ. Most commercial units put out 420V to 525V for the primary.

(S. M., Invercargill, NZ).

The voltage can be increased by another 75V using an extra 75V zener in series with ZD1-ZD4. We do not recommend going above this voltage as the capacitor is not rated for more volts. You will need to add about 50 more turns on the secondary of T1 as well.

Degaussing circuit explained

Could you tell me the function of the dual posistor in a TV set as mentioned in the Serviceman NEC TV set story featured in the September 2002 issue?

(C. D., via email)

The posistor is a positive temperature coefficient (PTC) thermistor connected in series with the degaussing coils around the periphery of the CRT. When cold, the posistor has a low resistance and this lets current flow in the degaussing coils. After a few seconds the posistor heats up, its resistance goes high and cuts off the degaussing current.

Speed controller for Meccano motor

I would like to use your 10A 12VDC motor speed controller to run a 6V Meccano motor using the original 6V battery pack. Is it possible to modify this circuit to run off 6V DC? Failing that, do you have another project that can be modified for the task?

(C. C., via email).

The 10A 12V motor speed controller cannot be used at 6V as the TL494 requires more than that to provide its internal 5V reference. Our suggestion is to use is the Mini PC Board Drill Speed Controller described in the January 1994 issue of SILICON CHIP, with some slight modifications.

The 9.1V zener diode should be replaced with a 5.1V type and the 220Ω resistor changed to 27Ω 1W.

Sound card interface is noisy

I've recently assembled a sound card preamp kit from the August 1998 issue of Electronics Australia, with the intention to use it as an oscilloscope. After assembly, I've done all the functionality testing as per the instructions, with the equipment working fine.

The problem is that the preamp is putting out some form of background noise (ie, with no CRO probe connected), which seems to be generated in the preamp circuit. Unfortunately, this makes the unit unreliable.

Would you be able to tell me if this was maybe experienced in the circuit built by Electronics Australia in August 1998 and if so, what did they do to rectify it? There is a paragraph in the article referring to the LM324 chip as noisy. Is this what I am seeing? Also, there is an instruction referring to fitting a higher performance TL074 quad op amp. Will this fix the problem?

(R. C., via email).

Our sound card interface presented in August 2002 is far superior to that presented in the August 1998 issue of EA. At the time of publication, we assumed that the kitset suppliers would automatically kill off the old project and present the new but it didn't happen.

Unfortunately, we do not know what problems were specific to the EA circuit. If you want low-noise performance, build the one published in SILICON CHIP. Note also that our article states that the "smallest voltage you'll be able to measure accurately will be in the mV range". Our prototype had less than 1mV RMS noise but that will depend on your sound card and your computer.

Speed alarm signal pickup

You have probably been asked this a hundred times before but can you tell me if it is possible to connect the kit directly to the computer for speed sensing. I have a VP Commodore with analog display speedometer. The speedo is driven by pulses from the computer. What I would like to do is use this signal instead of the magnet/coil method.

(M. G., via email).

Yes you can use the speed signal from the engine management system. The speed signal connects to the signal input on the coil input terminal on the speed alarm. The second coil input (shield connection) is left open.

Possible damage to 12V amplifier

I have recently constructed the 12V Mini Stereo Amplifier kit (SILICON CHIP May 2001). It has a few problems when it is operating. With the bass control anywhere in the boost region, the speakers start to behave strangely. Occasionally and randomly the cones violently pull downwards, and at the same time all sound is lost for about half a second.

If I pull out one of the RCA connectors (left or right channel) and have the sound coming out of just one speaker, when the other speaker pulls down, the one which has no sound coming through mimics it. There is also a lot of hiss produced, even with no input and the volume turned fully down.

The treble control does not seem to make this phenomena occur and neither does the volume. Even with the volume very low and the bass high, the intensity of the "pull down" remains the same.

In an effort to fix this problem, I have replaced IC1, the two 100μF 16V electrolytics, the two 2200ΩF 25V electrolytics and the 470ΩF 16V electrolytic. This has partially solved the problem but not fully. I have a feeling that all of this occurred when I connected the amp to a car battery and the wire sparked and the fuse blew. I am now running the unit off a 12V 3.4Ah SLA battery.

(D. F., via email).

It seems you might have damaged either the power amplifier or the op amps used for the tone controls. Perhaps changing the op amps will solve the hiss problem.

Also the amplifier requires heavy gauge wiring for the power connections between the amplifier PC board and the battery. Otherwise, the amplifier will exhibit a tendency to mute or "motor-boat" on loud signals, due the supply voltage dropping. The minimum to use is 7.5A rated wire. Replace the supply leads before you do anything else, as this may fix the problem.

Electronic wind vane decoding problem

Back in March 2000 you described an electronic wind vane with a 16-LED display. I have had the PC boards on hand for some time and have finally got around to putting the project together. But one problem has me a little confused.

At switch-on I am greeted with one LED, that being the North LED. Rotation of the Gray Code disk did not result in any other LED becoming illuminated. A quick check of the A, B, C & D inputs found them to be all low which would explain the illumination of the North LED.

The voltages on the input lines were, on average, + 0.02V (not illuminated) and +0.9V (illuminated). The inputs appear to be going high but not high enough. The application of +12V to the inputs results in all the correct LEDs on the display board lighting, proving that the decoding is working correctly.

There is around 3mA flowing through the infrared LEDs which appears OK. Any ideas as to why this is happening would be greatly appreciated. The IR LEDs I am using are DSE Z-3235 and Z-1956.

(B. C. Ballina, NSW).

The current to the infrared LEDs can be increased to provide greater drive to the infrared diodes. Use a 470Ω resistor in place of the 1.8kΩ resistor. Also, change the 10kΩ resistors for the infrared diodes to 47kΩ, to allow a higher voltage when exposed to light.

Make sure the LEDs and diodes line up for maximum light transfer. Also, make sure the gap between the LEDs and infrared diodes is at a minimum.

Kill switch for rev limiter

I have built a rev limiter from the April 1999 issue but I also need to have a switch which will kill the engine completely. The car is used for motor sport where a kill switch is required by the regulations. It seems that this can be incorporated in the ignition switcher circuit in the following two ways.

(1) Installing a second capacitor at C1 which can be switched in to kill a very high number of sparks, causing the engine to stall; or

(2) Install a switch between terminals 8 and 3 of IC1. When the switch is closed, transistor Q3 will be turned on to kill all sparks. Solution 2 would be easier to install. What do you recommend?

(D. M., via email).

Solution 2 would be best as it completely shuts down the ignition.

Low-cost oscilloscope probe

I am interested in building a sound card adapter kit from Electronics Australia, on sale at one of the kitset suppliers for $30. However, I reckon I need oscilloscope probes so that I can use the adapter, right?

When I checked prices of probes at Jaycar or Dick Smith Electronics, they were around $44 each, much more than the price of the kit. That is too much for a student budget. Can we make one probe on our own?

(D. B., via email).

We featured a low-cost, low-capacitance scope probe in the August 1989 edition of SILICON CHIP. It utilises a short length of coax cable, a resistor, trimmer capacitor and a few other bits & pieces you'd probably find in your junk box.

Door alarm uses electret microphone

I have searched all the indexes, including those on your website (excellent site, by the way) and I cannot find what I seek. It is an alarm using a microphone as a sensor. It does not operate on sound but on changing air pressure as a door or window is opened. I am certain it was in SILICON CHIP and I have every copy from issue number one but I just can't find it. Can you help?

(R. C., via email).

The article appeared in the July 1995 issue. It used an electret microphone as a pressure sensor. We can supply the issue for $8.80 including postage.

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