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.

Knock sensing for programmable ignition

I wanted to know if the Programmable Ignition kit from the June & July 1999 issues has provision to be used with a knock sensor to retard timing if detonation occurs? Does the kit in its current form allow the use of vacuum advance or is it purely just what you put in the program? What mods do you have to make to the engine or distributor to make this work?

(P. S., via email).

You might want to have a look at our Knock Indicator project from the April 1996 issue. However it does not act to retard ignition. Nor does the programmable ignition setup act on the vacuum advance.

Speed alarm cooks resistor I recently decided to build the Speed Alarm (November & December 1999) as I had a close call with a speeding fine. I talked my friend into also buying a Jaycar kit and set about building the two of them. Turn on resulted in the 10Ω 1W resistor smoking away. I later found out that I had reversed the polarity of one of the two 47μF electrolytic capacitors. Thinking this was the culprit, I replaced the electros, the 7805 regulator and the 10Ω resistor but the resistor still smokes. What have I missed, so I can finish my speed alarm and build the second one? I hope that the microprocessor is still OK as they sure are expensive to replace. (P. C., via email). Assuming that you did not reverse the polarity of the DC supply when you first connected your project, the most likely reason for the smoking 10Ω resistor is that ZD1, the 16V input protection zener diode, is reversed.

Speed control for LGB trains

With regards to the motor speed control project in the June 1997 issue, would it be feasible to use this as a variable speed control unit for LGB type model trains? Could the REF from pin 14 be used for the reference on a suitable DAC and the output of the DAC, with a suitable buffer, be used in place of VR1 which is 5kΩ pot?

(R. M., via email).

We would not recommend this circuit for model trains. We have published quite a few train controllers over the years. Have a look at the design featured in the October, November & December 1999 issues. Its output can be made compatible with the higher voltage needed by LGB models by substituting a transformer with a higher secondary voltage.

Speed control for a golf buggy

I have taken over an electric golf buggy business and need help in designing a new speed controller. The one used at the moment is unreliable. The buggy is run by a 12V 180W DC motor and the electronics has two MJ802 transistors which are prone to blowing under load. The motor can pull up to 45A. What sort of circuit would your suggest? (J. E., via email).

Mosfets are the answer. Have a look at the 50A speed control published in the May 2000 issue.

Voltage checks when building the Theremin

I am constructing the Theremin which appeared in the August 2000 issue of SILICON CHIP. I have only a basic knowledge of electronics, so I have a few questions.

Firstly, both the DC socket and the S1 power switch have three connectors and in the component overlay in Fig.5 of your article only two connectors are featured on both of these for wiring. I am not sure which connectors on these components I should be using or does it even matter?

Secondly, when setting up or tuning the Theremin you describe checking voltages at various points. For instance, "there is a nominal +6V between the case of one of the transformer coils and pin 8 of IC2 & pin 6 of IC3. The voltage should be between +5.6V and +5.8V." Plus, there are other references to checking voltages at the cathode of diode D1 when aligning the volume plate, etc.

As I am a newbie, can you tell me exactly how I go about doing this? I have a multimeter but am never exactly sure where to put the probes to check all of these things.

In fact, a beginner's article on exactly how to use a multimeter to check your work and find faults in a circuit would be greatly appreciated in your magazine.

(Z. C., via email).

The switch should be wired so that its two terminals are closed when set to the ON position. You can check this by selecting "ohms" on your multi-meter then using the two terminals that show a short circuit (zero ohms) when the switch is on. Make sure that the terminals become an open circuit (high resistance) when the switch is set to off.

The best way to check the DC socket is to plug in the plugpack and measure the voltage on the socket terminals. Do this before soldering the DC socket to anything. The positive terminal is the one which shows positive voltages on the multimeter when the red multimeter lead is connected to it. The black multimeter lead connects to the negative terminal on the socket.

Measuring voltages on the Theremin circuit is done similarly. Connect the black multimeter probe to the case of a transformer, then measure voltages with the red probe. They should be similar to those quoted in the article.

If you are having trouble with measuring voltages on the circuit, you could just try the Theremin without voltage checks and adjust the cores of the transformers as described while skipping the voltage measurements.

Turbo timer count-down problem I purchased a Turbo Timer kit as described in the November 1998 issue of SILICON CHIP and upon testing it I discovered that when the ignition switch is left on the timer is activated and starts counting down, so when the ignition is ultimately switched off there is no countdown. What should I do? (G. B., via email). Try increasing the capacitor value at pin 4 of IC1. This 100μF capacitor holds the timer reset until after a set time determined by the 10kΩW resistor connecting to the 12V. A value of 470μF should be enough to extend the reset time so the circuit will not trigger with power on.

Minimitter tuning is odd

I've had a strange problem with the FM Stereo Minimitter described in the April 2001 issue. Adjusting L1 works fine but L2 seems to have no effect. The radio receiver I'm using indicates FM stereo regardless of the tuning of L2. Could you please advise me on this?

(B. M., via email).

We suspect that coil L2 is not connected electrically to the PC board. Check that there is continuity through the coil by measuring its resistance between the relevant tracks on the PC board (using a multimeter). The most likely cause is that the enamel insulation has not been cleaned off the wire ends.

Alternatively, the capacitor across L2 may not be soldered correctly or it is the wrong value. It should be 47pF. Or maybe the 3-10pF trimmer is shorted or L2 is wound or terminated incorrectly.

Tachometer with shift indicator

Jaycar has suggested that I contact you with my need for an automotive tachometer with a bargraph display. I am not interested in knowing the actual value of the engine rpm, just an indication when approaching the upper and lower limits of engine speed and guidance in selecting the best speed for gear changing. The bargraph display, if mounted remote from the rest of the electronics, could be quite small and unobtrusive, yet very effective if mounted, say, with double-sided tape centrally just outside the glass of the instrument panel.

(J. B., via email).

Have a look at the Rev Limit Controller that we published in the April 1999 issue. While specifically intended as a rev limiter project, it can also be used as a Shift Light. We can supply the April 1999 issue for $7.70 including postage.

Optical pickup for Rev Limiter

My car is a 1993 Nissan Silvia which uses a LED system inside the distributor. I was just wondering if there is a different pickup circuit for the Rev Limiter described in the April 1999 issue. If not, which of the two systems would work, reluctor or the Hall Effect system?

(G. H., via email).

We published details of how to use an optical pickup in the Ask Silicon Chip pages of the August 1998 issue and the Circuit Notebook section in the October 2000 issue, page 58. The August 1998 version should suit.

Question on modifying PC power supply With respect to the article entitled "Use your old PC power supply for high current outputs" in December 1998, how high is the "high current". Also, have you ever described the theory behind high power Mosfet auto amplifiers? Can the output current of the Power Supply for Amateur Transceivers described in May/June 1991 be increased to around 30A relatively easily? (T. C., via email). The December 1998 article dealt with modifying a standard PC power supply to deliver slightly higher voltage; eg, 13.8V instead of 12V. Typically, you can get 8A from the 12V rail and 20A from the 5V rail. We have not published any theory behind Mosfet car amplifiers. They are just standard Mosfet amplifiers powered by a DC-DC inverter. The output of the 13.5V 25A supply cannot be increased without substantially upgrading the major components.

How to connect a subwoofer amplifier

I'd be the first to admit that I am a novice in the field of electronics. With perseverance and a lot of reading I had thought that I had developed a basic understanding of audio amplifiers, until now.

I have built a couple of amplifier modules supplied by Dick Smith Electronics, namely the 100W module (Cat. K3442) and the more powerful 300W beast, primarily to power subwoofers for my home-theatre system. Each amplifier module has the Sub Bass Processor preamp module (DSE Cat. K5403). My dilemma is this: if I use the subwoofer out jack on my surround receiver I get virtually no signal through either amp module. If I connect the same subwoofer output to my small 30W bookshelf stereo and get this to power my subs, I get thumping bass but only 30 watts worth.

Obviously there is signal coming out of the sub out jack, so why is it lost between here and my speakers?

(S. F., via email).

The sub-bass processor should not be connected to the subwoofer output of your surround sound receiver. The sub-bass processor is intended for those people who do not have a subwoofer output from their receiver. Try connecting the subwoofer signal directly to your 100W amplifier. You should get heaps of bass.

Bridging audio amplifier modules

I've bought two power amplifiers from Dick Smith Electronics (Cat K3442) and I would like to bridge them. What do I need and can it be done?

(E. P., Vermont, Vic).

There is a problem in bridging these amplifier modules. As they stand, they will deliver 100W into 4Ω or 50W into 8Ω. If they are bridged, they will only deliver 100W into an 8-ohm load. They cannot drive a 4-ohm load in bridge mode, since each module cannot drive a 2-ohm impedance.

How to eliminate the distributor

I wish to convert my car ignition system to distributor-less operation, ideally using one or two Hall Effect sensors to trigger the electronics to fire the coils or ignition systems. I'm not sure how to achieve the electronics part to count the pulses from the Hall triggers to fire the correct coil/cylinder.

Naturally, there are quite a few aftermarket manufacturers who produce such complete packages but they are quite expensive. Can you help?

(P. W., via email).

Have a look at the High Energy Ignition circuit in the July 1998 issue. It can be triggered by Hall Effect pickups and contains the necessary interface circuitry. If you are going to use one coil per cylinder you will need one Hall Effect pickup per cylinder. If you only want one Hall Effect device, operating from a toothed vane on the harmonic balancer (say), then you will need counter circuitry and some method of identifying the firing point for cylinder one, for correct timing.

You also need to allow for ignition advance. Without going to a full engine management system, you are really facing complications. Our preferred method would be to convert the distributor to Hall Effect or reluctor pickup.

Temperature compensation for pH meter I am writing to inquire if it would be possible to modify the pH meter for swimming pools (published in April 1988) so that temperature could be compensated for automatically. I was thinking of putting an LM355Z in series with the 6.8kΩ resistor (insulated and on a length of wire so it can be placed in the solution) but I'm not sure of the correct way to do this. I want to feed the output voltage to an ADC so I can automate the pH control system and log the data on a PC. The temperature will vary from day to night and this is why I want to compensate the temperature automatically. Do you have any suggestions? (J. E., via email). The temperature compensation in the pH meter changes the slope characteristic from the sensor. In other words, just adding or subtracting an external voltage will not alter the slope. This would require some sort of variable resistance which changed with temperature instead. You may be able to use a thermistor which is altered with parallel resistance to set the resistance change with temperature. Alternatively, the measurement can be altered in your computer to follow the graph in Fig.1 of the pH meter article. A separate temperature input for the computer would be required. Apart from this, the pH probe does not change much in output over the normal range of temperatures expected to be found, especially in Australia where temperatures are reasonably stable over a season. Maybe your automation can tolerate ignoring any temperature changes?

Optocoupler breaks down in welder

If possible, could you please help me with the following problem? We have an inverter welder that has been imported from Europe. The machine has an input rating of 230VAC and the manufacturer has stated that running the welder at 240VAC would pose no problem.

The problem is that a MOC3023 optocoupler is breaking down and letting AC flow back through the circuit and cause the solenoid to chatter. The optocoupler drives
a gas solenoid rated at 230VAC 13.5VA and incorporates an RC circuit which consists of a .022μF capacitor and a 22Ω watt resistor connected in parallel.

My question is, would the 10V increase in the primary generate a sufficient increase in the breakdown voltage to cross the protection threshold and blow the optocoupler? And is there any formula to work out what the break-down voltage would be?

(P. D., via email).

The MOC3023 only has a 400V blocking rating so depending on what it is driving, it could easily be fragile, especially in a welder. Given that its load is only a low current, the designers probably thought that it would easily handle the job.

Our approach would be to use the MOC3023 to drive a 600V Triac but one with a low holding current or alternatively, shunt the solenoid with an incandescent lamp to ensure reliable operation. You may also need Varistor protection in the circuit.

Notes & Errata Touch/Infrared Light Dimmer, January & February 2002: the circuit diagram (Fig.3) should show the .01μF capacitor and 1MΩ resistor connected to pin 6 (RB0 input) of IC1 and not to the A2 terminal of TRIAC1. The PC board pattern and overlay diagram are correct.

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.

You can buy products mentioned in this article here : KC5279 : IMPR. SPEED ALARM KIT 11/99

Share this Article: 

Digg this story

Seed Newsvine

Post to Reddit

Save on del.icio.us

Email