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Ask Silicon Chip

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 an email to us.

How do servos work

I understand the basics of the theory of proportional radio control but have you ever published an article about what the inside of a servo looks like? Obviously, the servo plugs straight into the receiver of the radio control unit but I was wondering what the motor is like, what limits its movement, etc. (B. T., via email).

We published an article on servos, including an exploded diagram, in the March 1991 issue. We represented the article in November 1997.

Power rating for speed controller

I built your motor speed controller from the November 1997 issue (Dick Smith Electronics kit). It works well in testing but I haven’t loaded it up yet. I notice in the inset titled "What Motors Can Be Controlled?" you state that the controller is only suitable for "intermittently used power tools". I guess that means it’s not rated for continuous use. What is the duty cycle of the controller? (C. C., via email).

Our article makes no mention of intermittent use. Provided the case does not get hot to the touch, there is no reason why it cannot be used continuously.

Bigger transformer for 500W amplifier

I was just wondering about the limitations of the 500W amplifier designed back in August, September & October 1997. The reason I’m interested is that I bought eight of these from Jaycar as an investment. At the time, I didn’t have the cash for the power supply but I now have the funds to finally finish these pieces of art. However, I have noticed that the original 800VA (57V-0-57V) toroidals are no longer available. Would two 500VA (65V-0-65V) do the trick for each amplifier?

Considering these toroidals have a higher voltage, would I need capacitors with a higher voltage capacity? Would higher capacitance help as well? I’m not expecting huge gains from the amplifier; just another 50-100W of headroom. Will the amplifier drive 2-ohm loads with the higher spec components? (M. G., Glenorie, NSW).

First, you cannot substitute a transformer with 65V windings as this will increase the amplifier supply rails to over 90V and increase the overall amplifier dissipation by 30%! We suggest you contact Harbuch Electronics on (02) 9476 5854. They can wind suitable transformers to order.

We do not recommend operation with 2-ohm loads as it will exceed the safe operating area ratings (SOAR) of both the output transistors and the driver transistors – see Fig.2, page 27 of the August 1997 issue.

Can SLA batteries be topped up?

Some time ago, I built the Mk1 version of your Universal Battery Charger and have a few queries:

(1) I have a 6V 5Ah SLA lantern battery which I have difficulty in charging. The charger 100% LED will light when this battery is connected, even if it is discharged.

(2) It is not recommended to top up Nicad batteries because of the "memory effect". However, is it possible to do this with an SLA battery so that my lantern torch is always on maximum capacity when I take it away on trips?

(3) I am interested in radio control modelling and use my charger to charge transmitter (9.6V), receiver (4.8V) and field packs (7.2V). I would like to be able to use the charger in the field. Therefore, is it possible for you to design a modification for both Mk1 and Mk2 models to operate from a 12V car battery? (D. M., via email).

The Mk1 version of the charger sometimes had problems with particular cells or batteries. Your charging problem could be solved by upgrading to the Mk2 version.

You can top up SLA batteries as these are "sealed lead acid" chemistry and do not have the Nicad memory effect.

The charger is not suitable for operating on 12VDC as the circuit has been optimised for use with raw DC (from a transformer and bridge rectifier).

Tone controls for bass guitar

I had success in assembling the guitar preamp and a 120W power amplifier and was delighted that it worked first time. The unit is for a bass guitar for my son and this is where my question arises. Can you supply me with substitute values for the tone control stage to suit a bass guitar? I am not sure what the treble and bass mid-points are but possibly 400Hz and 4kHz? (J. P., via email).

We don’t know which guitar preamplifier you assembled: the January 1992 design or the November 2000 design. Either way, we would not change the tone control. There is very little treble from a bass guitar and increasing the treble boost will only increase the residual noise. Nor would we change the bass crossover as it is likely to lead to premature overload.

Parts for 24V battery charger

I have been searching for a suitable 24V battery charger for some restored railway carriages and have found the "Automatic 10 Amp Charger" in the June 1996 issue. There are no suppliers for this project and some of the components are proving difficult to source. They are:

(1) "E-type ferrite transformer with bobbin, Jaycar LF-1270 or similar". The current product with that Jaycar Cat. No. is an inductor and not the said transformer.

(2) ETD 29 transformer assembly.

(3) BYV32-200. I presume there is an updated equivalent but not sure how critical it is. (R. P., via email).

The ETD29 is available from Farnell Electronics. Their catalog number is 305-6375 for each core (2 required),

178-506 for the bobbin and 178-507 for clips (2 required).

The T1 transformer is not available from Jaycar now. You could use an EFD25 which will almost fit in the PC board holes but redrilling the holes will allow this to be used. The Farnell catalog numbers for this are 200-300 for the cores (2 required), 200-311 for bobbin and 200-323 for clips (2 required).

The BYV32-200 is also available from Farnell. The Cat. number is 250-650.

Calibrating the digital fuel mixture display

I am building the Digital Fuel Display described in the September & October 2000 issues of SILICON CHIP.

In the instruction section for calibrating the air/fuel meter, it describes the procedure for setting the trimpots for a Bosch EGO sensor with an output of 0-1V. It also states that if you have the output specs for the EGO sensor in your car you can trim these pots to suit.

In the workshop manual for my 1994 Ford Falcon it says that the EGO sensor has an output from 0.33V to 1.1V. Can I tune the meter for this and do I make the span from 0.33V to 1.1V instead of 0V to 1V? Also, will this change the lambda to a different value? (B. B., via email).

You can set the span and offset to 1.1V and 330mV if you need to. However, the actual curve shape will still follow the Bosch sensor as programmed. A complete curve match would require adjusting the curve lookup table in the program.

The 0.6V stoichiometric point should not change much with a different offset voltage.

Is CDI dangerous for a motor bike?

I built the HEI (high energy ignition) kit for my Falcon not too long ago. I am very impressed with it. Now I would like to try something different and build the Multi-Spark Capacitor Discharge Ignition (CDI) described in September 1997, for a motor bike. I have a few problems that you may have answers for. The bike that I would like to put this on has a 4-cylinder 250cc 16-valve engine.

My main concern is that it is such a small engine and the high energy spark may arc to the aluminium head and damage it in the long term. The spark plugs under the normal twin-coil system produce a rather pathetic spark at higher revs.

Spark plug sizes are somewhat smaller than that of a car. The cylinder has a small 30mm stroke. Peak revs are at 17,000 rpm after which it redlines to 19,500 rpm. The cylinder configuration is two up, two down; ie, two outer cylinders up while the two centre cylinders are down. They fire in a 1 3 4 2 sequence.

Do you have any comments or suggestions?

While the CDI does have high energy, the actual spark voltage depends on the cylinder pressure. When the engine is under load, the cylinder pressures are higher and the voltage across the spark plug will rise to a higher voltage before it fires. But there is no danger of damage to the cylinder heads, even if there was an arc-over, which is highly unlikely.

How to wind coils with enamelled wire

I have a quick query on the MiniMitter (April 2001) that I am currently constructing. I am not quite sure how to construct coils L1 and L2. Does the enamelled copper wire need to be soldered to the pins on the former? If so, how is this done? I have tried unsuccessfully, only managing to deform the foot of the former with the heat. (I. C., via email).

We generally give fairly detailed info on winding coils but this time we forgot. You have to scrape the enamel off the copper wire before you can solder to it. Scrape it off, tin it with solder and then solder it to the pins on the coil formers.

Gas sensor project

Did you ever feature a gas detector in your magazine? A fellow worker seems to think he saw one in one of your magazines. (M. M., via email).

We published a carbon monoxide (exhaust gas) monitor in the July 1989 and May 1999 issues. We also published a breath tester in October 2000. We can supply these issues for $7.70 each including postage.

BASIC is still free

In "Ask Silicon Chip" for the July 2001 issue, you stated that GWBASIC/QBASIC interpreters were supplied free until DOS 6. In fact, Microsoft freely supplies the MS-DOS 6.22 version of QBASIC, together with on-line help, in the \other\oldmsdos folder of the Win95 CD.

I suspect that the Win98 CD may also include it. Another interesting Win95B freebie is HwDiag.exe which is found in the \other\misc\hwtrack folder. (F. Z., via email).

Oops, you’re right. On the Win98 CD, it’s in the \tools\oldmsdos folder. The two files required are QBASIC.EXE and QBASIC.HLP.

Kit wanted for rust prevention

On many occasions when I have been to car shows, I have seen a product that prevents rust by electronic means. These units are generally housed in a small aluminium case and are only about the size of cassette for a motorbike and double that size for standard vehicle. They connect to the frame and use the 12V wiring from the vehicle. I absolutely have no idea how it is possible to completely stop rust, especially by electronic means!

If they work as well as the representatives say, I would like to know whether it is feasible to make a kit that would do the same job? I’d assume it would be a popular kit, especially for those of us that own older "non plastic" cars.

Also, it has been seven months since I converted my Ford XC V8 to the High Energy Ignition plus Hall Effect sensor system. It has been the best investment that I have made on the car so far! The engine is extremely smooth and quiet on idling. I dare say that the fuel economy would compare to the latest engines found in 4WD and bigger family wagons! (A. P., via email).

We do not believe these electronic rust preventers can work since they do not involve a sacrificial anode. In any event, they are usually fitted by dealers to new cars (gives them extra profit) and since new cars take years to rust, few people would complain that the rust preventer does not work.

Glad the HEI works well – it certainly does make engines much smoother.

Updating the multi-purpose charger

I previously built the original Fast Multi-Purpose Battery Charger as described in the February & March 1998 issues of SILICON CHIP. I now want to update it to your new design but I am a bit confused about the winding of the main switching inductor L1. In your original design, you require 10 turns bifilar wound of 1mm copper wire, thus a total of 20 turns of wire.

In your latest design, you call for 20 turns bifilar wound which, by my interpretation of Fig.3 on p70 of the July 2001 issue, would make a total of 40 turns. Which is the correct number of turns to use? (B. H., New Town, Tas).

You can leave the number of turns on L1 as per the original version. (10 turns). The circuit will operate with either winding setup.

Clifford the cricket is mute

I am building Clifford the Cricket, as described in the December 1994 issue. I have put all the components in correctly and have also tried replacing the IC and both transistors but the circuit still refuses to work. The LEDs light but the buzzer just provides a low constant clicking sound. I have checked the buzzer and it seems OK. Can you suggest anything? (C. M., via email).

The oscillator for the piezo buzzer is possibly too fast for any chirping sound to be heard. Try changing the .047μF capacitor between pins 8 & 9 of IC1 to a larger value. A 0.22μF capacitor might be a good start. Also, the flashing rate for the LEDs can be varied by adjusting the 2.2μF capacitor value at pins 11 & 12 of IC1. A larger value will slow down the rate. A smaller value will speed it up.

More information on the PIC Testbed

I have some questions on the PIC Testbed published in the January 2001 issue. I know that I have successfully downloaded the program (.asm file) to the testbed (16F84). I assume that I need to set the jumper on the testbed for the clock when programming and I know that the program was downloading to the chip (by connecting the LEDs when programming. By the way, if the LEDs are left connected when programming, the chip will not program successfully). But I am unsure as to how to set the program in motion when it is downloaded.

Also, I have questions on the COM port on the board. I tried to use the COM port (built onto the board) before realising that it was nearly pointless doing so as it is not supported by any of the software I have tried so far and also because of the differences in support protocol. (M. C., via email).

The program should run (meaning the LEDs should chase) as soon as you disconnect the programming adapter cable and press the reset button. From your description, we assume you have selected the 4MHz crystal as the clock source, which is the correct option for the DEMO & TESTBED programs.

We have recently updated the little DEMO & TESTBED programs, as well as created a slightly modified version that will work with the RC oscillator.

Regarding the serial port, you’re quite correct in your assumption that you need to download a program to drive the port. A good starting point might be a little terminal program that receives characters from the serial input line and displays them on the LCD. You will find numerous examples to get you started on the ‘net. Check out Microchip’s application notes first at

Deep cycle vs auto batteries

I’m under the impression that car-type 4WD starter batteries do not like being drained of more than 20% of their capacity. I think that they where intended for starting only and not storage. As I have a 4WD and no extra battery, I thought a deep-cycle battery might be better. That way, more capacity can be used without damage to the battery. I know that a deep cycle battery would be more expensive. (G. M., via email).

As a general rule, all car and truck batteries have a very poor life expectancy if they are frequently subject to heavy discharge. In normal conditions, the vehicle alternator takes all the electrical load and so the battery does little except for its starting role which normally only causes a brief and very shallow discharge. If you are having electrical problems, the solution may be to install a bigger alternator.


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.

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