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).
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
(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).
Mk1 version of the charger sometimes had problems with particular cells or
batteries. Your charging problem could be solved by upgrading to the Mk2
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
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).
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).
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
The BYV32-200 is also available from Farnell. The Cat. number
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).
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
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
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
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).
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
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
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
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
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
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
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
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
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.
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
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
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 www.microchip.com
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
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