Tacho regulator burnout
A friend of mine recently bought his second Digital Tachom-eter
which I think was in SILICON CHIP. The
first one he tried he fried and on this second one he burnt out the voltage
regulator. For some reason it was getting too hot so I replaced it and put a
heatsink on it.
I tested it on a power pack set to 12V and the three zeros
flashed up and all the voltages tested out OK, so we tried it on his car. He
assured me it was calibrated. It worked once.
The second time we tried we got 8000 revs without the engine
running. So I set it up with the power pack again and it showed 8000.
He is really annoyed because he has paid a total of $60. What
could be wrong? (Damien, via email).
Without really knowing, we assume that you are
referring to the 4-Digit Tachometer published in the August 1994 issue.
The regulator should not burn out as these are short cir-cuit
proof. Perhaps there is a short circuit on the PC board somewhere causing the
regulator to overheat. Also the short could be causing the display to show an
"8" if there is a connection between the "g" segment and another segment on the
most signifi-cant display.
How to charge lithium ion batteries
I have just built the Multipurpose Fast Charger from the
February & March 1998 issues of SILICON CHIP and it
works well. Now how can I get it to charge li-ion batteries which are popular in
phones and cameras? (N. S., via email).
We published information on using the charger with
li-ion batteries in the Circuit Notebook pages of the June 1998 issue. We can
supply the back issue for $7 including postage
FM Mini-mitter misbehaviour
I have purchased and built three of the FM Mini-mitter kits
(SILICON CHIP, October 1988) over the past
10 years, along with one Baby-minder. Most have been successful, bar this and
the previous one. The previous problem was a centre frequency off the FM band
and was solved by increasing the 47apF capacitor on pin 10 to 59pF.
Now I've built the third unit. It delivers a stronger signal
than the others but does not trigger the stereo lock on receivers. I've checked
that the capacitor on pin 12 is .001μF as per the errata. I've also
checked the components around the mixer (pins 12, 13 & 14). Also, there are
about five points on the receiver dial around the best signal where a reasonable
signal is heard.
This leads me to suspect that I may be receiving a harmonic of
the main signal and that the centre frequency is again off the band. I tried
increasing the 47pF capacitor in the main oscilla-tor to 59pF by adding a 12pF
in parallel. This only gave about 10MHz shift. Can you suggest steps for further
diagnosis? I have no CRO. (J. C., via email).
You are probably correct in assuming that the
receiver is locked onto a harmonic. Try carefully tuning across the FM band with
the receiver well away from the transmitter. You will find several spots where
the signal is received. Select the one which gives stereo.
If you cannot find any other positions on the receiver, then
try retuning the transmitter to another frequency and try again.
Another possible problem with lack of stereo reception concerns the 38kHz
crystal. In the past some kits were supplied with 40kHz types by mistake. If you
do not have any means of checking this frequency, try another crystal
Cordless phone backup battery project
I’m keen to build the backup battery unit described in the
October 1999 issue, for my new Panasonic cordless phone. The plugpack is
nominally 13.5V but is actually putting out 22.5V. The base sta-tion handset
charger puts out 8.75V (no load) with a 3.5V NiMH battery in the handset.
I’m unsure how to set up the LM317 to achieve an output
equivalent to the existing plugpack. The R1 and R2 values in the article are all
for lower voltages. What values should I use? (G. T., via
As far as we can see there is no simple solution to
your problem since the plugpack output is a great deal higher than a 12V SLA
Reluctor problem with Multi-spark CDI
I have just put together the Multis-park Capacitor Discharge
Ignition as published in the September 1997 issue of SILICON
CHIP and I have come across some problems. There is no output from the coil
(standard type) and the transformer I wound makes a buzzing sound.
Here are the DC voltages I measured (all taken from left to
right (Q1 on the left) looking down on the devices bolted to the heatsink):
Q1 – 9.5V, 12.3V, 15.5mV.
Q2 – 1.4V, 12.1V, 17.1mV.
Q6 – 13.8V, 298.3V, 14.1V.
Q7 – 9.4mV, 14.2V, 9.1mV.
IC1 – pin 1 8.3V, pin 2 8.0V, pin 3 0.9V, pin 4 14.7V, pin 5
1.5V, pin 6 15.7mV, pin 7 8.12V, pin 8 8.3V.
IC2 – pin 1 15.3V, pin 2 22.2V, pin 3 12.5V, pin 4 9.1mV, pin 5
9.1mV, pin 6 13.2V, pin 7 13.2V and pin 8 15.1V.
The current draw without the coil is about 3.04A. The ignition
pickup is a standard magnet/reluctor from a late-model Chrys-ler 360 engine. (M.
K., via email).
The 300VDC supply is being correctly produced by
your cir-cuit as there is 298V at the drain of Q6. This means that all the
circuitry, including IC1 and Mosfets Q1 & Q2, is operating cor-rectly. It is
normal for the transformer to buzz as it is switched on and off to maintain
regulation. We assume that the 14.7V at pin 4 of IC1 is actually 14.7mV as it
should be close to zero.
It would seem likely that your problem is in the reluctor
pickup circuit. Check that ZD5 has 5.1V across it and that you have the correct
value resistors inserted. You can simulate firing the coil by connecting a
momentary short between collector and emitter of Q8. Alternatively, connect a
momentary short between chassis and the anode of diode D12.
The coil should give a spark provided that there is a path-way
from the high tension output to ground. To provide that, insert a paper clip
into the coil’s high tension output and bend it so that there is about a 2mm gap
to the coil’s negative terminal. We do not recommend having an open circuit high
tension output (no spark pathway) as the coil may break down internally.
Revised software for Speed Alarm
I have constructed the Speed Alarm as published in the November
& December 1999 issues and it is great other than the 159km/h limit on the
speedometer. I take my car out on the track occasionally and would like to use
it to check the accuracy of my speedo at 200+ km/h and the 888 display isn’t
really of great help. Have you written any software for the PIC that can
accom-plish this? (B. C. via email).
We have now revised the software so that it allows
the speedometer to operate up to 254km/h. Speeds above 254km/h will be displayed
as 888 to indicate overrange. The speed alarm fea-ture can now be incremented in
5km/h steps from 0 to 255km/h. The 255km/h setting will prevent alarm operation
for speeds up to 254km/h. The hysteresis between the alarm switching on and
switching off is now 1km/h rather than the previous 1.25km/h.
Options such as repeat alarm and upper and lower alarm
threshold selection as well as speedometer disable are still available. The
calibration procedure remains the same.
The revised software is called SPEED254.ASM and SPEED254.HEX
and is available for download from our website at www.siliconchip.com.au
LED ammeter has high earth strap resistance
I built the LED Ammeter (SILICON CHIP,
January 1999) which uses the earth strap voltage to display current status. I
can't fault the theory behind it but I can't get it to work properly.
On the bench it works perfectly using the testing method set
out in the instructions, In the car I turn VR1 fully anti-clockwise and adjust
VR2 for the green and yellow LEDs. I then turn on the parking lights and try to
adjust VR1 so that the yellow LED is on. This is where it all goes out the
window be-cause when you adjust VR1 it immediately progresses up the red LEDs;
ie, the wrong way.
I thought the circuit must have too much gain but looking at
the input circuit (IC1a) it only has a gain of 10. So I changed the
feedback resistor to 5kΩ and it worked better but still adjusting VR1 is of
no use at all and turning on the headlights sends the meter off the scale.
My vehicle has an extremely short earth lead of about 10cm but
I thought this would mean that we would just need more gain, not less. I then
swapped the two leads across the earth lead around and it did adjust up in the
exact opposite way. Is there anything I can do to make this work? (K. S., via
Possibly the short earth strap in your vehicle has a
higher impedance than the design allows for. This would mean you need less gain
and the value for VR1 is way too large. Try using a link for the 10kΩ resistor at pin 2
of IC1a and change VR1 to a 22kΩ trimpot.
Alternatively, you could try increasing the input resistor from
reduce the gain.
Electronic house number with large LED
Has SILICON CHIP ever
published a circuit for an electronic house num-ber? Something along the lines
of a rechargeable battery powered circuit would be good, using those large 70mm
LED dis-plays, either flashing or stable. The circuit would come on
automatically at night and the battery could then be charged by solar cells
during the day. Is there any plan for such a project? It doesn’t sound too
complicated. (P. L., via email).
We published a "LED light House Number" in the
October 1988 issue. It could be adapted to 70mm displays without problems. It
was powered from a plugpack but could be run from a solar charged battery. We
can supply a photostat copy of the article for $7 including postage.
Pro-Logic surround sound decoder
I built the Prologic Surround Sound Decoder published in the
November & December 1995 issues of SILICON CHIP and I
have a weird fault.
The processor seems to be switching between two modes all on
its own. Hooking up a CD player into the 5200 and then some speakers out through
the surround terminals, you would hear the CD playing for a few seconds and then
it would seemingly switch into another mode.
The switching is pretty obvious, as it is louder and clear-er
in one of the modes. The processor will stay in each mode the same length of
time before clicking into the other. What do you suspect may be the problem and
what are your suggestions? (M. L., via email).
The mode changing problem in your Prologic Decoder
does seem to be a strange fault. You do not say at what rate the mode changes
from one to another. At this stage we can only assume that it at the noise
Check that the pushbutton switches (S5, S6 & S7) are not
sticking on when pressed. Also check the mode switch S4b. The wiper of S4a will
connect pin 31 of IC1 to 4V when in the sur-round mode and to 0V when in stereo
mode. S4b should apply 5V to pin 33 of IC6 when in surround mode and 0V when in
3-stereo or stereo mode.
Finally, check the wiring between processor IC6 and IC1,
particularly the noise test inputs at pins 23, 24 & 25 of IC1.
Fence tester only works at night
I recently built the Fence Voltage Tester (SILICON
CHIP, May 1999) which I purchased from Dick Smith Electronics. It works
perfectly but there is one drawback. I finished building it at night and I
wanted to test it out straight away. So I went and tested it out on one of our
fences and for some unexplained reason you can only see the neon light flashing
when there is a light source present.
When you take that source away and it is pitch black, the neon
light either does not work or is not visible. I tested out my theory with a
torch and also with a light globe inside a shed and I found the same result.
I am puzzled at this and would appreciate it greatly if you
explain it. (T. H., via email).
It turns out that since the high voltage pulses are
so short, they are not enough to cause the tester to light up in total darkness.
It actually needs the extra photons from daylight (or from your torch) for the
neon gas to break down and discharge!
Apparently, there is a similar problem in gas arrestors used
for transient voltage protection. These devices have a relatively long response
time and will not work with very short spikes. Some manufacturers incorporate a
small amount of radon gas (a radioactive emitter) in the gas arrestors to
improve their response times.
Anyway, the circuit does work but only if there is some ambient
Connecting a ceramic phono cartridge
Is the auxiliary (AUX) input on a modern audio amplifier
suitable for the direct connection of a ceramic cartridge (for playing 78 rpm
records) or is some additional circuitry advantageous or neces-sary? (L.B.,
Depending on the output of the particular cartridge
and your amplifier’s gain, it may be possible to use the AUX input, pro-vided
each channel of the cartridge is shunted with a capacitor of say 470-1000pF to
improve the bass response. This would be a "quick and dirty" connection which
may be good enough.
However, it is more likely that you will need a high im-pedance
preamplifier with a gain of around 5 or so. To give good bass, typical ceramic
cartridges need an input impedance of at least 2MΩ. It would be possible to
produce a suitable stereo preamplifier using a dual low-noise FET-input op amp
such as a TL072.
Audio signal generator amplitude problem
I have completed constructing the Audio Signal Generator from
the February & March 1999 issues and it appears to be work-ing, at least as
far as my frequency counter and milli-voltmeter can tell. However, without being
"picky", I do have some ques-tions regarding its setting up.
(1) Should VR4’s setting be critical? For mid-frequencies, the
display either locks or it doesn’t. However, if VR4 is set for reliable
operation on the highest frequency range (>50kHz) then for the lowest
frequency range (<20Hz) it needs to be readjusted so that the display reads
correctly; but then the display doesn’t lock on the highest frequency range.
On the three highest ranges, frequency coverage is a multiple
of about 77-106Hz (this after adding a 0.82μF capacitor to pin 2 of the
555). However, I cannot get the lowest range to work/display reliably below
Below 20Hz my millivoltmeter indicates a slightly rising low
frequency output but the display shows all zeros. Adjusting VR4 will restore the
display but then I have problems at the highest end of the high frequency range
(2) The amplitude of the output signal rises as frequency
de-creases; in fact, at 20Hz it is up by about 2-3dB compared to 1kHz.
Attempting to adjust VR3 either kills the low frequency oscillation or allows
the circuit to oscillate supersonically (the AC output hits 10V on the
For stability (25Hz and up), trimpot VR3 is hard clockwise.
(3) Using a digital frequency meter, I had to add a
ca-pacitor to pin 2 of the 555 timer to get the display to read within 5% of the
frequency. Would you expect to need this amount of capacitance increase?
I have checked component values and the DC voltages are all
within spec. Incidentally, for the benefit of other constructors, my kit failed
to function at switch-on because a track on the PC board was missing; it was the
+5V supply to IC3 pin 11. (N. H., via email).
You cannot expect to obtain a flat response if the
oscilla-tor is not set up to operate over all the ranges. Some construc-tors of
the Jaycar kits have needed to change the 12kΩ resis-tor connecting to LDR1
to a 560kΩ resistor with a .0047μF capacitor placed in parallel with it. This will
allow the oscil-lator to be set up to oscillate over the full frequency
VR4 will be critical to set if the output level does vary with
frequency. Once you can set the oscillator correctly, this adjustment will be
The capacitance change at pin 2 of IC11 to obtain a
satis-factory frequency accuracy does seem extreme. Perhaps the origi-nal values
are out of tolerance.
Parking radar has low sensitivity
I have built the Ultrasonic Parking Radar kit (SILICON
CHIP, February 2000) with mixed results. I find the unit has low sensitivity
due to the hysteresis on pins 12 and 13 of IC1d. This gate switches from high to
low when the voltage on its input pins reaches 3.8V but will not turn off again
until the voltage drops back to 2.5V. The result is very low sensitivity with
poor long-range detection.
Also, the LED on the output refuses to conduct with the values
shown in circuit. Q3 switches OK but does not fully satu-rate. Also, I am only
getting 5.7V across the zener diode instead of 6.2V. I cannot make out the zener
numbers and it may be an incor-rect zener as supplied in the kit. Any ideas? (B.
C., via email).
The sensitivity of the radar is set by VR1 rather
than the hysteresis of IC1d.
You can obtain more range by separating the ultrasonic
transducers as described on pages 40 and 41 and in-creasing the value of VR1. To
some extent the gain is set by adjustment of VR2 which sets the threshold
trigger point for IC1d. Set this too high, however, and IC1d will remain
triggered (output low) as its lower input threshold will never be reached.
Perhaps your low sensitivity is due to the low supply vol-tage.
You would expect the LED to light when powered from 12V and 2.2kΩ resistor even if
Q3 does not fully saturate. Check that the LED is inserted the right way around
on the PC board. You could reduce this value to 1kΩ for more brightness if you
A 6.2V 400mW zener will probably be marked as 1N753A while a
5.6V zener will be marked 1N752. You can check the zener by placing it in series
with a 2.2kΩ resistor across a 12V supply. Measure the zener voltage with a meter. If
the zener voltage is OK, you may need to supply it with a little more current in
the radar circuit. Try reducing R8 to 390Ω.
VHF PAL demodulator circuit
Have you published an RF VHF PAL demodulator circuit or project
in SILICON CHIP? What I need is to convert a
VHF signal in RF mode back to composite and/or S-video (Super VHS) output(s) so
that they can be connected to a video camera for recording. Video cameras do not
have a TV tuner built in. S-video output is not important; composite video
output is mandatory. (M. O., via email).
We have not published a VHF PAL demodulator. In
fact, what you are asking for is virtually a complete TV set front end. Have you
thought about using a VCR to do the job? Even a machine in which the transport
is no longer working could be used for your job.
TVI caused by FM receiver
Last year I became a listener to ‘tube radio’ in my home. The
FM stereo decoder is solid-state, from Studio12 in Wales UK. The receiver is a
modified and realigned Kenwood W8, a stereo valved receiver, originally only
‘mono’ on FM, from the 1960s.
The problem is as follows: when I’m listening to just one FM
stereo station, on 102.3MHz, I get TVI (interference) on one VHF TV channel
(TEN, picture centred on 182.258MHz) and on one UHF channel (UHF 31 picture
548.25MHz). Note that the FM stereo sound via the Kenwood on this station
remains excellent, notwith-stand-ing the RFI it is generating.
The interference disappears if I disconnect the antenna lead
from the Kenwood to the wall outlet. It is a "shot-silk" effect, very like FM
transmitter sourced interference as pictured in various books on this subject.
The tuner does not have this effect on any other local FM station.
It seems to be the local oscillator in the stereo tuner, not
the IF in the TV receiver. The tuned frequency is 102.3MHz and with an IF of
10.7MHz the local oscillator is 91.6MHz; its second harmonic = 183.2MHz, slap in
the middle of VHF10 and the 5th harmonic is slap in the middle of UHF 31.
A friend of mine says that the strength of the station is
probably irrelevant and I’ve since confirmed this, at least with a variable
attenuator. That particular frequency, 102.3MHz or very nearly, may be
triggering some resonances somewhere in ‘my’ circuit.
So is there really a resonant problem with the local
oscil-lator when tuned at or near 102.3MHz?
The following actions are possible. I could use separate FM
cables from the TV cables, use two separate split-ters and use single rather
than combined wall-plates everywhere. Apart from the extra expense, the
disadvantages of this solution are having to crawl about under the house and get
up on the roof and lastly, it doesn’t actually fix the RFI/TVI near the
Alternatively, I could get someone skilled in the art to fiddle
with the operating characteristics of the valve tuner. Unless it is radiating
from the tuning gang, extra HT bypasses might help.
I could also filter the harmonics from the FM stage so it
doesn’t go back up the coax to the splitter, without affecting the signal
strength going in; ie, a steep low-pass from about 109MHz up into the UHF band
to 600MHz. Maybe Kingray could help here. What do you think? (T. B., via
We doubt very much that you can easily suppress the
harmonic radiation from your tuner’s local oscillator. Valve local oscil-lators
had a much stronger signal than transistor oscillators and they didn’t have a
clean waveform; ie, harmonics were pres-ent.
In fact, it is doubtful whether the designers actually ever saw
the waveform on an oscilloscope since that would have re-quired scopes with
bandwidth out to beyond 200MHz – such scopes probably did not exist when your
tuner was designed. Even today, with a good scope you might have great trouble
improving the oscillator’s waveform sufficiently to remove harmonic
Our first suggestion is to try the separate splitter ap-proach
and if that doesn’t work, an approach to Kingray might be the only solution.
Spring reverb unit has phase reversal
I have purchased and assembled the Spring Reverb unit
de-scribed in the January 2000 issue of SILICON CHIP. I have
achieved successful reverberation but now have a polarity re-verse/out of phase
problem with respect to the input source. All connections seem correct. Do you
have any suggestion to solve my dilemma? (A. C., via email).
The Spring Reverb module does invert the signal at
IC2b which is an inverting mixer stage. You would need to add another inverting
stage at the output to return the phase to the same as the original input.
However, the phase inversion of the signal by the spring reverb
should not present any problems. In fact many amplifiers invert the signal, as
do mixers and preamplifiers.
Increasing the turbo timer period
I have just bought a Turbo Timer kit from Jaycar (SILICON
CHIP, November 1998). Could you tell me how to modify it so that it makes my
car idle longer, or even make it adjustable like the really expensive ones? (I.
B., via email).
You can change the idle period by altering either
the 220μF capacitor at pin 6 of IC1 or the 390kΩ resistor. Increasing either
value will extend the period and reducing it will shorten the idle time. The
capacitor should not be increased much past 1000μF in value while the resistor
should be in the range from say 10kΩ up to 1MΩ.
If you want to make it adjustable, you could use a
potentiometer in series with a 10kΩ resistor, in place of the 390kΩ resistor.
Spring reverb frequency response
I recently constructed the Spring Reverb kit for use with an
electric guitar. The construction and electrical testing of the unit seemed to
go pretty much by the book. However, I found that when it came to a comparison
between plugging directly into the guitar amplifier and plugging in via the
reverb unit, I did notice an appreciable attenuation of top end frequencies,
although I was quite happy with the quality of the reverb effect itself for a
unit in this price range, .
I realise that this is not a high-end professional unit but am
wondering whether this is an idiosyncrasy of the circuit itself or maybe I’ve
missed some detail in the construction. Any ideas? (C. M., via
The frequency response of the reverb signal is
limited to 5kHz, as detailed in the specifications panel on page 32 of the
January 2000 issue. The response of the undelay-ed signal is up to 19kHz which
should be more than adequate. Without too much re-verb, the overall response
will be dominated by the 19kHz band-width.
If you find the high frequency end is dropping off without much
reverb being mixed into the signal, you have possibly used an incorrect
capacitor value across one of the feedback resis-tors. Check in particular the
capacitor between pins 1 & 2 of IC2b. It should be 33pF. The capacitor will
either be marked as 33 or 33p. Also the capacitor across VR1 should be
(3n9 or 392 on the capacitor).
There is no point in trying to increase the frequency re-sponse of the reverb
signal itself as the response of the spring unit is limited anyway; any increase
in bandwidth would just increase the residual noise.
Query on cordless phone backup
In the article entitled "Backup Battery for cordless phones"
from the October 1999 issue of SILICON CHIP you
have shown the charging circuit as simply being a diode and a 100Ω resistor. Would
this be OK if the unit is working 24 hours a day; ie, charging the SLA battery
I thought that SLA batteries should be charged gradually and
when they reach full charge, the charger switches to trickle charge. I
understand that the 100Ω resistor provides that trickle charge continuously.
Isn’t that damaging to the SLA battery? It would take quite a while to initially
charge the SLA, wouldn’t it? (O. N.)
It is OK to trickle charge an SLA battery and that
is what this circuit does. Yes, it would be a good idea to have the battery
fully charged before installing it in the circuit.
Plastic stereo power amplifier wanted
I like the 175W amplifier modules described in the April 1996
issue. Can I use two modules to a make a stereo amplifier? How do I go about it?
(T. W., via email).
You would need to use the suggested power supply
circuit on page 28 of the April 1996 article but with a 300VA transformer. The
modules would need to wired up in the same sort of layout as we used for the
stereo power amplifier featured in the February 1988 issue. We can send you a
photocopy of this article for $7 including postage.
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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