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comparator to pin 5 of IC2 LM3914?
(S. S., Adelaide, SA).
• The knock sensor should operate
on the Multi-Spark CDI if the ignition
coil input is altered to handle the
higher voltages. This would involve
using 1W resistors for the 15kΩ values
connecting to pin 6 of IC1d.
An alternative method of driving
the knock sensor would be to use
the ignition coil input from the tacho
meter signal on the Multi-Spark CDI.
Unfortunately, the signal will need
to be in
verted using another common-emitter amplifier in a similar
arrangement to Q5. Connect a 10kΩ
resistor from the collector of Q5 to
the base of another BC337 transistor.
Connects its emitter to ground and
the collector to the 12V supply via a
10kΩ resistor. The collector output
will be an inverted tachometer signal
suitable for driving the ignition coil
input of the knock sensor.
Remote control
extender not happy
with Mitsubishi
I have built up three of your IR Remote Control Extenders, as described
in the July 1996 issue of SILICON CHIP.
I have found them to be very useful in
most applications. However, when I
use one of these units for controlling
Mitsubishi TVs or VCRs then the only
way they will work is if the actual
remote control is within 1cm of the
IR extender.
I suspect the IR receiver in the
circuit does not have sufficient bandwidth and/or sensitivity for the Mitsu
bishi units. Are you able to help by
recommending a circuit modification
or by suggesting a better IR detector
which I could use? (G. T., via email).
• We don’t have any information on
this problem. Does anyone know of
a solution?
Converting the
insulation tester
A few years ago I built the Insulation Tester described in the May
1996 issue but have not found it
very useful as the minimum resistance it measures is 1.4GΩ. A
reading of less than this implies a
fault but in fact the AS3100 requires
a resistance of >1MΩ so a reading
of <1GΩ may be still be well within
the standard.
It was therefore with interest that
I saw the article detailing a Megohm
Tester in the January 1999 issue. I
noted that there were some similarities between the two circuits and
wondered if it would be possible
to convert the older design to read
LED ammeter green
LED always on
I have a problem with the LED Ammeter described in the January 1999
issue of SILICON CHIP. Everything is
fine with the circuit apart from the
extreme RHS green LED being continuously on after power up. No amount
of adjusting VR2 can change this.
I have replaced IC1 but it made
no difference. I also noted that the
10µF electrolytic capacitor’s positive
electrode is connected to D1 in the
overlay diagram but is connected
to pin 4 of IC1 in the schematic. I
oriented the capacitor this way but
still no difference was evident. (N.
P., via email).
• The 10µF capacitor should have its
negative electrode connected to pin 4.
The circuit diagram is wrong. Thanks
for bringing this to our attention. As
far as LED1 is concerned, when pin
5 of IC2 is at 0V, LED1 should be
in MΩ instead of GΩ. Is it possible,
without great effort or expense, to
do this conversion and if so, could
you please tell me how? (D. R.,,
Mansfield, Vic).
• As you have noted, the circuits
are quite similar in principle. They
differ only in the impedance of the
leakage testing circuit. To make
your Insulation Tester read in
Megohms you need to replace the
circuitry involving IC4 and IC5 with
that involving IC2b and IC3b in the
Megohm Tester.
It would not be necessary to
change the op amps as the CA3140s
can perform exactly the same function as IC2b and IC3 in the Megohm
Tester circuit. The LM3915 circuits
in both testers are identical.
off. Adjust VR2 to get 0V at pin 5 of
IC2. You can check this with your
multimeter. Also check that -5V is
present at pin 4 of IC1.
Notes & Errata
Digital Capacitance Meter, February
1999: On the circuit diagram on page
68, trimpot VR4 should be connected
to the +5V rail, not 0V. This is correctly shown on the wiring diagram
on page 70. Also, on the circuit, the
resistor between pins 5 & 6 of IC1b is
shown as 20kΩ but appears as 22kΩ
on the wiring diagram; either value
will work.
Command Control Encoder, February
1998: Under some circuit conditions,
the buffer involving op amp IC8a may
act as an inverter and this upsets the
circuit operation. To avoid this, connect a 1MΩ resistor between the +12V
rail and pin 3 of IC8. No other circuit
changes are necessary.
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
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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
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JUNE 1999 93
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