How do I cut pulse signals in half?
How do I cut pulse signals in half?
(OP)
I have an electronic tachometer on an engine that reads the pulse from one of the ignition coils. RIght now, the coil is fired once per engine revolution. I have a new elecronic ignition I would like to install, but it fires the coil twice per revolution, so the tachometer will read 2x actual engine rpm.
The ignition manufacturer says I can "adjust the value of the resistor in the tach", but what does that really mean?
Bearing in mind I am not an Electronic Engineer, can someone explain this to me, or offer an alternate method of getting the tach to read every second pulse?
Thanks
Rick
The ignition manufacturer says I can "adjust the value of the resistor in the tach", but what does that really mean?
Bearing in mind I am not an Electronic Engineer, can someone explain this to me, or offer an alternate method of getting the tach to read every second pulse?
Thanks
Rick





RE: How do I cut pulse signals in half?
Is it a 2-cylinder (4-stroke) engine !?!?
"...from <~>one<~> of the ignition coils."
Ah, a GM motor with a coil for each two cylinders ??
Many commercial aftermarket tachometers intended for cars will have a 4/6/8 (cylinder) switch inside. The switch simply calibrates the analog meter - trivial.
If you adjust the same resistor value, you could make the tachometer work with just about any ignition-equipped engine (for example 1, 2, 3, 5, 10, or 12-cylinders).
This approach is far easier than dividing the incoming pulse train by two (a flip-flop plus ~much~ signal conditioning).
RE: How do I cut pulse signals in half?
The tachometer is also a Honda tach, so there is no switch to adjust it.
Can you elaborate on "adusting the resistor value"?
Rick
RE: How do I cut pulse signals in half?
Another approach to consider would be to connect the tach to the original points (presumably still with the original pulsing) before - rather than after - the new ignition module that provides the double pulsing to the coils. This assumes that the new module 'loads' the points with voltages similar to the original circuit (perhaps not likely). It is most likely that the new circuit uses the points as a low voltage switch (~5 volts, not 12 volts). Then again, it is quite possible that the tach might accept the lower voltage signal as a valid trigger.
It is also possible that the tach would ignore the double pulse if the timing was sufficiently fast that the second pulse could be interpreted as point bounce. Then again, this approach might only work over a limited rpm range.
If I were in your shoes, I'd work backwards through the options listed above.
1) Try it to confirm it doesn't work properly as is.
2) Rewire it to the points, instead of the coil.
3) Modify the tachometer resistor value.
For option three, see if anyone on the WWW has information about the correct value to make it easier. You'll need to Google the subject using model numbers (motorcycle, ignition module) and the words 'tach' or 'tachometer', and 'resistor'. You might have to bang on Google a few times with various search terms to see if anyone has crossed this bridge before or not.
RE: How do I cut pulse signals in half?
However, Japanese manufacturers many times used di-arsonal movements and completely different circuits. Sometimes, Japanese products sold in the US use tachs custom-branded from the US manufacturers, but usually only in smaller markets where volume is low.
The input to a tach contains a first stage that is a filter and/or transient limiter (if pickup is directly off a coil). Usually just a R/C followed by a zener. This then drives a transistor or schmitt-input device to square the input and avoid extra pulses. The switch theshold is usually 4 to 10 Volts.
If you need exactly a divide-by-two, you could attempt to do a little circuit - RC filter into a transistor or comparator - and then into a flip-flop configured to divide-by-2 - and then into a transistor (level-shift) into your tach. However, this will take a little playing around, some basic electronic knowledge, and possibly a scope to do it.
RE: How do I cut pulse signals in half?
Typically some basic input filtering (C + R + diodes), a one-shot (not a chip, perhaps a JFET?) triggered by the ignition pulse, and perhaps a filter on the meter to still the quivering needle at low rpm.
YMMV.