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OT: Electronics question, tach drive adaptor

I

Indiana Ken

Well-known Member
I have an electronic tach I would like to use on a single cylinder 4-stroke engine. My engine has a crank trigger and fires once each 360 degrees of crank rotation. The tach is for a single cylinder 4 stroke however, it appears to be calibrated for a spark every every 720 degrees of crank rotation.

The tach requires 12 volt power and takes a signal from the (-) terminal on the coil. It works fine except, it reads 2 times the RPM the engine is actually running.

Is there a device I can buy or a circuit I can make that will count to two then output one signal, such that the tach will read actual engine RPM?
 
If possible mount a pickup sensor where it will pickup a signal from the cam shaft gear,sprocket.That would give you 720% per 360% crank rotation.
 
Just a guess, but if this is a universal tach,
there may be jumpers, different input lead
combination, or dip switch setting inside the
case. Could pull up instruction manual online?
 
An electronics person could design a circuit around a T flip-flop. Should already be an adapter out there. Try googling for "tachometer adapter divider"
 
The problem to make an adapter is the input to your tach expects to get a sharp pulse from the coil. If you run that pulse through divider circuit, the output of a digital device may not be strong enough to trigger the tach.

I googled "tachometer pulse divider" and got a lot of hits, but none of them appear to do what you need: take the pulse from the primary side of a coil, condition it so it doesn't blow up the rest of the circuit, divide it by two, then generate a pulse strong enough to trigger the tach.
 
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