looking for circuit to convert signal from coil to ttl input for tachometer

[Build-A-Burger]

I want to find a circuit to level translate the output of the coil (12v) to the input of a microprocessor.

 

[BradtheRad]

The coil... Does this refer to the spark coil? It is risky to give it a direct connection to low voltage circuitry.

Instead consider placing a sensor near a spark plug wire. It might be a simple coil of wire.

Or, I once wrapped a bit of aluminum foil around the main spark wire, then jumper-clipped it to my oscilloscope. I triggered the sweep off #1 cylinder. Then I could watch the intensity of spark to all cylinders, one after the other.

Be careful not to squeeze spark plug wires. High voltage can penetrate the insulation. That is why your sensor should not touch the wires, but be located at a small distance.

 

[Build-A-Burger]

Well that makes sense. Someone once told me a small coil of wire in parallel to the spark plug wire would be enough. I imagine then a 5v zener diode and an RC in parallel?

 

[BradtheRad]

Well that makes sense. Someone once told me a small coil of wire in parallel to the spark plug wire would be enough. I imagine then a 5v zener diode and an RC in parallel?

I suppose the zener is to make sure the microprocessor is not exposed to greater than 5V.

The capacitor might be necessary, to sample-and-hold a signal long enough for the microprocessor to detect it. (It is common to add a diode to prevent discharge back through the pickup device.)

The spark is so brief, it's a natural question to ask, why not tap into the 12V ignition circuit? That would give you a more usable signal. So if you know exactly which wire to tap into, and you can do so with no risk of disabling your vehicle, then that might be worth a try.

However it seems easier to place a sensor near the spark plug wires. You'll just need to experiment with a few pickup devices. Also some kind of waveform conditioning circuitry.