I have a tacho reader that i have working on a 5v car circuit, but I am trying to get it to work on another car with a 12v circuit.
The pickup on the 12v circuit is from the coil(-) on the car. It's a pulse from 12v high to 12v low.
The circuit that works on the 5v car circuit is basically just a 5v zener facing to limit nothing (positive) coming in, and only earth going out, and that is pulled high with a 10k and connected to an attiny input. So every pulse the input line goes low then comes back up. I know there's other ways of doing it but this limits interference with any other tacho on the same line.
But the problem is how to stop 12v from coming through the 5v zener on the 12v circuit?
The first time i hooked it up, the zener let everything through and the attiny broke...
Currently it goes from the 12v coil (-) through a 4k7 to where it meets two zeners, one to ground, and one to the input line which is pulled high with a 10k. Both zeners (the lined end) are facing towards the 4k7. Now the attiny (new one) only chucks a fit when the line is connected but still works again when not connected....
I'm more of a programmer than a electronics guy..Does anyone know the right set up to use for this case?
Basically the 12v should clip low to 0v. I have a couple of schottky's 1n5819 would they be better?