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Originally Posted by ianbartie
.....The original pulses from the ECU were a perfect 12V square wave 50% on & 50% off which only varied in frequency. The tacho in these newer VWs are a stepper motor type so I am guessing it was a PWM signal the ECU was outputting to drive the tacho.
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If your sure it was a constant 50% duty that only varied in frequency then it can't be PWM to drive tacho. Lets just approach it then as varying frequency with fixed duty.
Quote:
I am thinking along the lines of some sort of 555 circuit that outputs the correct 50% duty cycle square wave with the frequency being altered by the Cherry sensor input.
Simple, huh - except I have no idea how to do it.
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I'd use the sensor wheel you made up with 4 equal spaces per revolution. Feed cherry output into the attached circuit (input pullup required as well as decoupling cap on pin 14). It is a divide by two flip-flop made from a CMOS 4013 (dual, but we use one). Output will be 2 pulses per rev at 50% duty regardless of frequency.