I upgraded the flywheel in my car and this replaced the 2x per rotation pulse to a 36-2 crank angle tooth pattern, so the (special) variable reluctance (VR) tacho that was in the car didn’t work anymore. To fix this situation, I built a little circuit to take the analog VR signal and turn it into a 5 volt pulse using an LM1815n chip, and the use pulse counting on an attiny microcontroller to turn the rather high frequency input into a lower frequency output, which in turn I connected to a ‘regular’ (12v pulse counting) tachometer pcb to move the galvano motor.
The circuit on a piece of perfboard. LM7805, some caps and ISP header on the right, 8 pin socket for the ATTiny, and a 14 pin socket for the LM1815n chip. Transistor to send pulses to the tacho not yet placed.
Back of the board.
Sadly, I never finished the project before I changed cars, though the LM1815 lives on on board of my Speeduino ECU, which will soon control the engine of my race car.