I agree with circuitous. Most tachometers just take a pulse train from the ignition, so all you need to do is vary the pulse frequency instead of the pulse width.<p>On my Honda NC30 project bike, I&#x27;m feeding the stock tachometer with a variable-frequency pulse train from an Atmega328. The tacho just wants its input pulled to ground with a frequency equal to 4x the RPM, so one external transistor is all that&#x27;s needed.<p>(The 328&#x27;s main function is for talking to the wideband O2 sensors and sending that data over the CAN bus to a Microsquirt ECU, but it works nicely for also reading engine RPM over CAN and outputting the signal to the tacho.)