I have a cheap optical tachometer that I would like to convert to read the RPM of a chainsaw motor by sensing the ignition pulses or the field of the flywheel magnet. My plan is to use a small induction coil with a series connected diode (for polarity), a 'snubber' diode to take care of any reverse EMF spikes and a series resistor to avoid overloading the existing circuitry. If I break the positive lead of the light sensing diode and insert an SPDT switch, a lead from the coil circuitry plus a separate negative lead to the new circuitry, this should allow me to use the tachometer in either optical or induction mode. Does this sound feasible and would the suggested simple 'add on' circuit work? I have tested the principle by winding a small coil and measuring the frequency with a DMM and get good results - reliable (and sensible) readings with the coil just resting on the (plastic) engine cover over where the spark plug/HT lead are. Same for placing the coil close to the flywheel magnet. However, these reading are of Hz not RPM and, as it's recommended that a chainsaw is not run at full RPM under no-load conditions for more than 20-30 seconds, there would not be sufficient time to do the math conversion whilst adjusting carburettor settings. I would be grateful for any input/suggestions. Dave.