Tachometer mod ?

Discussion in 'The Projects Forum' started by Potter, Jun 1, 2012.

  1. Potter

    Thread Starter New Member

    Oct 16, 2011
    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.

  2. wayneh


    Sep 9, 2010
    So, you already have something that works but you need to multiply its output (Hz) by 60 (to get rpm)?

    I'd just make a small lookup table or chart. Example: Read 50Hz, find 50 in the table, read off 3000rpm. Done. Before long you won't even need the table.
  3. Potter

    Thread Starter New Member

    Oct 16, 2011

    I had tried that but it's a very 'clunky' solution. If you just want to measure the RPM then it's OK but trying to make tiny adjustments to the speed of a chainsaw that is revving at around 13,000 RPM and with the time restriction also, it's just too complex.

    Anyway, as no-one else had offered an opinion, I went ahead and knocked the circuit up just to test it. It works fine and with a magnet attached to the chuck of my minilathe it matched the lathe tacho exactly, well at least up to 1000 RPM. I was not prepared to take it any higher as even though I had taped the magnet on as precaution, the thought of what could happen if it let go is scary.
    If anyone else is interested, the series resistor is not needed.