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Thread: Motorcycle tacho using teensy electronics guidance.

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  1. #1
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    Motorcycle tacho using teensy electronics guidance.

    Hello all.
    I got myself a dirt bike and would like to put a tacho on it to display rpm and log it along with engine temp, maybe some other stuff who knows.
    What is a good way to do this?

    The comercial ones seem to use the primary or secondary of the ignition coil for this. Either resistive connection or inductive non contact setup.

    My bike (suzuki DR-Z250) has CDI ignition and I could tap into the signal coil(>1.2V), pickup coil (>5v)or generator coil (>75VAC unloaded)or ignition coil primary(measured >250V)

    The lowest current (emmiter side) optocoupler I found was a H11L1M.
    I'm probably being silly but I'd rather not put any load on signal or pickup coils for fear I could somehow alter ignition timing or spark voltage. Is that silly?

    So I'm thinking the generator windings are the safest bet as power draw isn't an issue (about 1W I'll be using) and its voltage is much lower than ignition.
    My motorbike manual says this should over 75VAC unloaded, a limp kick start gives me 15VAC.
    I drew up a circuit that I don't think would damage anything if I came off two generator windings.
    Worst case i'm consuming 1W so if I spec everything high it should be reliable all siliconed up in a tight spot?

    Let me know your thoughts.
    If you've already done this tell me how you went about it.
    Thanks.
    Click image for larger version. 

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  2. #2
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    Note that the typical motorcycle voltage regulator works by shorting out the alternator. But I think enough of the waveform will remain for this to work. Circuit looks OK to me.

  3. #3
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    Thanks for the tip. That will work out good if thatís the case. Iíll start it up tomorrow and check how it works and double check my specs and resistor values. They may need slight change as lowest voltage I allowed for was kickstart with ignition off. Might pull lower with headlights etc on.

  4. #4
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    Works well. triggers on at 6.8Vdc and off at 5.4Vdc on 15vAC generator coils.
    https://www.youtube.com/watch?v=fUtgJjUKdVQ

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