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Looking for RPM circuit to read fan speed

Discussion in 'Electronic Basics' started by Junkmail789, Oct 9, 2003.

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  1. Junkmail789

    Junkmail789 Guest

    I'm looking for a circuit that would read the speed of a dc fan that runs at
    5hz. I would like the speed indicated on 3 7 segment displays with a
    decimal after the first digit. The circuit would need to be able to be
    mounted on the shaft of the fan.

    Can anyone point me in the right direction?

    Would it be possible to design such a thing using a rotary encoder?


    Matt S.
  2. John Fortier

    John Fortier Guest

    A mirror on the fan shaft, a reflector opto coupler and a counter circuit.
    If you want more accuracy, use more mirrors. for example, with eight
    mirrors at 5 Hz, you will get 40 counts per second. Assuming that by 5 Hz
    you mean 5 revolutions per second.

    From your description you seem to want to mount the whole circuit on the
    shaft. Won't that, at 5 Hz, make it a bit difficult to read, unless you
    grab the fan and swing round with it! So I'm assuming you only want the
    sensor on the fan.

    For the counter and display, there are purpose built modules available, at
    relatively low prices.


  3. Junkmail789

    Junkmail789 Guest


    I'll double check today but I believe there is a little bit of the shaft
    protruding from the rear of the assembly. These are small fans (about the
    size of those used in computer power supplies). So yes you are correct in
    that I only want the sensor part on the shaft itself.

    Thanks for the response, this is a great group :)

    Matt S.
  4. Then there was the old "shining a light through the fan blades intro an opto
    transistor or device to your choice " trick ??
  5. Spajky

    Spajky Guest

    on my site under electronics there is a schematics for non tach wire
    PC fans adapter for measuring Rpm on computer, maybe can help that too
    4 you before counter with display ...

    -- Regards, SPAJKY
    & visit site -
    Celly-III OC-ed,"Tualatin on BX-Slot1-MoBo!"
    E-mail AntiSpam: remove ##
  6. John Fortier

    John Fortier Guest

    Assuming the blades don't overlap.

  7. Then you set them at an angle to see between the blades.
  8. Wade Hassler

    Wade Hassler Guest

    Are there any disturbances on the power-supply leads that you could
    read to determine the RPM?
    (from one who avoids mechanical design)

  9. I think I've seen a typical app in one of Linear Tech's op-amp data sheets
    which does exactly that. But I'm not sure what type of motor it is suitable
    for. I think the rpm info should be present in the power lines, it's just a
    matter of how easy it is to clean it up.

    Smart idea!

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