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Tachometer and frequency division issue.

Discussion in 'General Electronics Discussion' started by highvoltagefeathers, Dec 6, 2012.

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  1. highvoltagefeathers


    Dec 6, 2012
    Hey guys!

    I'm new to this forum, but I'll try to keep the noobishness to a safe level.

    I've built a gas turbine engine using an automotive turbocharger as many have, and now need an optical tachometer.

    I've bought an omron E3X-A11 photosensor unit and an E32-D24 2M fiber kit.

    I'm basically just painting the compressor wheel nut (the hardware that holds the rotating assembly together on the intake of the engine) half bright white, and half black, and then running the fiber set (light source and fiber running to detector) right up to the side of the nut.

    The unit simply put out a 12V square wave signal, of one cycle per revolution.

    To make for a simple tachometer setup allowing me to quickly and accurately monitor the operating speed of the engine, I'm planning on using an aftermarket automotive tachometer (0-8000 RPM)

    The tachometer I'm looking at accepts a 12 volt square wave signal from the ignition system, and (on its "4 cylinder" setting") will assume one cycle per revolution.

    Only problem is, the turbine engine runs just shy of 80,000 rpm.

    I needed to make a frequency reduction circuit, to reduce say 1330Hz to 133Hz, which will be correctly (reduced by a factor of 10) interpreted by the tachometer.

    I'm planning on using the 4017 decade counter, and its pin 12 square wave division feature.

    Will this counter do its magic on a straight 12 volt system, or do I need to reduce the signal to 5 volts, and then amplify the 5 volt output to the 12 volts the tachometer is expecting?

    The amplification I can handle in a blink with a mosfet, but I'm not sure a 5v regulator would operate smoothly at those frequencies.

    Any input is appreciated!
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The 4017 should work from 12V. Just make sure that the 12V is clean (i.e. no spikes and other transients).
  3. highvoltagefeathers


    Dec 6, 2012
    I see. There are several (rather "rough") pwm motor controll circuits running on the same power supply.

    Would I be safe running a choke on the supply running to the counter, and the photosensor?
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