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Tachometer Input

Discussion in 'Electronic Basics' started by Richard, Oct 9, 2004.

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  1. Richard

    Richard Guest

    I have a device which requires a signal for a tachometer. We have this
    device in the training room for showing / teaching operation to mechanics.

    The device gets it's normal input from a Magnetic Pickup (Inductive pickup)
    used on the flywheel. It goes thru a series diode then then passes by a 5
    volt Zener, then goes into a microcontroller and displays RPM as well as
    other functions.

    I need to make a simulator which I can adjust from 0hz to 5khz to work on
    the above. Currently we are using a gear on a drill press, which works,
    but we would like a small electronic design. I was thinking about a 555
    type circuit, but a 555 puts out a digital signal and the mag pickup outputs
    AC. Does anyone have any simple ideas or schematics I could use for this?

  2. Rileyesi

    Rileyesi Guest

    I have a device which requires a signal for a tachometer. We have this

    You might be able to use a diode between the common of the pickup and the
    common of the 555 circuit to 'trick' the pickup to think it is seeing an AC
    signal. The diode would raise the common of the pickup to a voltage potential
    that is higher than the 555 circuit.

    Just a thought.
  3. Ban

    Ban Guest

    I would try first to just use the output of the 7555 (an improved C-Mos
    version of the 555) with a supply of 5V the output I would capacitivly
    (100uF) couple to the meter . As you have described it, the unit apparently
    only triggers on the 0-crossings, so the waveform is probably irrelevant.
    If this doesn't work you either have to build a real sine generator, or
    filter the 7555 output with a tracking lowpass filter. But 0Hz are not
    A simple sine generator can be made with a pic with filtered PWM output,
    this can also make very low frequencies (or DC), which require
    huge(infinite) capacitors when done analog.
    You're welcome
  4. Richard

    Richard Guest

    I think this should work directly off the 7555 because on my scope, I have
    the frequency output pin used for calibration which outputs a 2khz signal
    which looks like 5vdc square wave (if I remember correctly it was 5 volts)
    and the tachometer did indeed pickup the signal,
  5. Yes, I'd certainly have expected it to. Are you confirming your
    problem is now solved, and that you have a 555 astable connected
    directly to your Tachometer?
  6. Richard

    Richard Guest

    No, it's not solved yet because I haven't built the 555 circuit yet. I just
    sent the cal. freq. from my scope to it to see if it would read a digital 5
    volt pulse.
  7. Rich Grise

    Rich Grise Guest

    Well, build the 555 circuit, and just put it to the same place you put
    your 5V. pulse. Power the 555 from 5 volts, and you're done. (you did
    say that it did count the 5V pulse, right?)
  8. Richard

    Richard Guest

    Built a 555 circuit and tested it by hooking it up to my scope. The 555 is
    working fine however
    I do not get a RPM reading on the device. So I gues it is expecting a zero

    BTW, my calibration frequency on the scope was 0.5vpp and it does not
    trigger the device consistently either.

    Anymore ideas?

  9. Ban

    Ban Guest

    Put the capacitor between the input and the timer, like I had described in
    my first post, this will shift the DC-level, so the input signal goes below
    0, even when the timer is fed from only +5V.
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