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Build a stroboscope for measuring the RPM

  1. Jun 19, 2009 #1
    Hey, I'm trying to build a stroboscope for measuring the RPM of a few different fans.

    So far I've tried using LEDs but haven't been very successful. I tried using a function generator with square pulses (I tried both 20% duty cycle and 50%). So I started at 1 Hz and kept on increasing the frequency. Unfortunately, the small square of tape that I put on the face of the spinning fan (as well as the sticker) never really came into focus. I could see it kind of slow down, but then it never quite got to the point where it was strobing at the same frequency (I adjusted down to a tenth of a Hz).

    The fan I'm using as a test subject does indeed have a tachometer so I know I was running it at about 4000 RPM. If I'm not mistaken I should have been able to get the strobe in synch at about 67 Hz. Unfortunately, I could never really get it to work.

    The circuit itself is just a few LEDs in series with a 100 ohm resistor, I adjusted the voltage to make them fairly bright. I put this on a breadboard, but I'm assuming that the capacitance of the breadboard shouldn't have an effect at such a low freq (0 to 100 Hz).

    Does anyone have any hints on how to accomplish this? As far as I know LEDs would work for this purpose. I feel like it would be better if the duty cycle were lower, but my function generator only goes down to 20%.

  2. jcsd
  3. Jun 19, 2009 #2


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    Staff: Mentor

    Re: Strobe

    Were the lights all out? Also, you are right that a shorter duty cycle should sharpen the focus of the strobe effect. Can you make a one-shot circuit that is triggered by the rising edge of the signal generator output? Something like a flip-flop with RC feedback or a 555 circuit. Both will need a power amp out, like a 74AC245 or similar gate.
  4. Jun 19, 2009 #3


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    Science Advisor

    Re: Strobe

    You'll never, ever get it to freeze dead. Slight variations in line voltage and air currents will ensure the fan speed continually drifts up and down.
  5. Jun 20, 2009 #4


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    Science Advisor

    Re: Strobe

    If the LED was on for 20% of the time, the fan would rotate 72 degrees in this time, even if it was synchronized.

    Your sticker would appear to rotate one way then the other as you changed frequency, although it would not look like a sticker.
    It would look like a bar of white light occupying an arc of about 72 degrees.
    But you might be able to just watch for one end of the bar to stop rotating one way and start rotating the other way.

    Commercial stroboscopes do a good job of freezing a fan, so it is possible.
    One time I tried it, I could see that the shaft and the fan were not turning together so the fan blade was slipping on the shaft. Probably never would have suspected this.
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