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Measure spindle speed with a Multimeter???

Discussion in 'General Electronics Discussion' started by Les Fackrell, Nov 26, 2016.

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  1. Les Fackrell

    Les Fackrell

    Nov 26, 2016
    I am in the market for a replacement multimeter and I have noticed that some have a frequency counter. This made me wonder if a simple circuit using a light dependent resistor, some shiny sticky tape and a battery would make a signal that the meter could Count? If the reflective tape were stuck on the spindle and a light shone on it, the LDR would change its resistance depending on the reflected light. Would this make a signal that the meter would pick up?
    You will probably guess that I am a complete novice in electronics. (I am aware that you can buy a tachometer cheap as chips on Ebay but I just wondered)
  2. Bluejets


    Oct 5, 2014
    Think you will find it requires a changing voltage.
    A hall effect (A3144), magnet, collector resistor and a battery should work.
    Take the signal from collector to ground.
  3. Alec_t


    Jul 7, 2015
    LDRs are rather sluggish, so you would get unreliable results at high spindle speeds.
    (*steve*) likes this.
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    If you wanted to go with an optical solution, an opto-transistor would be a better bet.

    Some problems you might have to deal with are ambient light levels, 2 x mains frequency flickering of lights, and enduring you get enough light to the sensor.

    Some form of chopper that interrupts a light source pointing directly at the sensor may be better if you can achieve it.

    CDRIVE Hauling 10' pipe on a Trek Shift3

    May 8, 2012
    Does your spindle have any ferrous gears or other ferrous protrusions?

  6. Nico Orru

    Nico Orru

    Jun 7, 2017
    I have obtained reasonably accurate results:
    - gluing a tiny neodymium magnet to a broken bit
    - wiring up a U18 hall sensor (latching, but should not be necessary)
    - using an oscilloscope with frequency meter to read the result. You can probably achieve the same by using a multimeter (with freq.meter) instead.

    20170607_213312.jpg 20170607_213329.jpg
  7. FuZZ1L0G1C


    Mar 25, 2014
    Many DMM's these days have built-in frequency measurement.
    Found (on Major-Tech and Top-Tronic DMM's) that they read sinewaves more reliably than square (maybe as square contains harmonics*).
    Also, according to their own manuals, voltage level should be at least 10V RMS AC.
    * Reads the fundamental of square-wave, but 'hunts' up & down as well.
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