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Building a Frequency to Voltage Converter

  1. Jan 12, 2009 #1
    I found an old circuit diagram online but have never been able to find it again.

    Here is what I am trying to do:
    The three-phase DC motor I am working with is a 4-pole motor. Therefore there is a direct relationship between the frequency between the motors phase-leads, and the current RPM of the motor. IE - 2 magnetic cycles per revolution

    What I need to do is take frequency measurements between the leads and convert that to a voltage that is representative of the motors RPM, which i can then display on a LCD voltage meter. I believe this to be a much cheaper method of obtaining a tachometer (plus it is an interesting project).

    Interesting side-note: Using wheel radius and gear ratio I could easily output MPH using op-amps, although it would be inaccurate while the car is gliding in neutral.

    If anyone has any ideas or links to a good circuit diagram it would be greatly appreciated. My background is in Physics, so my ability to design complex circuits like this is very limited without extensive research.
  2. jcsd
  3. Jan 12, 2009 #2


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    Staff: Mentor

    You can make an analog frequency-to-voltage converter with a differentiator stage, but I'd be inclined to do this project digitally instead.

    The function that you are asking about is basically a "frequency counter". Check out this page and the links at the bottom to see if they are of help (especially the "building your own frequency counter" link):


  4. Jan 12, 2009 #3
    The simplest approach I can think of is to use a 555 timer to generate a pulse slightly shorter than the period of your highest frequency. Pass the frequency through a Schmidt trigger so you have a clean waveform to trigger the 555. Read the output of the 555 with a digital voltmeter. The voltage may be a little erratic at the lowest frequencies.
  5. Jan 12, 2009 #4


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  6. Jan 13, 2009 #5
  7. Jan 13, 2009 #6
    I guess I need to be more specific, the frequency I am reading comes from a MC that is outputting 91V pulses, from what I read that national semiconductor model will not like having an input voltage that high.
    Correct me if I am wrong.
  8. Jan 13, 2009 #7
    R1 along with D1 & D2 will limit your input voltage to +/- 0.7 volts. It'll probably still work but R1 should be at least 1/4 watt.
  9. Jan 14, 2009 #8
    What is the frequency to voltage ratio you want? In other words what are the lowest and highest frequencies you expect and their corresponding voltages?

    The circuit you reference is similar to what I was suggesting above but is set up to produce a 200 uS second pulse maximum. This seems a little a little short for your application. Depending on your frequency range you will probably have to increase C4 significantly.
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