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Frequency and Capacitance meter

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  • Nicolae Sfetcu
    Frequency and Capacitance meter From: kenny@m.cs.uiuc.edu An [...] idea is to use the 555 as a monostable, and trigger it with a fixed frequency clock. Duty
    Message 1 of 2 , May 29 1:24 PM
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      Frequency and Capacitance meter

      From: kenny@...

      An [...] idea is to use the 555 as a monostable, and trigger it with a fixed frequency clock. Duty cycle will be proportional to capacitance.

      Circuit will look something like:

      +5 ---+---------------+---+
            |               |   |
            R            +----------+
            |(see below) |  8   4   |
        +---+------+-----|7        3|------/\/\/\---+------- Vout
        :          |     |  LMC555  |               |
        C to test  +-----|6         |             -----
        :                |          |             -----
      ground       +-----|2        5|----+          |
                   |     |    1     |    |        ground
                   |     +----------+  0.1 uF
                   |          |          |
         Clock ----+        ground     ground
           ^
      (see | next section - CMOS Oscillator)
      

      The ON time for the monostable is about 1.1RC, so component values that should work would be a 50 Hz clock, say a 1 Hz low-pass filter on the output, and R = 9.09K, 1%. That combination will give an output of one volt per microfarad. Switch R in decades for smaller capacitors. Trim R for calibration.

    • Nicolae Sfetcu
      Frequency and Capacitance meter From: kenny@m.cs.uiuc.edu An [...] idea is to use the 555 as a monostable, and trigger it with a fixed frequency clock. Duty
      Message 2 of 2 , Oct 30, 2001
      • 0 Attachment

        Frequency and Capacitance meter

        From: kenny@...

        An [...] idea is to use the 555 as a monostable, and trigger it with a fixed frequency clock. Duty cycle will be proportional to capacitance.

        Circuit will look something like:

        +5 ---+---------------+---+
              |               |   |
              R            +----------+
              |(see below) |  8   4   |
          +---+------+-----|7        3|------/\/\/\---+------- Vout
          :          |     |  LMC555  |               |
          C to test  +-----|6         |             -----
          :                |          |             -----
        ground       +-----|2        5|----+          |
                     |     |    1     |    |        ground
                     |     +----------+  0.1 uF
                     |          |          |
           Clock ----+        ground     ground
             ^
        (see | next section - CMOS Oscillator)
        

        The ON time for the monostable is about 1.1RC, so component values that should work would be a 50 Hz clock, say a 1 Hz low-pass filter on the output, and R = 9.09K, 1%. That combination will give an output of one volt per microfarad. Switch R in decades for smaller capacitors. Trim R for calibration.
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