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From: kenny@m.cs.uiuc.edu
Newsgroups: sci.electronics
Subject: Re: Freq. and Cap. meter questions
Message-ID: <21000096@m.cs.uiuc.edu>
Date: 30 Aug 90 21:24:00 GMT
References: <33045@unix.cis.pitt.edu>
Lines: 36
Nf-ID: #R:unix.cis.pitt.edu:33045:m.cs.uiuc.edu:21000096:000:1389
Nf-From: m.cs.uiuc.edu!kenny    Aug 30 16:24:00 1990


Earlier today I wrote:
>For a capacitance meter, put the capacitor under test into the
>control circuit of a 555 astable.  The output frequency is in inverse
>proportion to capacitance, and you can take it from there.

A better 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|---------------^v^v^v---+------- Vout
  :          |     |  LMC555  |                        |
  C to test  +-----|6         |                      -----
  :                |          |                      -----
ground       +-----|2        5|----+                   |
             |     |    1     |    |                 ground
             |     +----------+  0.1 uF
             |          |          |
   Clock ----+        ground     ground
(see below)

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.

Kevin, KE9TV
kenny@cs.uiuc.edu
...but moving to...ke9tv@nrtc.northrop.com