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Simple Capacitance Measurement

People on the ladyada.net forums have all sorts of projects they work on. Mike Witt wanted to measure capacitance but his meter doesn’t have a C measurement mode.
However, he remembered that in 555 timers, the frequency is dependent on capacitance so he whipped up this super-simple capacitance meter. Initially I suggested going with the AC waveform method but this is probably just as good and much easier. Just vary R1 and R2 to get different frequency ranges for measurements

(I forgot to add values, R2 and R1 are both 1k ohm
Q is the output, that’s what I hook to my scope.

C_HI is the high end of the cap (if you want to test a polarized cap)
C_LO is the low end. –these don’t matter on standard ceramic caps

Vcc should be between 5-9v, but can be up to 15v safely. 5v is a good TTL level.
To find out C, use the equation C=t/R (from t=RC)
Since my scope only gives frequency, I substitute t=1/freq
We know R (R=r1+r2), so we get
C=t/2000
In my test case, I get freq=2.25676kHz
that means that freq=2256.76Hz
So…
C=(1/2256.76)/2000
~0.00044/2000
~.00000022155 F
~.22 uF

The Cap I used was labeled 224J, which makes it a .22uF +/-10%

Many multi-meters have frequency counting too


Read more about it on the forums



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1 Comment

  1. You are very kind as to share such valuable information with us mere idiots.

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