Posted on Feb 5, 2014

U1a is an oscillator and U1b the measurement part of the circuit. It converts unknown capacity into a pulse-width modulated signal the same way an automotive dwell meter works. The meter is linear so the fraction or percentage of time that the output is high is directly proportional to the unknown capacitance (CX in the schematic). Meter M1 reads

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the average voltage of those pulses since its mechanical frequency response is low compared to the oscillator frequency of U1a.

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