Dual TLC555 Capacitance Meter 0.1pF Resolution

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Two 555 timers turn capacitance into an analog voltage readable on a DMM. With a modern 600.00mV scale, that is readable to 10femtofarad.

steven-j-greenfield

Steven J Greenfield

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Clearer schematic and block diagram added
01/05/2022 at 21:56 • 0 comments

I have redrawn the schematic so it is more clear. I also drew up a block diagram with waveforms and explanation of circuit operation.

I welcome questions and comments.
Some parts have drifted
01/04/2022 at 20:08 • 0 comments

It has 5 scales based on capacitance per mV. 1pF, 10pF, 100pF, 1nF, 10nF. AKA full scale 200.0pF, 2nF, 20nF, 200nF, and 2uF. In addition, the highest scale is useable to 20uF if you let the meter autorange to 2V full scale.
I noticed the middle scale, 20nF, is not in agreement with the other scales. I suspect that resistor has drifted. It is, after all, about 30 years old.
My intention is to replace the resistors with modern metal film 1% and recheck it. Then build a surface mount version as a multipurpose kit, useable as a capacitance meter, capacitive sensor or switch including for remote use, and as a liquid level measurement with continuously variable output.