I have a capacitor in the picofarad range. That capacitor will change slightly ±5 picofarad. How can I detect that change?
I used two parallel plates (about 5cm gap) that gives 18pF.
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\$\begingroup\$ Look here hyperphysics.phy-astr.gsu.edu/hbase/electric/capac.html and you will get an idea. \$\endgroup\$– JuniorCommented Dec 3, 2015 at 7:23
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1\$\begingroup\$ Are you trying to detect the change (yes/no) or to measure displacement precisely? If the latter, how precisely? And also how fast? \$\endgroup\$– Spehro 'speff' PefhanyCommented Dec 3, 2015 at 8:11
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1\$\begingroup\$ Two basic approaches are to use an AC bridge or to make an oscillator and measure the frequency. \$\endgroup\$– Spehro 'speff' PefhanyCommented Dec 3, 2015 at 8:35
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What I did was build an oscillator with the unknown capacitance as the C in a LC circuit.Remember the unknown cap is not precisely known but because you have a ball park value you can design a LC osc like I did .My capacitance was higher than yours so you will want less inductance than the 100microhenry that I used.Aim things up for a sensible resonant frequency like say a few MHz .You dont need some exotic osc or expensive transistor and its easy to bread board and test .I used another transistor to square up the osc sinewave to logic level. For me that was job done because the software people wrote code based on counting the pulses.The osc that I used was a modified dynatron but you may want to use your own design.If your cap isnt a cap that equates to low Q so you could arrange the osc to stop as a simple method of detecting this condition.
