How to use a capacitive sensor to trigger a relay?

I know how to use an Op Amp to sense a differential input and trigger a relay. This works great for resistive and voltage based sensors. I imagine that I need to do something with AC, but im not certain. Perhaps use the capacitor to phase shift then rectify it for a variable voltage output?? Do you think 60hz... show more I know how to use an Op Amp to sense a differential input and trigger a relay. This works great for resistive and voltage based sensors.

I imagine that I need to do something with AC, but im not certain. Perhaps use the capacitor to phase shift then rectify it for a variable voltage output?? Do you think 60hz AC would work or do I need to generate a higher frequency?

What are your thoughts? Thanks for your help!
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