How to build a human chain detection circuit?
Here is a DC circuit that I built and seems to work. So, oldfart was right. I tested it with two 47M in series plus my own hand to hand resistance. My fingers work really well, other parts of my hand/arm do not, dry skin has a very high resistance. You should test the circuit with a lot of different people.

The cap must be very low leakage, ceramic or film, do not use an electrolytic. The op-amp must have extremely high input impedance, anything with a BJT input isn't going to work. The worst case offset voltage will change the threshold significantly, but it should still work. You could try to find an op-amp with lower offset voltage. 

I recommend running on batteries.

The circuit takes a long time to settle out at power-on, like one minute. So, if you are driving a solenoid, you may want to add a lock-out on power-on

Sorry, Circuit Lab keeps crashing on me. This is the best I can do right now.



I still believe that an AC circuit might have advantages with dry skin. But, I couldn't get anything to work with a solderless breadboard. I had 60 Hz pickup, the circuit wants to oscillate, etc.