Is it possible to use a PCB pad as a button? I think of using it to switch on a curcuit that is only supposed to be enabled, when the user holds it in its hands.
As inspiration, I used the pads that are being used on soft touch buttons on keyboards or in calculators:
I know that the human body has a quite high resistance, so what would be an appropriate circuit to detect the touch input? Bare hardware only. I don't want to use any microcontroller here.
For reliability reasons I wouldn't go for an open-gate design and rely on the 50Hz noise. It probably might work, but your idea of using interleaved fingers should work quite well.
The resistance of dry skin is somewhere between 1k and 100k, so you could think of an NMOS transistor (find one with ESD protection) and a large pull down of e.g. 1M ohm. Then you can use the finger as a pull-up resistor to turn on the mosfet.
simulate this circuit – Schematic created using CircuitLab
You could also use a bipolar (or darlington) transistor, they are less vulnerable to ESD defects, but cannot supply a large current at the output if needed so you would need to buffer the output.
A capacitive sensor would be an alternative solution, but requires a more complicated circuit.