I came across a file that I saved a while back of an exchange on the forum that is, I think, quite funny.
OP wrote:
deadbolt detection
I have a simple, standard deadbolt -
I was going to try to take the plate off and chisel in a microswitch, but the wood in the area is not that great, and it would be better to do something temporary. Any creative ideas for trying to use some sort of "pillow"-like switch that would be activated by the deadbolt itself sliding into place and "crushing" it? Or other semi-crazy mostly-temporary options?
I answered:
Do you have a piece of conductive foam like ICs come in? Sandwich the foam between a couple pieces of conductive foil (aluminum, copper). Hook wires to the foil. Now you have a crappy force sensor that, when mashed by the dead bolt, should change resistance enough to detect with an analog input. The sensor will need to be part of a voltage divider.
reply from BulldogLowell:
"Now you have a crappy force sensor that, when mashed by the dead bolt, should change resistance enough to detect with an analog input."
hehe
AKA the cheezo-electric effect.