Yes, being able to start a self bondage game by closing a magnet is very convenient, because then you don't have to guess how long your preparations will takelobster wrote: As for the relay, I had another idea. Going with a MOSFET allows be to regulate the strength of the magnet using PWM. For getting into the bondage it can be very helpful to run the magnet at say 10% strength. If I include a current sensor in my design, I might be able to detect if the magnet is being put under load (as per the increase in current). This way I have a very natural point to fully engage the magnet and start the count-down timer.
I'm pretty sure the magnet draws the same amount of current irrespective of the force exerted on the anchor plate. What you will be able to detect though, is changes in inductance. The magnet's coil's inductance is significantly larger with the metal plate on the armature (cuff closed) than it is with the plate removed (cuff open). When switching on the magnet, this difference in inductance causes the current through the coil to rise slower with the plate on it than without it. My Raspberry Pi program (see this topic) measures this time difference by means of a fairly simple driver circuit.