If you put a capacitor in paralel with the coil and pulse it the circuit should resonate. The frequency will be different with the plunger in than with it out. select a higher voltage coil than the micro can trip directly and you can detect the state through a high impedance voltage divider so that the input is protected when you trip the reset. Since the coil has a very low duty cycle you can use an r/c time constant on the base of the reset driver to prevent spiking and emf from the coil when activated no additional snubber or flyback diode would be needed Terry