Circuit at rest (dark) state operation the electromagnet M current flows, once there is sufficient strength so that light shines on the phototransistor is turned on, that is th
e final stage transistor is in the OFF state, the solenoid release. Because by 1M ohm feedback to the photosensitive base of the transistor effect, so that even light cancel time-sensitive transistor is still turned on. Until press the reset button T, in order to return to the initial state. Darlington transistor which can be used in place of an amplifier greater magnification.