Circuit photoelectric switch S1 is gated or luggage room light control switch. A point is in its closed operating voltage is positive. If S2 is closed, regardless of whether or
not S1 is closed, the output terminal voltage U3 are high, transistor is turned on, the relay does not pull, the light emitting diode LED is not illuminated. S2 is a normally open contact, if S1 is closed (door open), the Cl slowly charged through Rl. At a certain time (adjustable by potentiometer RP can not exceed 60s) inside, U3 suddenly goes low, then the relay is turned on, the alarm is working. If the alarm S2 is closed, and the U3 is high, the relay releases.