Timer alarm circuit diagram for car or room

S1 in the circuit is the door control photoelectric switch or the luggage room light control switch. Point A is the positive operating voltage when it is closed. If S2 is closed, regardless of whether S1 is closed or not, the output terminal voltage U3 is high, the transistor is turned on, the relay does not pull in, and the light-emitting diode LED does not light up.

S2 is a normally open contact. If S1 is closed (the door is open), Cl starts to charge slowly through Rl. Within a certain period of time (can be adjusted by potentiometer RP, no more than 60s), U3 suddenly becomes low level, so the relay is turned on and the alarm works. If S2 is closed during the alarm, U3 is high level again and the relay is released.