Alarm delay control circuit

Working principle of the circuit: When the courtyard door is opened, S2 is in the open state and S3 is in the closed
state. When S1 is connected, the entire circuit does not work because S2 is disconnected. After the owner goes out,
he closes the courtyard door. At this time, S2 is closed, S3 is disconnected, and the power supply charges C1 through R1.
C. The voltage at both ends gradually increases. After 2s, VT1 begins to conduct, VT2 is saturated, and the collector
outputs high level. Because S3 is disconnected, the rear-stage delay circuit loses power, VT3 and VT4 are cut off, and the
alarm does not work and is in an alert state. .