This security alarm detects the change in the condition of the sensors. When the activation selector is open the alarm is off because the IC2 output is high.
When the selector gets closed the IC2 output switches off after a certain time, set by the C5 capacitor, so the security alarm turns on. If a sensor changes condition, an EXOR logic gate gets high for an instant. Then the IC1 timer is triggered by the Q3 or Q4 transistor, while the Q7 collector gets off. Once the C3 capacitor charges the IC3 timer turns on. If the circuit is not deactivated, the IC3 output triggers the alarm sound, and turns off the Q9 transistor. The C4 capacitor charges, so the IC4 output switches on and the circuit backs to the start condition.
With the values specified in the circuit diagram the security alarm should feature a time delay of about 10-15 seconds. In the video I used smaller C3-C4 capacitors. Do not modify the value of the R15-R19 resistors.