This 2003 circuit uses two types of sensors: by reed-switch and by infrared. If the magnetic sensor (reed) is opened, there will be no immediate activation of the siren, but only after some time.This time is determined by the second half of CI 556 (double 555 - monostable) through the resistor and capacitor connected to pins 12 and 13.Time changes can be made by changing the values of these components. After the sensor breaks, the 556 (first half - stable) will send a positive pulse for the count. The first and second outputs count only the time while the third output will activate the siren. The fourth output (pin 7) of 4017 sends a high level to pin 15, resetting the count. When the magnetic sensor is closed, it resets the 4017 by locking the count.
