You can use this circuit to prevent theft.It alerts you when someone tries to enter your home or office by knocking on the door, pushing the door or knocking.The sensor element is a capacitive microphone, which is mounted on the entrance door of the house, preferably on the door frame.Knocking, pushing, or knocking on the door can make some noise.This is detected by the microphone and fed to the preamplifier section of the circuit, which is connected to the buzzer via a trigger.Therefore, when someone knocks or knocks, the buzzer will go off.