This is another circuit of IC 555 simple burglar alarm mini project. It has been fully tested in our lab and its PCB layout is also given below.
The circuit uses the simple trick that whenever, pin-4 i.e. the RESET pin of IC 555 is grounded, all its functions stop. But when it is raised to +ve supply, then the IC works normally.
The circuit is basically used as Astable multivibrator circuit. But since pin-4 of the IC is grounded it remains in dead conditions till the burglar loop is broken.
As shown in the above circuit diagram, when power supply is connected to the circuit, it remains silent. This is because the pin-4 i.e. the RESET pin of the IC is grounded through the burglar loop, shown on the left side of the circuit.
Now when the loop is broken by the burglar, pin-4 is raised to +ve supply through resistor R3. So the circuit starts working as astable multivibrator and the output of the IC becomes HIGH and LOW periodically.
The buzzer connected across the output and ground start producing whistle like sound. You can check the actual working of the circuit, in following video.