This loud door security burglar alarm circuit can be used to secure the doors to detect intrusion or any unauthorized entry. The circuit will activate a loud alarm when the door is opened and once the alarm is activated it will not closed if the unauthorized person closes the door.
The circuit is using two
ICs, which are a 555 timer IC, and other is a UM3561 four siren sound generator IC. But in this circuit the UM3561 is wired to produce only one sound. A NC (Normally Closed) reed switch is also used in the circuit that is the main starting point of the circuit. A small piece of magnet will be required to attach at the other part of the door, in front of the reed switch to make it open.
Working of the circuit is simple when an intruder or an unauthorized person will open the door the distance between the magnet and the reed switch will increase due to which the reed switch will become in its normal condition that is NC (Normally Closed) and trigger's the 555 IC. When the 555 IC is triggered its output goes high and activate the UM3561 alarm circuit. The 555 timer IC is working here in time delay mode so once it is activated it will not be deactivated until the preset time period is completed no matter if the door is closed or the reed switch will become open. The preset time period can be set with the 2M variable resistor. Increasing the value of capacitor Cx can further increase the preset time period.