Here is a very sensitive and long range laser burglar alarm circuit for security purposes that can detect a small interruption in the laser beam coming to the laser receiver circuit and activate a loud alarm for upto one minute. The circuit is very useful and can be used for many anti theft and security
purposes. It can also be used as a long distance proximity sensor or detector. The circuit is using a cheap ready made 3V laser pointer available in the market. By arranging / aligning the laser beam with different mirrors a very efficient security system can be made.
The circuit is divided in to two sections first is a laser transmitter or laser light and second is a laser receiver with time delay and alarm or siren circuit. An LDR (Photo resistor) is used to sense the laser beam. When any person or object will come in between the laser beam the laser beam will be cut or interrupted therefore the LDR will not sense beam on its surface that increases the resistance of the LDR due to which the circuit become activated and produces a loud alarm. The sensitivity of the circuit can be adjusted with 1K variable resistor.
Aligning the circuit:
It is important to properly align the circuit to work smoothly. After completing the circuit fit it in a suitable box and make a hole in the box to receive the laser beam on the surface of the LDR. Make sure to align the LDR inside the box in a way so it will not receive any other outside light except the laser beam. If the LDR will receive the outside light then the circuit will not work properly.
As described above a laser light can be purchased from the market, remove its batteries and connect it with a 3V power supply to make it continuous switch on.