It is easy to construct a Laser based Relay activator to operate Alarm systems or to switch on CCTV or Auto Camera when there is any attempt of theft. It can be used to guard, doors, entry passages etc. The circuit uses Laser beam so that the range of the circuit is more than 10 meters and there is no false triggering.
Laser diode is similar to LED and emits Monochromatic narrow laser beam that passes long distance through air. The Laser diode requires 3 volts and around 100 mA current for getting maximum range. So the current limiting resistor must be selected as per the range. A China make low cost laser torch can be used for the purpose.
The Phototransistor L14F1 is the darling NPN transistor that has very high sensitivity and has a lens system to focus the light into its pn junction. When the Laser beam hits on the Phototransistor, it conducts and pulls the base to T2 so it remains non conducting. Variable resistor VR1 adjusts the base bias of T2. When T2 is off, the Monostable based on IC1 will be off keeping the relay driver T3 off. When the Laser beam breaks, T1 turns off and T2 turns on as indicated by the glowing LED. When T2 conducts, Monostable will be triggered to activate the Relay. Load can be connected through the common and NO contacts of the relay. The relay will be on depending on the values of R3 and C2 and with the given values it will be around 3 minutes.
China make Laser torch is available at low cost around Rs.30. Remove the battery and solder wires to the battery points. Connect the wires to the circuit. Laser diode and Phototransistor should be fixed at the opposite sides with correct alignment so as to make the alarm off. LED can be used as a visual indication for this.
Caution: Laser is harmful to eyes, so do not look into the laser light and don’t direct it to the body. It should not be used in home if there are small children. Fix the laser diode and Phototransistor just above the ground level to prevent accidental looking into it.