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You can use this laser-based intruder alarm conveniently at the entrance of a hall or any other similar location to protect the same from unauthorised access as and when desired. Its range can be extended further to protect the perimeter of a  building from unauthorised intrusion by using a cascade arrangement of multiple alarms. The circuit consists of a transmitter and a receiver. Circuit diagram : Fig. 1: Transmitter circuit Diagram The transmitter circuit (shown in  Fig. 1) is nothing but a laser diode (LD1) driven by a 9V PP3 battery. The  output of IC1 is regulated 5V as long as  its input remains equal to or more than  7.5V, thus ensuring a constant drive  current for the laser diode. The battery (Batt1) is connected to the circuit  through switch S1. The laser diode (LD1) can be  replaced with a laser pointer (torch)  emitting red laser beam. The laser  pointer itself can be used as a transmitter. The pointer has in-built series  resistance, on/off switch and battery. Ci...

The circuit below is similar to the one above but can be used with a laser pointer to toggle the relay rather than a push button. The IR photo transistor Q1 (Radio Shack 276-145A) or similar is connected to the set input (pin 6). The photo transistor should be shielded from direct light so that the voltage at the set input (pin 6) is less than 1 volt under ambient conditions and moves to more than 10 volts when illuminated by the laser pointer or other light source. CMOS Toggle Flip Flop Using Laser Pointer Circuit Diagram The reset time is about a half second using a 4.7uF cap which prevents the circuit from toggling more than once during a half second interval. The 10K resistor and diode provide a faster discharge path for the 4.7uF cap so the circuit can be retoggled in less than 1 second. The 3K resistor in series with the photo transistor may need be adjusted for best performance. The relay shown is a solid state variety to be used with lights or other resistive loads at less than...