Things which you can learn:
*Application of NPN transistors for switching purposes.
*Use of the Photo-Darlington 2N5777 for sensing purposes.
*Application of the NE555 timer IC as a multivibrator to generate oscillations of a particular frequency.
The range of the Burglar Alarm is approximately 30 meters, which means that the transmitter and receiver units can be placed up to 30 meters apart. Since the laser torch can transmit light up to a distance of 500 meters, this range can be increased by proper orientation of the photo-transistor. However, to avoid false triggering by sunlight, the photo-transistor should be mounted in such a way that it doesn’t directly face sunlight.Build this circuit and prevent theft in your premises.
The transmitter circuit is powered by 3V DC. The multi-vibrator built around NE555 timer (IC1) produces a frequency of 5.25KHz. The body of the laser torch is connected to the emitter of the NPN transistor SK100 (T1) and the spring-loaded lead protruding from inside the torch is connected to the ground.
The receiver circuit is powered by 12V DC. It uses the photo-Darlington 2N5777 (T2) to sense the laser beam transmitted from the laser torch. The output beam signals from the photo-Darlington are given to the two-stage amplifier followed by a switching circuit. As long as the laser beam falls on photo-Darlington T2, the 12V relay (RL1) remains unenergized and the buzzer does not sound. Also, the indicating LED (LED1) doesn’t glow.
When anyone interrupts the laser beam falling on photo-Darlington T2, NPN transistor BC547 (T6) stops conducting while NPN transistor SL100 (T7) is driven into conduction. As a result, LED1 glows and relay RL1 is energize to sound the buzzer for a few seconds (the alarm duration being determined by the values of resistors R15 and capacitor C10 in the circuit). At the same time, the large indication load (230V AC alarm or hooter for louder sounds or any other device for momentary indication) also gets activated as it is connected to 230V AC mains via normally opened (N/O) contact of relay RL1.