Saturday, April 13, 2013

Laser Based Intruder Alarm

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 :
Transmitter circuit 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.

Circuit diagram :

Receiver circuit
Fig. 2: Receiver circuit Diagram

The receiver circuit (shown in Fig.  2) is basically a current-to-voltage converter built around IC LM356 (IC3).  The output of IC3 is fed to the monostable built around 555 timer (IC4).  The high output of the monostable  drives the piezobuzzer to sound an audio alarm.  The receiver section operates off 5V  DC generated from another 9V battery  and voltage regulator IC 7805 (IC2).  The battery (Batt.2) is connected to the  circuit through switch S2.

When the laser light transmitted  through LD1 falls on phototransistor  T1, the output of the op-amp (IC3) at  its pin 6 remains high. In this condition, the output of IC4 remains low  and the buzzer does not sound.  When the laser beam falling on phototransistor T1 is interrupted by someone, the output of op-amp IC3 goes low and IC3 produces a pulse. This pulse triggers monostable IC4 and its output goes high to sound the alarm for a time period of about R8xC8.

Assemble the transmitter and  receiver circuits on separate general-purpose PCBs and enclose in suitable cabinets. Mount the transmitter and  receiver units on opposite pillars of the  entrance, aligning the two such that  the laser beam from the transmitter  directly falls on the phototransistor.  Block the laser beam with your hand  and measure the op-amp output. It  should not be low. At pin 3 of IC4, you  should get a positive-going pulse of  one-second duration beginning with  high-to-low edge of the trigger pulse  appearing at pin 2 of IC4 or collector  of transistor T2.

Source :

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