Here is a simple and cheap Laser Based Security System, which costs below Rs. 100. It is a simple circuit using LM358 OP-AMP and LM555 Timer. This circuit is triggered when the Laser beam falls on the LDR interrupts, which sounds the buzzer connected to it. This can be used to protect your valuable items, money, room etc.. By using different mirror arrangements you can form different security systems. This can also be used to protect more than one objects by suitably arranging mirrors. One possible arrangement is shown below.
In this arrangement Mirrors B,C,D are arranged such that the reflected beam is making 90° to incident beam and the Mirror A is placed such that the beam form Laser falls on the Mirror B and the beam from Mirror D falls on the LDR.
In this circuit the IC LM358 ic is working as a comparator and IC LM555 is wired as a bistable multivibrator. The transistor BC548 is wired as a switch to provide the negative trigger at pin 2 of LM555. The 50K pot is used to adjust the sensitivity of the circuit.
When any one interrupts the laser beam, the LDR will go to darkness which increases the resistance across LDR, which in turn increases the voltage across it. When the voltage at Non-Inverting terminal (PIN 3) will greater than Inverting terminal (PIN 2) the output of the comparator goes high. Thus the output of transistor goes low and triggers the 555, which sounds the buzzer.