This is a project of  smiple laser security alarm circuit. The circuit can be used as a laser door alarm, laser motion detector, laser proximity sensor, trip wire alarm and for other simple home security purposes. The circuit is using only few low cost components. You can use any cheap toy laser available in the
market, remove the internal button cells from the laser and connect it with power supply or external batteries for continuously running.

An LDR or photoresistor is used to sense the laser. The transistor is working as a switch. When any person or object will come in between the laser beam the transistor will become switch on for a short time period and charge the 1000uF capacitor and activate the buzzer. The buzzer will still run for few seconds from 1000uF capacitor after the transistor will cutoff. For buzzer you can use any 9V to 12V loud peizo buzzer. The sensitivity of the circuit can be adjusted with the 50K variable resistor. For properly operating the circuit make sure that the LDR will not receive other light except the laser beam, this can be done by fitting the circuit in a small box and making a small hole in the box from which the LDR will receive the laser beam.
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Simple Laser Security Alarm Circuit Diagram