Protect your home with the Easy Laser Alarm kit. It uses a 741 op-amp and 4017 decade counter as the brain and is powered by a 9V battery. If you like, connect a 9V wall transformer to the power connections to save your batteries. The kit works by sensing the light from the laser hitting the photoresistor which lowers the resistance, once the beam is broken the resistance increases and the 741 chip sends a pulse to the 4017 which sets off the buzzer. To reset the circuit the laser must be back on the photoresistor and then you push the reset button.
If you add mirrors to reflect the laser you can protect multiple entrances at once.