A laser alarm system operates by projecting a beam of invisible laser light across a doorway or window opening. When the light is broken, it activates a buzzer or alarm. The principles are very similar to those of more low tech burglar alarms. A laser alarm requires only slightly more sophisticated electronics and can be put together by anyone with a soldering gun and a knack for tinkering with basic circuits and transistors.
Connect a photocell to a 5K resistor. This will pick up the laser light and enable the alarm.
Use a 2N3904 transistor to link the resistor, a 1000uF 35v capacitor, a 12v siren and a 9v battery clip. The attached schematic should demonstrate how the parts come together. Use an IC board to hold them in place and make sure the board is small enough to fit into a project box. Do not connect the battery yet.
Cut a small hole into the side of the project box big enough to fit the photo cell. Glue a short (1-inch) length of black plastic tube on top of the hole. This will protect the photocell from any ambient light.
Mount the photo cell carefully in the length of tube, running the wires back in to the project box. Mount the IC board in the project box using hot glue, making sure there is enough room for all the components.
Cut 3 small holes in a second project box. The first should be large enough to hold a 5mw 12X30mm laser, the second should be large enough to hold a toggle switch, and the third should be just large enough to fit the wires from an AC adaptor.
Slide the positive wire from a 3.2v AC adaptor into the hole and connect it to the positive outlet from the laser. Positive wires are often marked by red insulation.
Connect the negative wires from the laser and the AC adaptor into a standard toggle switch. Secure them all inside the project box with glue.
Use tape, glue, or strips of velcro to mount the two project boxes on either side of the door you wish to protect.
Connect a 9v battery to the first project box and replace it in the exact position is was in.
Turn the laser on with the toggle switch and make sure it lines up perfectly with the photo cell. The alarm is now activated. If anything breaks the beam, the siren will go off.