[Rul] built a nice laser trip wire alarm for use in Airsoft matches. Just place the enclosure so that the beam crosses a doorway and it will sound an alarm when the beam is broken. The only problem with this setup is that you need a reflective surface on the opposite side that can be positioned to direct the beam of light back to a photoresistor. But wait, there’s a second option. [Rul] also added a leaf switch that can be connected to an actual trip wire instead of using the laser.
A PIC 16F688 controls the laser module and monitors the photoresistor and leaf switch. When power is first switched on the box goes into a setup mode where it waits until the laser is detected by the photoresistor, switching on an LED to signal that you’ve got the beam aimed correctly. One press of the push-button puts it in activate mode which will sound the hacked window-alarm inside when the beam is broken. A toggle switch lets the operator chose between laser or wire operation.
You certainly won’t miss the alarm when it’s set off. Watch it scare the life out of a poor house cat in the clip after the break.