Laser trip wire in an easy-to-use form factor

[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.

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Laser trip wire – the bare essentials

[Gordon] sent us a tip about this simple laser trip wire system after reading yesterday’s post on a more complicated laser security unit. That build did a lot to provide functionality, such as a system to disarm the trip wire, and a robust light detection circuit. This time around there’s more happening with smoke and mirrors than with electronics.

[The Timmy] built this simpler version based on a laser trip system from Afrotechmods (video of that one is embedded after the break). He had a bag full of small square mirrors which he attached to a wall with some poster putty. A laser module shines a beam of light onto a cadmium sulfide sensor after it bounces around the optical network for a while. That CdS sensor controls an N-channel MOSFET, switching it off when light is detected and on when the intensity of the laser is absent. This example just turns an LED on and off, but since it uses logic-level voltages you can choose to add a microcontroller to the mix if you have other plans in mind.

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Arduino security with frickin’ laser

[over9k] used his Arduino to set up a laser trip wire. The laser is mounted along side the Arduino, reflects off of a mirror, and shines on a photoresistor that interfaces via a voltage divider. The signal from the voltage divider is monitored for a change when the laser beam is broken. [over9k] set things up so that a webcam snaps a picture of the intruder and Twitters the event for easy notification. Video after the break walks through each of these steps.

This build is a bit rough around the edges but unlike other laser trip wires this keeps all the electronics in one place. The laser interface could be a bit more eloquent, and we’re wondering just how much current it is pulling off of the Arduino pins. But if you’re bored and have this stuff on hand it will be fun to play around with it. [Read more...]