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.


  1. pppd says:

    The beam should be disabled immediately after it’s interrupted and then enabled for a split second to test if the path is clear again until it actually is. Otherwise you are risking having a blind cat if it’s stupid enough to keep looking at the beam.

  2. Techie says:

    A little bit of an overkill in my opinion, but is OK.

  3. Dimitri says:

    Wooo, i need one !

  4. Grey Drifter says:

    Needs a pressure plate on its base to prevent improper disarming. Overkill would be a multi-point network of said devices.

  5. SparkyGSX says:

    Setting up the mirror seems like a lot of work, and you risk false alarms if it would, for whatever reason, move by only a tiny amount.

    I’d try using a small retro-reflective sticker as a reflector instead; it’s quick to setup, and I’d think it would be more reliable.

  6. alan says:

    The alarm is the down fall. When someone sets the alarm off, they will run and hide. How about sending an SMS alert so they don’t know you are coming to get them?

  7. Jon says:

    Maybe try an bicycle style reflector to make it easier to align the mirror?

  8. WJ says:

    This is an awesome project, but I wonder the life expectancy of the battery. I’m thinking that even a energy efficient laser is going to quickly run the battery down. Maybe doing another version where a PIR is used. In any case, This is a cool little tripwire.

  9. Hirudinea says:

    If your using it in AirSoft matches you should use an infrared laser and have the LEDs turn off after setup.

  10. eventhorizon says:

    You could try using proximity sensors, or sonars. It requires almost no set up at all. You place it where you need it and when you activate it make it check the current distance it’s pointing at, when someone walks pass it, obviously the range from where it’s pointing originally will be changed. no mirrors, almost no set up needed (except for placing it near a door and activating it).


  11. Snake says:

    Nice, now I have a reason to smoke indoors at your house.

  12. Mike says:

    Watched the vid to see the cat get the life scared out of it, all it does is turn it’s head kinda quickly!
    rage. haha

  13. pRoFlT says:

    Yah i liked the hack where cats got shot with a hose as they passed by. And caught on tape too.

    Maybe an air hose so you dont get water everywhere in the house.

    and Overkill could have been several mirrors that formed a preimiter around the house and coule trigger if some entered in through any door.

  14. Oscar says:

    This is a great idea. I had thought about something like this for my window. But nothing quite like this. I like to sleep with the window open, and I am on the main level of the house. I can place this in front of my open window and sleep sound. That’s until someone attempts to climb through.

    • KillerBug says:

      You should put a high voltage strip on the bottom of the sliding part of the window, and another on the base of the window…when the laser is tripped, 240V AC engages and the results are truly shocking! You wouldn’t even need an alarm…the smell of burning hair would wake you.

  15. Bigdeal says:

    Can’t it be disarmed by pointing a flashlight at the sensor?

    • Threeck says:

      It could be disarmed that way if the sensor only checks 1 and 0, otherwise if the sensor is used with a ADC the slight increase or decrease of light intensity received by the sensor could trigger the alarm.

  16. Rich says:

    Use two beams and you can discern the direction of travel.

  17. XiuiX says:

    I dreamed of owning one of these as a kid.

  18. Darren says:

    So how do i get one or how did you make it so i can make one???

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