Passcode Protected Laser Tripwire Alarm System


Sometimes security doesn’t need to be overly complex to be effective. Instructables user [1234itouch] recently built a simple laser tripwire alarm that can be mounted virtually anywhere, complete with a keypad for disarming the device.

He mounted a photo cell in a project box, along with an Arduino and a 12-button key pad. A laser pointer is aimed at the photo cell from across a gap, which results in a steady voltage being read by the Arduino. When the laser beam is broken, a drop in voltage is detected, and the alarm sounds until you enter the proper pre-configured passcode. Entering the passcode triggers a 15 second grace period during which the the alarm cannot be tripped again.

It might not be built with triple-thick steel doors and thermo-sensors, but it’s a simple device for simple needs. In its current form it could be pretty useful, and with a little reworking, there are a wide range of things it could be used for.

Continue reading to see a demo video of the tripwire alarm, and be sure to check out these other tripwire-based security systems.


13 thoughts on “Passcode Protected Laser Tripwire Alarm System

  1. I remember playing with ‘security’ devices as a child, using one of those ‘100 in 1’ Radio Shack boxes. The only thing I thought of right off for this project is: Why not mount a mirror opposite the laser, and keep the optical sensor in the package with the keypad?

    With interesting mirror placement, one could create a “Mission Impossible” style laser maze with no electronics modification, right?

  2. Including the laser in the device would add WAY more security! The way it is I could just use a second laser pointer, light at the sensor while I pass the thing and unplug it or something…
    With the laser in the device you could send a clock or even a random sequence of 1s and 0s, that way if you’d light at it with a second laser there are two possibilities:
    – the laser is not strong enough to cover the signal of the internal laser, if you break the laser beam it’ll give you an alarm
    – the laser is strong enough, covers the internal laser’s signal and it’ll give you an alarm
    What do u guys think?

  3. That reminds me of a neat trick I read about in that same kit, transmitting sound using a laser and a light detector. It works the same as an optocoupler but at a distance.

  4. @tokyodrift
    I believe the trick of pointing a secondary laser won’t work because two lasers will increase the voltage of the photocell – which ends up being detectable by the uC.

    @andar_b for transmitting sound in a laser the easiest way is to do amplitude modulation. It is very prone to background noise though..

    Btw: is there anything preventing a thief of disconnecting the device from power? Like someone said, the whole security of a system is given by the weakest point in the system.

  5. i was thinking of building one of these, Dealextreme has red laser modules for the low, low price of $0.90 (if you buy in bulk) i bought 1 at full price and a green 20mW, which i broke. (was going to extend the diode away from the driverboard, Tip: don’t try to solder on that brass, it doesn’t stick and if you keep trying, you burn up the diode) red one still kickin’ running on an 18650 cell @ 4.11v, so i’m thinking of buying 10 of them, and integrating them into various projects, such as: ..laser tripwires. i would say laser sights, but green lasers are amazing, and would be much more intimidating, if you were the target.
    Any tips on how to get solder to attach to that brass-type stuff that lasers are made of? (sure, i could wrap a wire around it and electrical tape, zip-tie, etc.. and i wish i had, but there’s gotta be a way to solder to it. maybe i should have tried sanding it first.)

  6. @Jeditalian – regular old tin/lead solder should work.. make sure you clean the surface and use flux. Also, if it’s a large chunk of brass you might just not be getting enough heat into it. Use a larger soldering iron tip so that when you touch it to the brass you don’t suck all of the heat immediately out of the iron (heat sink effect). You could also preheat your brass housing with a heat gun, but be careful of the heat. If you can remove any electronics first, do so. Here’s a video of soldering brass tubing… not quite the same but it shows that it can be done:

  7. You don’t need to solder to the brass, the laser diodes ground pin is connected to the case. You can solder to the ground pin directly or through the PCB which is undoubtedly already connected to ground in some way.

  8. @Tokyo Drift: Put the Lazer and Sensor on an Serial IO lines.
    Then do:
    Serial.Print(randomString); Serial.Read(returnString);

    if(randomString1 != returnString2) { //OH SNAP }

    Then just push MD5 sums all day long at 9600baud.

  9. The problem with any tripwire system is that it only works if a trespasser is not expecting it to be there. Otherwise, be it laser or physical wire, it can be easily stepped over. If you manage to conceal the two devices on the wall, fine. But if you’ll have two small gizmos on the wall with wires running into (at least on of them), the only thing the setup might catch is a cat or dog. Unless of course it cannot be avoided (like a door opening into it), but there are simpler ways to sense that sort of thing. Oh, and modulation is compulsory, period.

  10. @Max – what about modifying a barcode reader so the laser scans a range several times a second?

    Those, IIRC, produce a fan shaped area. You could mount that on the top of the doorway and put the reflector at the bottom. Provided the laser is strong enough of course.

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.