Laser Tripwire Alarm System Uses Mirrors To Increase Coverage


Instructables user [EngineeringShock] has been hard at work building a laser trip wire security system, complete with a combination lock. The security system works just like you see in the movies, employing an array of mirrors to bounce the laser across an opening several times in order to secure the space.

A PIC18F1220 micro controller sits at the center of the alarm and handles the majority of its functions. It takes input from the laser detection circuit, triggers the buzzer, as well as arms and disarms the entire alarm system. An LS7222 digital lock handles the passcode verification side of things, taking input from a 16-button matrix keypad, and telling the PIC when the proper code has been entered.

As you can see in the video below, the alarm system works and the buzzer is quite loud. There is one small problem however – the alarm only arms itself after the proper code has been entered and the lights have been turned off. The light sensing circuit he uses is too sensitive and can only operate in darkness, though he discusses the ability to add a more accurate sensing solution.

If you are interested in reading more about laser tripwire security systems, check out this similar passcode-based system, this alarm system built into a toy, and this Arduino-based alarm system.


15 thoughts on “Laser Tripwire Alarm System Uses Mirrors To Increase Coverage

  1. Its not a bug its a feature
    now the baddies have to try to break in in the dark ;->
    (plus it’ll make the cool Hollywood red laser + smoke effects way niftier)
    for bonus points should have used a green laser, i mean lets face it they are way cooler ;->

  2. If he wraps some tape around his light sensor to block off some of the ambient light and point the laser directly into the sensor then it would probably work quite well even with room lights on.

  3. Um… How about using coded LASER pulses? As it stands I could shine a flashlight or my own LASER pointer on the photo-resistor, make entry and rob you having never triggered the alarm.

    A neat project and a good start but please don’t mistake it for secure. I’d love to see rev 2 with a sharper eye toward security.

  4. Red lazer, how 1980’s.
    Green lazer, how girly.
    Blue laser! now that would be cool.

    as for “tone decoding” again easy to defeat I can easily sample the beam to figure out what the tone is. Same for the encoded beam.

    The solution is simple and elegant. Laser detector, something looking for coherent light at the frequency the beam color is. These can be purchased.

    and the detector shrouded by a long and narrow shroud so you have to break the beam on order to try and fool the sensor.

    The long narrow shroud will stop 99% of all attempts, you need to go to the laser detector to stop the guy with a 9,000,000CP floodlight from simply overwhelming the sensor.

    Now that could be another way, installation calibration looks as the light power of the beam, anything above or below = trigger alarm. that way the 9,000,000 flashlight will not work as well.

  5. Nice construction. To make it serious, just separate the keypad from the rest of the circuit. Then hide the controller circuit somewhere and embed the keypad in the wall with the connections running inside the wall.

    As other people referred you can also PWM-modulate the laser beam and detect tampering.

    I also suggest the use of first-surface mirrors to waste less laser power into secondary reflection beams.

  6. Nice idea, room for improvements. As others said, at least modulate the laser, so you don’t have to care for ambient light and it will work during the day as well. Right now I could use a strong enough flashlight, point it at the right spot and walk right through without being detected.

    @ spork: I believe emperor Palpatine is borrowing it right now :P

    @ fartface: whatever floats your boat, but I’m sick and tired of seeing blue LEDs everywhere when we are making everything in the color spectrum today. If we are talking security, infrared lasers would be a better choice, and they are cheaper to boot. Why let the thief know about the location of the alarm? (bright red spot on the wall)

  7. Poits taken.
    However, I rushed to make this for a contest.
    1) I could have done the whole keypad circuit with the 18F1220, but it would have taken more time. Time that I didn’t have.
    2) Robbers would not likely see the laser, or know to shine a light into the sensor. I added in a circuit at the end of the instructable that would have been a better laser circuit to use, which would have aftually activated if a robber did just that. However, time was a factor.
    3) I agree. Next time I will just demo. However, it was posted in on instructables, and it is asked that you go into as much detail as possible.

    Room for impreovements, yes. Lots of room. For the time spend designing it, putting it together and making an instructable? Easider said than done.

  8. perhaps next time do the cool demo first with the freekin lazor beams, then follow up with the details? you need to hook the audience before you try to educate them ;->

    anything with lasers is still pretty cool, ooh add a mist curtain so you can see them for added cool.

    I so have to do this for something, a solution in search of a problem

  9. you can add some code to turn off the laser light in case of trespassing, therefore the perp cannot identify easily the location of the sensor to tamper with based on the light path

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