Laser Trip Wire With Keypad Arming

Most of us have had a sibling that would sneak into our room to swipe a transistor, play your guitar or just mess with your stuff in general. Now there’s a way to be immediately alerted when said sibling crosses the line, literally. [Ronnie] built a laser trip wire complete with an LCD screen and keypad for arming and disarming the system.

The brains of the project is an Arduino. There’s a keypad for inputting pass codes and an LCD screen for communicating if the entered code is correct or not. [Ronnie] wrote his own program using the keypad.h, liquidcrystal.h and password.h libraries. A small laser pointer is shined at a Light Dependent Resistor which in turn outputs an analog signal to the Arduino. When the laser beam is interrupted, the output voltage drops, the Arduino sees that voltage drop and then turns on the alarm buzzer. The value that triggers the alarm is set mid-way between the values created by normal daylight and when the laser beam is hitting the LDR. [Ronnie] made his code and wiring diagram available for anyone who’s interested in making their own laser trip wire.

Hopefully, [Ronnie’s] pesky little brother didn’t watch his YouTube video (view it after the break) to find out the secret pass code. For a laser trip wire sans keypad, check out this portable one.

17 thoughts on “Laser Trip Wire With Keypad Arming

    1. Those attachment screws look inadequate to transfer the force / load to the device chassis. Someone could get hurt. I’d run some numbers, maybe beef it up with tape. And a pictoral warning label.

      1. There will be almost zero lateral force. The shell is not in a barrel so it will simply explode outward with only a very tiny amount being sent laterally before the thin plastic shell gives up containing the force and splits open.

    2. that’s going to burst!

      does anyone remember cap rockets?
      Hang one, I used a crappy pocket knife that had a blade and a corkscrew, you jam the blade in a convenient crack, hang the cap rocket off a pin through the corkscrew, run a trip wire/string off the pin.
      or
      if you have electric matches, get a wooden spring clothes peg, put 2 thumb tacks on the inside of the grippy bit of the clothes peg, run 2 leads, one from each thumb tack, to a battery and your electric match, then put something like an icecream stick between the thumb tacks, attach your trip wire to the icecream stick.

      for the warning device put an electric match in the bottom of a 50mm PVC end cap, get a short length of 50mm pipe, say 100-150mm long, in the end cap put some shotgun powder(don’t go nuts, 5 or 6 teaspoon fulls will do)
      cut some wads from cardboard, 5mm corrugated works well, you’ll want 2 – 4 wads, pack them in tightly over the powder, then put a few desertspoons of talc or coloured tempera powder paint on top of the wads.

      Don’t use flour for the “deterrent powder”!

  1. Not Bad. I would do as shakipu said and also make it a latching circuit so that the code is required to disarm. a small battery that is diode or’ed on power will prevent the unit from being disarmed when unplugged

    1. Maybe Zener diode to lower the voltage drop. Also, could feed the power into an input that will detect when someone tripped on the power cord or took it out intentionally. Then a small battery could give power enough time to trigger a notification/alarm

  2. My suggestion for further development would be an IR Laser and a Photo Diode instead of an LDR. That way it is not noticable for the intruder. Also, I would use a reflector on one side, so the IR Laser would bounce of the reflector/mirror and travels straight back to the Photo Diode, sitting directly above the laser. This way the distances can be greater and the circit only has to be on one side of the contraption. I admid though, that the setup of the reflector-to-photo-diode might be a bit fiddly.

    1. Used to do that until we started pulling knives on each other. We decided that it would make the holidays a little awkward, so we switched to blowguns with balls of modelling clay. More fun, less blood, less bitchin’ by Mom.

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