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How to Make a Laser Trip Line

A trip line can be as simple as a cord tied to an aluminum can full of rocks. When someone's foot strikes against the cord, the can tips and rattles, informing you of an intruder's presence. A laser trip line does the same thing to alert you to an intruder, just in a bit fancier style. But don't let the word "fancier" throw you --- the parts are commonly available and fairly inexpensive.

Things You'll Need

Laser pointer
Optical detector
Detector electronics
Buzzer or light
Power supplies or batteries

Instructions

- 1
  Place the laser pointer a few inches off the ground, high enough so that the beam clears all obstacles.
- 2
  Connect the power supply to the laser, if using external power. You can unscrew the battery compartment and clip leads to the battery connections. For a more permanent installation, you can solder wires to the battery connections.
- 3
  Place the optical detector at the opposite end of the area to be secured. Put the detector at the same height as the laser, with the photosensitive surface facing toward the laser.
- 4
  Align the laser so the beam hits the center of the detector.
- 5
  Connect the power supply to the detector circuit and the circuit to the detector, then hook up the alarm buzzer or light to the circuit output. You can use clip leads, soldered wires, or wires connected to terminal lugs.

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