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How to Build a Laser Tripwire

If you can learn anything from a Hollywood high-tech thriller, it's that laser tripwires make a fancy security barrier. If you aren't expecting a crew of sophisticated thieves to break into your place, a laser tripwire can be an effective security barrier as well. You can make your own with a handful of inexpensive parts.

Things You'll Need

Laser pointer
Wire
Solder and soldering iron
Photoresistor
Power supply
Detector circuit and indicator

Instructions

- 1
  Remove the case from the laser pointer, remove the battery, and solder wires to the two laser input lugs. The other end of the wires goes to your power supply. Use your own custom battery-powered supply or use a commercial variable power supply. Check your laser to determine what voltage it needs, and set your power supply to that value. Typically, laser pointers will use 4.5 or 6 volts.
- 2
  Attach your laser to a wall or post so that it points along the border you want to protect.
- 3
  Place the photoresistor at the opposite end of the border, mounting it so it intercepts the laser beam.
- 4
  Block the laser beam and measure the resistance of the photoresistor and let the beam strike the detector and measure its resistance again.
- 5
  Set the detector circuit to trigger the indicator if the beam is blocked, and do nothing if the beam strikes the detector. Knowing the resistance values, you can put the input directly into a computer, use a comparator to put a signal out when the resistance is low, or build a voltage bridge to balance when light is on the detector and give a signal when the voltage is out of balance. There are dozens of different ways to detect the signal.

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