How to Build a Laser Tripwire

Laser beams travel in straight lines, which makes them perfect tools for setting up a perimeter fence.

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.

Tips & Warnings

• You can add mirrors to bend the light and make your tripwire into a perimeter alarm. If you're using your tripwire during daylight and you have a long distance to cover, you might want to get a bandpass filter that will only allow your laser wavelength to get through to the photocell.

• That will make sure there's a big resistance difference between when the laser is on and off the detector.

• You can also buy ready-made kits that come out of the box ready to use, although you'll be paying a bit more for the convenience.

References

• Rice University: Sensor Design
• Electronics-Tutorials.ws: Light Sensors

Resources

• YouTube: Laser Tripwire Circuit
• Amazon.com: Wild Planet Lazer Tripwire