How to Make a Door Alarm by Using a Laser


Laser alarm systems are popular because of their accuracy and ease of use. They can be built at home with a few supplies that can be found at electronic supply shops. There are three basic components to this system: the buzzer or siren, the trigger and the power source. For a laser door alarm, the laser is the trigger. It is sounds the alarm any time the beam of light is broken.

Things You'll Need

  • 5-megawatt laser, 12-millimeter by 30-millimeter
  • 3.2-volt AC adapter
  • Battery-powered siren or buzzer
  • Batteries for the siren or buzzer
  • Photocell
  • Black plastic tube, 1/2-inch diameter, 1 inch long
  • Glue
  • Self-adhesive Velcro strips
  • Determine the height of the alarm. Mark the preferred tripping point on the doorway with a No. 2 pencil. You can erase this tripping point later; it's just to guide you when you're reading to install the laser. Position the laser alarm where you desire -- but the higher it is, the more out of the way it becomes for children and pets. Consider where the closest electrical outlet is; the laser AC adapter needs to be plugged into an outlet.

  • Slide the photocell into the black plastic tubing. The tubing protects the cell from ambient light. Connect the photocell to the buzzer or siren with glue. Let dry completely.

  • Use Velcro to mount the laser to the door frame on the side closest to the electrical outlet. Use one of the marks made in the first step as your guide. Plug the AC adapter into a wall socket.

  • Mount the photocell and buzzer -- opposite the laser -- with Velcro. Turn the laser on and ensure the light from the laser hits the photocell.

  • Insert batteries into the buzzer. Turn the laser on if it was turned off while you were checking the light earlier. Break the beam, either with the door or a hand. If the alarm sounds when the beam is broken, everything works as it should. If it does not, ensure the light reaches the photocell, the laser is plugged in and that the batteries in the alarm are working.

Tips & Warnings

  • Replace batteries in the buzzer every six months, just like a smoke detector. To remember this, replace batteries when the time changes to and from daylight saving time.

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