How to Make a Security Laser

Remove the housing from the laser pointer and remove the battery. Clip and strip the ends of the leads of the AC adapter, and connect the adapter to the battery connections using a soldering iron. Be sure to match the AC adapter specs to the laser diode requirements and wire the positive lead to the positive connection.

Attach the laser to the wall near the area you want to protect. For example, you could put it near the lower left corner of an open doorway, pointed to hit on the edge of the other side of the door. You can tape it, attach it with fabric fastener, or use whatever attachment method you prefer.

Place a mirror at the point where the laser hits. For example, you would stick it with tacky wax on the edge of the door so that the laser spot strikes it in the center. Adjust the angle to send the beam back across the door, to strike the opposite side about 3 feet up.

Place another mirror at the spot where the laser spot hits. Repeat this, creating a chain of mirrors, until the area you want protected is sufficiently covered. For example, in the doorway, you could have the original mirror in the lower right-hand corner, the second, 3 feet up on the left side, and a third, 5 feet up on the right.

Adjust the last mirror to point at a convenient spot for your photocell. If your alarm circuit is to be AC powered, for example, you would want your photocell close to an electrical power outlet. For the example doorway, put the photocell close to where the laser is, near the lower left corner of the doorway.

Measure the output voltage of the photocell when the laser beam strikes it, and when the beam is blocked using a multimeter. Record the two readings. For example, assume the photocell has 9 kilohms of resistance when it’s illuminated, and 110 kohm when the beam is blocked.

Set your circuit up to trigger when the resistance transitions from the “beam on” to “beam off” values. For example, you could build a voltage divider with the photocell connected to ground and a 47 kohm resistor connected between the photocell and +5 V. When the light is on, the voltage at the junction between resistor and photocell would be about 0.8 V, and when the light is off the voltage would be about 3.5 V. You can feed that voltage into a comparator and have it trigger the alarm.

Connect the photocell to the alarm circuit and put it in place. Again, tape, use fabric fastener, or any other appropriate method.