How to Make a Broken Beam Alarm

A broken beam alarm is triggered when someone breaks a beam of light -- usually the light is infrared (IR) so it will be invisible to the intruder.

Beam alarms detect intruders.Beam alarms detect intruders.
The system is somewhat counterintuitive -- if you break a circuit, it sends current to the alarm, and then when the beam is restored, the current that goes to the alarm does not go off. The behavior of broken beam alarms is created by using separate power supplies and two relays that control each other's access to power.

Make some fundamental decisions about your alarm system before you wire up anything. Decide how you are going to set up the beam and what kind of alarm you need. Figure out how high or low to set the beam. Each situation is different. The basic alarm choice is between a noisy alarm to scare off intruders, or a silent alarm to trap intruders.

Wire up the primary circuit. This is the circuit that includes the beam but not the alarm. The primary circuit is a simple series circuit -- a loop. The components in the primary loop include a power supply, the beam projector and receptor, the coil of the primary relay, the contacts of the secondary relay, and the testing switch. The primary circuit contains the beam and the secondary circuit contains the alarm -- each circuit turns the other circuit off. When the primary circuit is on, the secondary circuit is off and vice versa. The testing switch is in the primary circuit -- it is a push switch that is used to test the alarm.

Wire up the secondary circuit. This is the circuit that includes the alarm. The secondary circuit is also a simple series circuit -- a loop. The components in the secondary circuit includes a power supply, the alarm, the coil of the secondary relay, the contacts of the primary relay and a control switch. The relay in each circuit turns off the other circuit. The contacts of an NC relay (NC means normally closed) are closed unless there is current through the relay coil, which acts as an electromagnet to open the contacts. When the beam circuit is on it keeps the alarm circuit off. When the beam is broken, it turns the alarm circuit on. As long as the alarm circuit is on, the beam has no effect on the system. The alarm continues until the control switch is depressed.

Things You Will Need

  • Beam projector and receptor
  • Two power supplies
  • Two NC-type relays
  • Wire
  • Alarm
  • Control switches


  • Mount the control switch and the test switch near each other. This will make installation, testing and restoration easier.


  • If the beam projector and receptor are visible, an intruder can step over the beam or bend over to go under. If you cannot hide the beam equipment, you might consider a system or mirrors that reflect the beam one or more times across the port of entry before it reaches the receptor.