CAT Reference Material

Motion or Beam Detector?

If you're like most people, you don't want strangers walking up to your home unannounced.  The miracles of modern security systems make this once unavoidable scenario very preventable.

For outdoor perimeter security, you have two basic options.  A Passive Infrared (PIR) motion detector or a Photovoltaic Beam detector. 

The PIR operates by detecting changes in the temperature of the scene in its field of view.  The heat of a person passing in front of an empty deck will trigger the sensor to alert the alarm system.  Many PIRs have what they refer to as "pet immunity."  Basically, the bottom part of its field of view is blocked, so short animals won't register a disturbance.  They are reliable instruments and are hard to fool, but they often register false positives.  If the device is mounted outdoors, then wind, rain, leaves, or moths close to the device can trigger the alarm.  The problem isn't as severe inside, but false positives are still very common. 

Most of us have experience with Beam Detectors in our garages.  Powered garage doors have to have a beam detector on it to ensure that it will not close down on something if the beam is interrupted by car, person, or pet.  It operates with a transmitter and receiver.  One end transmits a beam, and the other receives it.  If there is something in between the two devices, the receiver will tell the door not to close, since something is in the way.  In a security system, the detectors should be installed so they are slightly above the height of most dogs, and thus anything passing between the two will interrupt the beam and signal to the alarm system that someone is intruding.

Which to choose?

PIR

Pros:  
easy to install
cheaper
very difficult to elude

Cons:
many false alarms

Beams

Pros:
few false alarms

Cons:
difficult to install
more expensive
possible to evade if you are aware of them; see Catherine Zeta Jones in Entrapment :)

Bottom Line:
Even indoors, the false positives from PIRs are tiresome.  Outdoors, they render the device nearly useless for an alarm system, but they are fine for security lights or even an exterior piezo.  If you want outdoor perimeter protection, spend the money and get the Beam Detectors.

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