Ultrasonic
and Microwave Motion Detectors
Ultrasonic and
microwave motion detectors work by transmitting an ultrasonic or microwave
signal and receiving the reflected signal back. The reflected signal
frequency is identical to the transmitted frequency if there is no movement.
Movement will cause an increase or decrease in the frequency of the
signal (known as the Doppler Effect), triggering an alarm. A microwave
motion detector uses a higher frequency than ultrasonic, allowing it
to detect motion through most interior walls.
Photoelectric
Motion Detectors
Photoelectric motion detectors (also referred to as a photoelectric
eye or PE) consist of two parts; a transmitter that emits an (invisible
to the human eye) beam of light, and a receiver that receives the beam
of light. If the beam of light is interrupted or broken by motion, an
alarm is triggered. PE's can be surface mounted or recessed and require
a straight line-of-sight between the transmitter and the receiver.
Passive
Infrared Motion Detectors
Passive infrared motion detectors (PIR) detect the heat energy given
off by (animal or human) bodies. If movement of the energy source (above
a certain velocity) is detected, an alarm is triggered. Because PIR's
transmit no signal, they can be positioned and adjusted to cover tighter
areas. To prevent false alarms caused by a pet, some units have a special
lens that prevent the PIR from detecting motion close to the floor.
Because they are passive (emitting no signal or beam), PIR's generally
provide the most economical form of motion detection.
Other
Motion Detectors
Floor mats are thin pressure switches that can be placed under rugs,
triggering an alarm if enough weight is applied to the mat. Stress detectors
mount on the bottom of the floor joists, triggering an alarm when stress
is created in the joists by movement on the floor. Magnetic detectors
placed on interior doors trigger an alarm when the door is opened.
Commercial Security |
Residential
Security
