Motion detectors come in several types such as Sonic or Ultrasonic, Microwave, and Passive Infrared (PIR).

The PIR sensor has two detectors in it, each one sensing IR. Each of the two detectors monitor a specified area. When nothing is sensed in the area, both detectors detect the same amount of IR in the area including the ambient amount radiated from the assigned room or walls. When a warm body enters the area, it is first intercepted by one half of the PIR sensor, causing a positive differential change between the two halves. When the warm body leaves the assigned area, the opposite happens, which results in the sensor generating a negative differential change. These changes are what triggers the detector.

Motion detectors must comply with applicable standards, such as ANSI/UL 639, Standard for Safety for Intrusion-Detection Units.

Below are some facts from UL 639:

Section 68 Movement Detector, Microwave Type

68.1.1 A microwave type movement detector system shall produce radio frequency radiation that covers a specific area and detects movement therein.

68.2 Sensitivity

68.2.1 The detector shall include provision for adjusting the sensitivity to intruder motion, area or distance range, or both, to cover areas of various sizes and configurations.

Section 69 Movement Detector, Sonic or Ultrasonic Type

69.1.1 A sonic or ultrasonic type movement detector system shall produce audible or inaudible radiation of sound waves to detect movement within a specific area or to create channels of radiation and detect movement through the channel.

69.2 Sensitivity

69.2.1 The detector shall include provision for adjusting the sensitivity to intruder motion, area or distance range, or both, to cover areas of various sizes and configurations.

Section 70 Movement Detector, Passive Infrared Type

70.1.1 A passive infrared type movement detector system shall detect invisible radiation emitted by a human body moving in the specified area.

70.2 Sensitivity

70.2.1 The sensitivity setting, range setting, or both of the system shall be sufficient to cover the areas of various sizes and configurations specified by the manufacturer.

Below is a sample of NFPA 731. For the complete section, see the actual NFPA 731 text at NFPA.ORG. Once there, click on the free access link to the latest edition of NFPA 731.

5.3.3.1.9 Motion Detection.

5.3.3.1.9.1 Motion detectors shall be in compliance with applicable standards, such as ANSI/UL 639, Standard for Safety for Intrusion-Detection Units.

5.3.3.1.9.2 Motion detectors used for protection of openings and building perimeters shall be installed in accordance with the manufacturer’s published instructions.

5.3.3.1.9.3 Motion detectors shall not be installed in environments that negatively affect the operation.

5.3.3.1.9.4 Motion detectors shall include, but not be limited to, the following:

(1) Two or more technologies combined in a single device

(2) Microwave

(3) Passive infrared (PIR)