Method of Operation

A typical occupancy sensor is comprised of a motion detector, electronic control component, controllable switch (relay) and power supply. Most detectors sense motion based on passive infrared and/or ultrasonic methods of operation. The electronic control makes a determination of where a human is present or not present and sends the input signal to the relay, which opens or closes the power circuit. Depending on the sensor, we can set the sensitivity of the sensor to adapt it to the normal amount of motion that occurs in the space. We can also set the time delay between the sensor detecting a lack of occupancy and turning the lights off (usually about 10-15 minutes).

Passive Infrared (PIR) Occupancy Sensors
PIR sensors respond to sudden changes in background heat energy by detecting the appearance of heat energy at a wavelength emitted by humans. They must be able to have a direct "line of sight" to an occupant to detect his presence. A curved faceted lens defines the field of view as a fan-shaped series of vertical and horizontal "cones" of detection projected from the sensor. The farther an occupant is from the sensor, the wider the gaps between these cones. Therefore, the sensor’s sensitivity to motion decreases with distance from the sensor.

Ultrasonic Occupancy Sensors
These sensors use a quartz crystal that radiates high-frequency (25-40 kHz) sound waves undetectable by the human ear. These waves are emitted into the sensor’s field of coverage, where they bounce off of objects, surfaces and people. When the waves bounce back to the sensor, their frequency is measured. Motion is detected via a slight shift in frequency (Doppler Effect), triggering an occupancy signal. Ultrasonic sensors can "see" around objects and surfaces if the surfaces in an enclosed area are hard enough to bounce back the sound waves for detection.

Dual-Technology Occupancy Sensors
Dual-technology sensors utilize both ultrasonic and PIR technology for maximum reliability and coverage with a minimum of false triggers. Both ultrasonic and PIR signals are required to switch on the lights and switch them off, while one or the other is required to keep the lights on.

New Technology
Occupancy sensor manufacturers are pushing the envelope with technology that improves aspects of performance such as adjusting sensitivity automatically and being resistant to false triggering due to airflow, coverage fluctuations due to temperature and humidity and blackouts caused by coverage gaps. Every manufacturer wants to be the best and builds additional value into its products; investigate the manufacturers to see what’s new.

 

Manufacturers of Occupancy Sensors

Passive Infrared

Ultrasonic

Acoustic

Multi-Technology

Photosensor

Ceiling Mounted

Wall-Mounted

Pole Mounted
     
     
     
     

 

PIR Sensors

Ultrasonic Sensors

A given sensor might be sensitive to:

Hand movement up to 10 ft.

Upper body movement up to 20 ft. Full body movement up to 40 ft.

 

 

Totally restricted to detecting occupancy within "line of sight"

 

 

More sensitive than ultrasonic in situations where the occupant is moving laterally in front of the sensor rather than towards or away from the sensor

Not sensitive to small changes in the space, resulting in fewer false triggers unless a sudden heat change occurs in the same wavelength range as that emitted by humans

More effective for restricted-coverage areas such as aisles, outdoor and high-bay applications

Costs a little less to purchase

A given sensor might be sensitive up to:

Hand movement up to 25 ft.

Upper body movement up to 30 ft.

Full body movement up to 40+ ft.

(In general, can cover a larger area than PIR sensors)

Less dependent on "line of sight" for occupancy; may detect occupancy behind objects/around corners if in enclosed space with hard surfaces

More sensitive than PIR in situations where the occupant is moving towards or away from the sensor rather than laterally

 

Ultrasonic sensors are more sensitive to small changes in the space, such as breezes from HVAC system or windows, resulting in more false triggers unless carefully calibrated

May result in false triggers in restricted-coverage areas, outdoor and high-bay applications due to "leakage" of waves

Costs a little more to purchase

MOUNTING

Indoors, occupancy sensors can be mounted on walls at the wall switch, on the ceiling or at the workstation.

  • Wall switch. These sensors are mounted in place of the standard manual wall switch hardware. Be aware, when specifying the occupancy sensor and assessing its performance, that the standard manual wall switch was located for easily accessibility, not for its effectiveness for automatic lighting control. However, wall-switch-mounted sensors are popular for private offices, conference rooms and closets/copy rooms.
  • Ceiling-mounted. These sensors, the first introduced and the most popular, utilize low-voltage wiring that connects them to a low-voltage power supply and to relays that perform switching in the ceiling plenum. Ceiling-mounted sensor installation costs more than wall-mounted, but can be economical and effective to cover large areas that would are either impossible to cover with wall-mounted sensors, or would require a larger number of wall-mounted sensors, such as partitioned office plans, large open office plans, warehouse aisles and corridors.
  • Workstation. Workstation occupancy sensors can integrate the local task lighting with other portable devices (such as radios and heaters) to switch off all of these devices together when a lack of occupancy is detected at an individual workstation.
  • Outdoor. Occupancy sensors can be mounted outdoors as well following manufacturer guidelines; for outdoor applications, PIR sensors generally experience fewer false triggers.

 

SWITCHING OPTIONS

For wall-switch-mounted and ceiling-mounted applications, these options are available:

  • Automatic-on. The occupancy sensor switches the lights on automatically when occupancy is detected and off automatically, after a preset time delay, when a lack of occupancy is detected.
  • Manual-on (switch-mounted). The occupant must manually flip the switch to active the lights, while the sensor switches the lights off automatically in case the occupant forgets when exiting.
  • Bi-level switching (switch-mounted). The two switches in a double-gang wallbox each control one ballast in each two-ballast fixture, enabling dual-switching between 50% and 100% of light output based on occupancy. The switches can also be connected to bi-level electronic ballasts--one that provides for 100% light output for all lamps and the other that provides a lower light output.

Other specialized switching options are available; check with manufacturers to learn more.

Introduction to Occupancy Sensors

Method of Operation

Applications