You may have entered a room at work or some other public place and the lights turn on automatically. The room has an occupancy sensor. Wouldn’t it be handy to have them at home? You can.
Occupancy sensors, also known as motion detectors, turn lights and other equipment on and off automatically, depending on whether people are in the room.
They play a large role in sustainability plans by reducing lighting, and therefore the electricity necessary to keep the lights on. They are now also available for residential use.
Residential occupancy sensors are both inexpensive and effective. Some utilities offer rebates to customers, although they are most likely to offer them to commercial customers. They are most useful for rooms that sit empty much of the time or rooms where people might forget to turn the lights off and they’re on over night.
Depending on the size of the room, the kind of lighting used, and occupancy patterns, they can reduce lighting costs from 20% (for a small office) to 65% for a locker room. They pay for themselves quickly, with or without rebates.
How occupancy sensors work
The detectors operate switches with three basic technologies: passive infrared (PIR), ultrasound, and acoustic.
Infrared technology detects body heat. Some PIR switches also detect visible light in order to adjust for daylight conditions. In order to work, infrared occupancy sensors must have a straight, unobstructed line of sight. They are most suitable for regularly shaped rooms, such as an office, or rooms with high ceilings, such as an auditorium.
Ultrasound technology uses a high-frequency sound above the human threshold of hearing. Because the sound waves reflect off walls and other obstructions, they are most suitable in irregularly shaped rooms and rooms with partitions, large furniture, or structural columns.
Ultrasound sensors require attentive calibration to avoid false-on conditions; otherwise small motions caused by the ventilation system can trigger the switches. It is also necessary to select a frequency that will not interfere with other ultrasonic devices or hearing aids.
Acoustic technology uses audible sound. Like ultrasound, it can work around obstructions. Acoustic motion detectors will not be set off by movement of the air when the room is unoccupied.
More sophisticated occupancy sensors combine two or more of these technologies in order to minimize false detection. Specialized areas, such as bathrooms, stairwells, or hallways require specific designs in order to work properly.
Occupancy sensors also differ according to where they are mounted. The simplest ones replace an ordinary wall switch. They can also be mounted higher on a wall or on the ceiling.
Each of these technologies has advantages, disadvantages, and recommended uses, as shown in this chart from the US Department of the Interior.
|Hybrid or Combina-tion Sensors
|Integrated Daylight Sensors
|Wall Switch Sensors
|Wall or Ceiling-Mounted Sensors
|Two or more technolo-gies to minimize false detection, usually PIR and ultrasonic or PIR and audio.
|PIR or ultrasonic sensors with a light-level sensor.
|PIR, ultrasonic, or combination/ hybrid sensor and control circuitry packaged into one unit, sized to fit in a standard wall box.
|PIR, ultrasonic, or hybrid sensors designed to be mounted separately from the control unit(s), usually in high locations.
|PIR or ultrasonic sensors designed specifical-ly for bath-rooms, hallways, and stairwells.
|Can be very foolproof, allowing wide coverage and applications.
|Can be wired to a dimming circuit to control room lighting based on available light and occupancy.
|Small, inexpensive, and easy to install.
|Can cover wide areas effectively; switching units can control a variety of equipment.
|Specifical-ly designed for these spaces.
|They can be more expensive (for small area applica-tions), and may require more adjustments since sensors contain more than one sensing unit.
|They can be difficult to adjust and require a dimming ballast or special wiring.
|Their range can be limited, and depending on the location of the switch, they can easily be obscured.
|They tend to be more expensive and often necessitate rewiring.
|Rewiring may be necessary if certain lights need to stay on.
|Large, open areas and areas with unusual occupancy patterns or work require-ments.
|Areas that receive large amounts of daylight.
|Smaller meeting rooms, individual offices, and store rooms.
Home occupancy sensors
The table and article assume some kind of business setting. Homes have somewhat different needs. For example, you’ll want a vacancy sensor in your bedroom to prevent tripping the switch as you move in bed. Vacancy sensors are occupancy sensors with a different setting.
In other words, with the unit set to operate as a vacancy sensor you’ll be able to turn the light on and off manually, but the sensor will turn it off after a specified time if you leave the room. You can also select wireless battery-operated occupancy or vacancy detectors to avoid the expense of hiring an electrician.
You’ll still have the choice of infrared or ultrasound and where the sensors are mounted. Your choice is largely determined by where you are installing them. A bedroom, bathroom, closet, laundry room, or garage all have different requirements.
Lighting: Occupancy Sensors / US Department of the Interior [Link no longer active as of 2/16/16]
Green Building: Guidelines for Design / City of Santa Monica, California
Light switch with passive infrared sensor. Public domain from Wikimedia Commons.
Occupancy sensor technologies. City of Santa Monica Office of Sustainability and the Environment. [Link no longer active as of 2/16/16]