They’ve been staples of business offices for years, but now occupanc y and vacancy sensors are increasingly installed in homes. Used to switch off room lights after occupants have left, their numbers are growing thanks not only to the green movement, but also because of state mandates for homeenergy savings.

Watt Stopper/Legrend in Santa Clara, Calif., was one of the first firms to produce room sensors for controlling lights. The firm branched out into the residential market in 2004. A basic Watt Stopper wall switch contains a pyroelectric sensor and lens system, an ambient light sensor, a microprocessor, and a switching element.

The switches can distinguish people in a room from other sources of motion such as pets or even drapes dancing in a breeze. The key to this filtering, says Watt Stopper Marketing Director John Null, is in microcomputer programming and in the Fresnel lens beaming room light to the infrared sensor. The Fresnel lens is a special design that has both vertical and horizontal elements. Room light passes through it onto a two-element IR detector. “We’ve refined the detection technique by making sure the IR source is moving across the face of the lens. The processor applies rules about speed-of-movement and the amount of energy in the target before it makes a decision about whether to open or close the switch,” says Null.

The same basic electronics go into both occupancy sensors and vacancy sensors. The difference is in the programming of the microcomputer. Vacancy sensors turn off lights after a preset time delay once people have left the room. Occupancy sensors turn on lights when they detect people moving around, then douse lights once everyone has left. Both include time delays to allow for people sitting motionless.

Setups get more complicated with multiway sensors, as would be the case with three-way switches at both ends of a staircase. “When one of these detects motion, the other has to know about it because you don’t know which is connected to the load,” explains Null.

The only downside to these switches is that their sensors must draw power all the time. “We generally leak power to the sensor through the neutral or through the lamp, though we can’t leak current through CFLs or fluorescents,” says Null. Fluorescent bulbs also need wall switches having relays rather than triac switching elements.

There are more developments in store for automatic light controls. Null says ultrasonic sensors may be the next item to come off the drawing boards as a way to detect motion out of the switch’s line of sight.

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