In outdoor lighting, photocells will respond to daylight levels and activate the outdoor light operation when darkness arrives. The photocell activates the lighting circuit for nighttime operation and the sensor turns on the light only when needed by responding to the presence of people. In rooms that do not have natural light, an occupancy sensor can prove highly conserving when connected to background lighting. The sensor will operate the lights only when people are in the room. Ultra-sonic sensors are best in rooms with partitions or dividers.
Infrared sensors are better for open areas.
Some sensors include both features. Integrated units that include the sensor and relay in a single housing that fits into a standard electrical box are priced more reasonably for residential applications. 2.3 Timing Devices and Dimmer Controls Timers turn lights on and off at predetermined times. Photocells respond to light levels and do not need rescheduling. Dimmers will reduce the light level and will only save energy when used consistently.