The TD-20U Indoor/Outdoor Sensor Transmitter from Optex offers two selectable detection patterns, a 17' fan pattern or a 50' beam pattern. The sensor uses PIR to detect motion, and once motion is detected it sends a signal back to the RC-20U Chime Box Receiver using RF signals. The device is powered by a standard 9V battery (available separately) which can last for approximately 2 years when the sensor is triggered about 250 times a day. There is a red LED that indicates when the sensor has detected motion and when the battery is low. Up to nine TD-20U units can be added to one RC-20U Receiver, enabling you to set up a full perimeter around your property.