Door/window sensor set. Magnetic type contact sensor. Notifies you when your door or window is open. Supervised Sensor: monitors the sensors and their battery condition automatically. GM-318R can work with up to 4 sensors. (additional sensors sold separately) Both audio and visual indicators. Operating range: 300 feet. Best of all, the House Link Door/Window Alert System is easy to set up, with the sensor alerting the receiver wirelessly.