Alerts you anywhere inside your house when your garage door is open! Garage door monitor set. Notifies you when a garage door 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 Garage Door Alert System is easy to set up, with the sensor alerting the receiver wirelessly.