The Aqara smart windows door sensor can detect the opening and closing of door and windows and send a notification to your phone via the APP. With the ability of matching with other Xiaomi smart devices such as gateway remote control, smart bulb, air cleaner, etc. this sensor is able to help you create a smart home. The windows door sensor consists of a sensor subject and a magnet. It detect the opening and closing of door and windows by sensing the near and separation of the sensor subject and magnet.