The eufy Entry Sensor E20 secures your doors and windows. You can connect your device to the app when you receive it. To use this sensor, you always need a eufy HomeBase (HB Pro, HB3, or HB2). As soon as someone opens a door or window, you get a notification via the eufy app. This way, you always know what happens. Even when you're not home. The sensor is small, so it fits on every window frame without it standing out. The battery lasts up to 5 years, so you don't have to worry about an empty battery. With this sensor, you're sure that your windows and doors are closed. This gives a safe feeling and prevents that your children or roommates accidentally leave a door open.