A free app for Android, by Nu-Kob.

Auto Connection is a handy utility that helps you save battery life on your Android device by automatically turning off your mobile data when it’s no longer needed. It’s a useful application that will save battery life and help you get a better signal when needed. 

The application can automatically turn off your data when the screen is off or when you’re using a specific application. This can be done by adding the app to the Auto Connection list. It can also be turned off manually when you want.

The app will also be useful when you’re using Wi-Fi as it will help you to turn off your data when the Wi-Fi is connected and turn it back on when it is disconnected.