AlwaysOn

This app is open source there is does not contains ads all other apps related to it on Google Play Store are highly paid or contains ads and have poor functionality or just battery hunger,
AlwaysOn is an app for every device with an AMOLED or OLED display. It keeps your screen on while the time and battery percentage are displayed but it does not drain your battery.
Also, icons appear when you plug in your headphones or start charging your device.
You can customize your always on display and toggle it on or off with quick tiles and widgets.
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Source code