Basic, simple, impractical, whatever.
Contains a little filtering and really simple, extra simple logic.
The concept is: power a Raspberry Pi Zero from a main supply of approx. 12V, when the main supply reaches a point where it needs charged and is removed, a logic level shifts from logic high to logic low, which the Pi catches and executes a shutdown command - cleanly shutting down and hopefully preventing data corruption.
Replace the linear regulator with a switching supply to increase efficiency.
