Änderungen

With R3/R4 the input voltage of 5V is simply divided by half and connected to GPIO pin 7. When the main power supply fails the level at this pin quickly drops to a low-level, otherwise the Raspi will detect a high-level. A simple shell-script using the "gpio" command is polling every second the state of this pin and trigger a shutdown if the level goes to zero.

== Result ==

It works pretty well but those big caps are expensive if you have to buy them. It could be done much better and with much smaller supercaps if a step-up-converter is used to generate the 5V or better directly the 3.3V that the Raspberry only needs (avoiding that power-wasting linear regulator).