The Raspberry Pi and other similar Linux-based single board computers simplify many projects. However, one issue with Linux is that it doesn’t like being turned off abruptly. Things have gotten better, and you can certainly configure things to minimize the risk, but–in general–shutting a Linux system down while it is running will eventually lead to file system corruption.
If your project has an interface, you can always provide a shutdown option, but that doesn’t help if your application is headless. You can provide a shutdown button, but that leaves the problem of turning the device back on.
[Ivan] solved this problem with–what else–an Arduino (see the video below). Simplistically, the Arduino reads a button and uses a FET to turn off the power to the Pi. The reason for the Arduino, is that the tiny processor (which draws less than a Pi and doesn’t mind being shut down abruptly) can log into the Pi and properly shut it down. The real advantage, though, is that you could use other Arduino inputs to determine when to turn the Pi on and off.
For example, it is easy to imagine a Pi in an automotive application where the Arduino would sense the ignition was off for a certain period of time and then go ahead and shut off the Pi. Or maybe the Pi needs to be turned on when a motion sensor fires and then turned off again once there is no motion for a particular time period. Any of these strategies would be simple to build with the Arduino.
We’ve seen a similar project that used an IR remote as the trigger instead of a physical button. If you are afraid the Pi will just lose power unexpectedly, you might consider a battery backup. If powering a Pi with regular electricity is too tame for you, try steam.