Hardware Design
The schematic is pretty simple. We use an ATTiny (either 45 or 85, whatever costs less) as the brains of our system. As a side note: It is actually quite impressive what you can press into the small memory of an ATTiny45.
We have the header H1 connecting to the Raspberry, the header H2 that allows us to connect the battery, switch, external voltage and button, and we have P1 that allows us to choose between the RPi voltage and the external connector. Interestingly, in earlier revisions I used a simple 2-pin header that simply cut the connection to the RPi. By using a 3-pin header instead we have a simple mechanical means to guarantee that only one voltage can be fed to the ATTiny and furthermore, that no excessive current can be drawn by e.g. connecting the EXT_V pin to ground while having the jumper connected.
To guarantee that no current can be fed back to the RPi we use a diode, and additionally we add a voltage divider that on one hand allows us to safely measure any voltage for which we choose the right resistors, and on the other hand guarantees that the ATTiny won't be powered parasitically through its protection diodes.
An LED with a current-limiting resistor and a decoupling capacitor round the whole thing off.
