Document use of pins to create a "case open" switch.

[brianredbeard]

Similar to older PC cases, we should evaluate a way to provide feedback to the Raspberry PI to know if the case it is in has been opened. At the same time, this largely predicates other things like:

• constant power
• stable storage

Similarly the "right" way to do this in the world of distributed systems is more along the lines of endpoint monitoring so that you know if/when the endpoint goes offline.

Thoughts?

[thatch]

Sure! If there aren't any requirements about shared ground (say, to use an existing switch in a piece of equipment) this is straightforward:

• Hardware, using the switch inputs documented at https://github.com/google/makerspace-auth/tree/master/hardware/pi-hat-1#user-content-pinouts
• Software, using a pullup oin the gpio and reading current state/an interrupt. If using our framework, the ThreePosSwitch does this in a pretty safe way with bounded delay that doesn't require debouncing, and can be used as a guide.

Were you suggesting this for the tool's case, or the authbox's case? The box we spec'd doesn't have a good way to mount things near the lid, but we're still looking for something better.

I worry about removing power to the authbox as a way around that; we don't currently have a battery backup.