Elegant overkill: a printer filament monitor

This will email you if your 3D printer runs out of filament. It will work with any printer that uses filament on spools - though you will need to use a spool holder that uses ball bearings like those referenced in the instructions.
In contrast to the minimalist filament monitor I created specifically for the Replicator 2, this will work on a broad range of printers (with the above provisos) - and is significantly more complex. It uses an Arduino Uno with an Ethernet board, simple custom elect