Fair less cumbersome and dimensionally dependant (remember, spools are all different sizes and widths...) to just do what is done over in the Filament Alarm section... which solves the problem for you. Microswitch, either on the rear frame or right above the extruder, that closes when the filament is removed from the path.

This then deals with tangles, stripped filament, broken filament and out of filament... and doesn't care what your 'spool' is (for example, Laywoo - doesn't even come on a spool..... )....

Don't let the above deter you... but theres always more to a solution for a problem than there appears If you are a little time-poor, I'd file this one for a solid rainy day, and in the meantime, just do the tried-and-tested filament alarm with a micro-switch....

... And I'm not aware of any 'drag' thats induced on the filament... particularly if you are using a roller microswitch.... *shrug*

I was thinking that idea might work for a filament out detection system.

Have some rings on the PVC pipe that holds the spool, and run something really light (balsa, drinking straw, etc) from the rings across the face of the spool. The filament coming off the spool would keep the straw from falling all the way down and rotating the rings, once the filament runs out, the straw could fall, the rings would rotate and something like two pieces of aluminum foil or coils of copper wire could make contact and alert you to the filament being out.

Make it light enough, and it would almost certainly involve less drag on the filament than the schemes that run it through a small hole past a microswitch.

I'll probably try this someday soon (the problem is fitting it in with all the other things I want to try :-).