See this thing in action on:
http://youtu.be/01HIZuBam00
http://youtu.be/MP64M0xKElQ

I wanted to create a filament holder where the extruder wouldn't need to pull the reel to get it's filament deliverd. So i made this motorized version. It's dead simple, two cheap toy motors rotate the wheel when there is enough pull on the filament.
The pull is detected by two microswitches, either one of them switches current to the motors. This way you never have to turn anything on or off. The filament feeder accepts different size of reels without having to adjust bolts and nuts.

Everything is purpose designed in OpenSCAD, with the parameters defined up front, so you can easily modify things yourself if you want to. The drive wheel is a modified version of: http://www.thingiverse.com/thing:883165

The two motors are overkill, either one of them if powerfull enough to drive the reel, but i like the symmetry and they are almost a cheap as the bearing i would otherwise need.

This was really fun to build, i had the idea in my had for a couple of weeks and then saw the challenge, so decided to build it. All parts where designed on the go, so while the build progressed it became clear what was needed. And thanks to 3d-printing it's so easy to create just that part that's needed. The whole thing was build in two days and this first version worked precisely as i imagined.