Filament Monitor for Prusa Direct Drive Extruder

Update:

I have finally completed this make but ended up using this sensor instead:

http://www.amazon.com/Effect-Magnetic-Sensor-Module-Arduino/dp/B013JM7ALC/ref=sr_1_6?ie=UTF8&qid=1450704779&sr=8-6&keywords=arduino+hall+sensor&refinements=p_85%3A2470955011

This sensor board also fits well in the redesigned idler and has the benefit of an LED to indicate it is activated. This makes it easier for positioning the magnet on the correct orientation.

You can see the video of the out-out-o