Low Resistance Optical Filament Runout Sensor

I needed a reliable and low resistance filament runout sensor for a printer that I'm designing. Clearly, credit for inspiration goes to the Prusa sensor design though i wanted to make the lever ratio a little higher to be less sensitive to tolerances.

It uses a pair of 605 (5mmx14mmx5mm) bearings to sense the filament and a spring scavenged off a ball point pen to provide the small amount of pressure required to follow the filament. It uses M5 bolts as axles, however if you're ambitious, you could cut some shafts :).