Filament RunOut Sensor for 3mm

As Marlin integrates a Filament Runout sensor control, I made it with a cheap Slot Type Optical Coupling Module 3.3V-5V LM393. You can buy it for 1,5€ aprox. in Ebay.
It's for 3mm filament.
Arm.STL is for install it in the free motor screw hole of a typical Greg's Extruder.
Not tested yet with Marlin. I tested it with an Arduino UNO and works like Marlin needs (LOW when filament out).