This is a prototype for a replacement of the Prusa i3 Mk3 optical filament sensor, with a ball bearing and a PINDA sensor (or another inductive proximity sensor as I didn't have a spare PINDA sensor)
This style of sensor is still robust (unlike a micro switch) and is unaffected by filament colour, shine or transparency (iron filled filament will falsely trigger the sensor though) . This also means you can make your extruder any color you want, it no longer has to just be black.
I don't personally own a Mk3 (I have a Mk2s) but I use a Mk3 at work almost every day. With some filaments, the optical filament sensor is fantastic, but sometimes, it's just not overly cooperative.
This lead me to designing this extruder modification, (from the original Prusa R3 extruder files) influenced by the FINDA sensor setup in the MMU2.0.
Since I don't own the Mk3 that I work with, I haven't been able to install the modification, but a simple bolt on sensor mod (Which can be found here https://www.myminifactory.com/object/3d-print-83081) I also designed was used to test the operation of the sensor / bearing combo.
The bearing moves along the Y axis (front to back) as the extruder does not and therefore the bearing won't move away from the sensor during normal printing conditions.
I still need to finish the required firmware modifications to replace the optical sensor with this style of sensor. After printing, you will require a 10-14mm m3 Socket cap screw, a 5.5mmx1.5mm square nut, an 8mm Ball bearing and of course a PINDA sensor.
Hopefully Prusa will move to a system similar to this in the future.
Good luck and happy printing!