Filament Runout Sensor

This is my filament sensor remix for [these switches from Amazon](https://www.amazon.com/dp/B00MFRMFS6/ref=cm_sw_su_dp). I also enlarged the filament hole to fit a length of 4mm PFTE tube, which should give a very smooth filament path, and it plugs nicely into my [reverse Bowden mount](https://www.thingiverse.com/thing:4779598). Just carve out in the spot where the roller hits it with a sharp hobby knife.