Other projects: https://www.instagram.com/engineeringns/ This runout sensor uses two ball bearings to ensure that there is no wear, it also allows the filament to be fed from both directions so that the printer can auto-eject it after the switch sees a runout condition. Bearings: 3mm-ID, 8mm-OD, x 3mm thick 5mm-ID, 8mm-OD, x 2.5mm thick https://www.amazon.com/5x8x2-5mm-Rubber-Shielded-Bearings-MR85-2RS/dp/B019I3PGWG/ref=sr_1_3?ie=UTF8&qid=1546484223&sr=8-3&keywords=5x8x2.5