TMOTW #8 Filament runout sensor

https://www.thingiverse.com/thing:2095911

This setup is for octprint. https://www.youtube.com/watch?v=QImHhx3egLA

Here is a video showing how to do it in marlin without octoprint. https://www.youtube.com/watch?v=BwdlDlPiaoA

There are a TON of these on thingiverse. So thats for you to chooose. https://www.thingiverse.com/search/page:1?q=filament+sensor&sa=&dwh=315a14c5b25c01b