A very small, one piece, printed filament-out sensor designed to fit a miniature snap-action switch. Mount this device on either the inside or outside of your printer between the filament spool and the extruder. Pushing the filament through the hole opens the switch. When the filament runs out the switch closes. You can use that to pause or stop your printer by connecting to the right pins on your controller board and programming the firmware to recognize the switch closing as a fault.