It uses an IR laser optical motion detector to detect filament movement. It then detects if there are any issues with the extruder.
The Duet3D Laser Filament Monitor uses an IR laser optical motion detector to detect filament movement. The detected movement is transmitted to the Duet 2 or Duet 3 electronic controller, which uses the indicated movement to determine if the filament has run out, jammed, or been ground away in the extruder. It is available for 1.75mm diameter filament only.
The monitor PCB comes with two Molex KK-style 3-way plugs and pins, along with ~60mm of Capricorn XS Ultra Low Friction PTFE tubing to act as the filament guide. The 3D-printed enclosure and M3 mounting bolts are not included. Please see the documentation for more information: Documentation for the Duet3D Laser Filament Monitor.
Note: The Laser Filament Monitor is sensitive to the filament type used: it has proved less sensitive to PETG of certain types. Please read the documentation for more details.
Warning: The Laser Filament Monitor is a Class 1 Laser device: Under normal conditions, the laser beam is always below the IEC/EN 60825-1:2014 maximum permitted output level of 716uW according to the manufacturer of the optical sensor; nevertheless, Duet advises against looking into the beam when the filament monitor is powered. You should most definitely not look at the sensor through magnification when it is operating. The laser beam is invisible infrared; therefore, your eye blink reflex will not protect you!