Principle of operation : two ss49e hall sensors work in differential mode, temperature drift same for all sensor. Temperature compensation not needed. Frame on the sensor for the dust filter
Used parts
two ss49e hall sensors https://aliexpress.ru/wholesale?SearchText=ss49e
one 5x2mm magnet https://aliexpress.ru/wholesale?SearchText=5x2%20magnet
two bearing roller 4x8 mm (you can use round part of 4 mm drill rod)
Spring from automatic pen
15-16 mm long M3 screw and M3 nut
Wires
Sensors in the body are located opposite sides !!!
If you using klipper firmware no additional details needed. Sensor support added https://github.com/KevinOConnor/klipper/issues/2426
For calibration you need have 2 known diameter. I use 1.5mm and 2 mm drill rod for 1.75 mm filament
Hall based filament width sensor assembly
https://youtu.be/TDO9tME8vp4
klipper setup procedure https://3dua.info/topic/146-datchik-diametra/
For using on marlin you need use additional arduino board from my previous thing https://www.thingiverse.com/thing:3096289