This is a clever and simplified filament sensor for 1.75mm filament.
It is well designed to fit most of 4mm OD PTFE tubes.

You just need to find de proper limit switch with the roller lever. I got the light blue ones but I've seen them in black as well.
Find the proper logic for your filament endstop or define it in your firmware. It depends on the contact you choose (NO or NC). Then activate the pulldown or pullup resistor via firmware. It is all related to your firmware logic so you have different configurations that can work.
If you encounter reading problems such as false contacts or erratic signal behavior (which are not common), use a little ceramic capacitor between the two contacts.

Materials:
-Small limit switch with roller lever.
-2x 10mm M2 screws, to attach the endstop to the case, they screw directly on the part
-2x 12-15mm M3 screws, to close the case
-2 M3 nuts
-PFTE tube. Length depends on your needs and where you'd like to install the sensor

CUTTING YOUR FILAMENT END WITH A GOOD ANGLE IS CRITICAL TO INTRODUCE THE FILAMENT WITHOUT TROUBLE, AS SHOWN IN THE IMAGE.