Note: There may be some minor differences between the images and the file as I make small improvements. The parts list will remain the same, but more than likely the Lid and Base will need to be reprinted if one of the changes.

Anyone who has been upgrading an ender 3 over the years likely has a bunch of spare parts laying around. I designed this filament runout sensor specifically to only use hardware from commonly upgraded/removed ender 3 parts (the extruder assembly and z limit switch from a v2 specifically, unsure if the parts are exactly the same on other models, but they should be close).

This model also has 2x M4 mounting holes which go through the entire model for use with wing nuts and mounting to extrusions. Zip tie insets are on the todo list, for now just run them through the holes... The model is 20mm thick with the lid, so you will need 2x 25ish mm M4 machine screws and wing nuts to mount it this way.

Aside from 2 (somewhat optional depending on your machines tolerances) M3 heat set inserts and the 2 printed parts, everything else is completely from the ender, including the screws and 2 other heat set inserts. These inserts can be removed by heating them up as normal, then quickly threading on whatever part was in it before and pulling it out. They are all fairly easy to remove. You will need:

-(1x) Z-limit switch from stock Z-Stop setup
-(1x) Idler wheel from stock extruder, along with its corresponding (1x) lock washer, (1x) m4 heat set insert, and (1x) m4 machine screw (there is also a longer m4 screw and heat set insert elsewhere on the extruder that may grab a bit more threads that will probably work better, you can use those as well). The lock washer is mandatory, do NOT lose it, or the idler will not be at the correct height to trigger the sensor/may rub or lock up.
-(1x) (optional) Boden tube fitting from stock extruder, along with its corresponding (1x) Boden heat set insert
-(2x) (optional) M3 Heat set inserts, sourced from anywhere. Many people who print have these laying around, if you don't, just go buy some. If you don't have any AND are broke, you may also see the screws section below.
-(2x) M3 Machine screws of 7mm or shorter. These also came from the stock extruder and went into the stepper. You may also use (2x) self tapping screws that hold the switch in its original mount, and no inserts, whatever works best for you.
-(1x) M3 Flat machine screw, sourced from extruder. This screw secures the Lid to the Base
-(1x) Black plastic tube thingi. No clue what this is called, but it came from the extruder as well. See the picture, you will know what I am talking about.

Assembly:
NOTE: All inserts have a small lip that they should be pressed up against. Be mindful to not over/under insert these.

1. Clean up any blobs, stringing, etc specifically where the board mounts and where the Idler pully rests.
2. Set the M4 insert. It goes under Idler Pulley, inserted from the build plate surface upwards. Make extra sure this one is straight and has a strong grab on the plastic, as when installing the Idler Pulley, the lock washer will be creating its own impression in the plastic to allow the bearing to sit level with the switch.
3. If using (2x) M3 machine screws to mount the switch, use the (2x) M3 inserts, inserted from the build plate surface upwards. You may need to clean up some extra filament, as these holes were made to also allow the self tapping threads. Just thread the M3 screw in from the bottom and it should push any blockages up for you to pluck/file.
4. Insert the largest heat set insert into the largest hole on the side of the print. This is for the Boden tube fitting. You may skip this if you don't want to run a Boden tube to your extruder.
5. Install the Idler Pulley. The lock washer goes between the bearing and the plastic, not under the screw head. You should not be able to see it when assembled.
6. Install the switch. You may use either (1x)/(2x) of the M3 machine screws or (1x)/(2x) stock mount self tapping screws. 2 screws are not needed, but I included a hole on both sides as the clearance of the plug may very from one switch to another and one side may not quite fit a screw once in the housing. The metal lever should have between 0.75mm and 0.95mm of clearance to the edge of the idler pulley. Any less may cause filament to not close the circuit. The lever should also have just a little bit of tension on it and rest directly on the plastic guide ramp under the pulley. This print is designed with very tight tolerances here, and the switch should honestly be just fine with a press fit of you really had to.
7. Insert black tube thingi into the top of the housing. This should be a snug press fit. If its not you may need to run a file/drill bit through it, or reprint and tune your "hole horizontal expansion" until it is.
8. Screw in Boden tube fitting, if using it.
9. Do a test run with some TPU or other flexible filament. It should require almost no "aiming" and should entirely self guide into the Boden tube with minimal force. The lever should put enough force on the filament to spin the pulley when moving the filament. If it doesn't you may have issues keeping the circuit closed aka false runout readings. Again, check your printers tuning/accuracy.
10. Secure the Lid to the Base using the M3 flat head screw. The hole is sized for this to self tap. The screw should site slightly below flush with the Lid. If it doesn't you need to check your machines/slicers hole tolerances.

Follow your boards instructions for wiring and setting NO/NC.