I tend to leave prints unattended and I like to have them pause if they runout or if the filament breaks. 
 

Additional items needed:

• D2F-FL or D2F-L limit switch 
• 5.5mm ball bearing. 
• Available board pins
• M3x3mm heated inserts
• Silicone insulated wires 
• Whatever 2-pin connectors you prefer. I used 2.5mm JST SM just because I had some on-hand. 

You'll need to remove the arm from the limit switch if present.

Print in a high temp resistant material. I went with Polymaker Fiberon PA612-CF but any good ABS/PA should work. Just be sure your filament profile is tuned in your slicer or you may get some inaccuracy. In particular, ensure that flow, temp, and shrink are dialed in. 

• 0.16mm layer height (0.2 should also work, but 0.08-0.16 are what I usually run for dimensional accuracy and improved layer adhesion)
• Infill 100%
• 5 walls
• 4 layers top and bottom
• Painted support needed in switch location and under filament exit extension. (I will adjust model to have built in supports in future)

1. Remove extruder from printer and unplug all connections to rear pcb.
2. Carefully remove all screws from rear housing and fan side housing (if you still use the stock fan setup). It may help to take a quick picture or label bolts as you remove them. There are a variety of sizes and lengths here and they are specific to their locations. 
3. Once you've printed and test fitted just to ensure clearance, you will need to drop the 5.5mm bearing into it's hole and confirm that it can move freely. 
4. Gently bolt the limit switch in place so that the exposed button contacts the ball bearing. 
5. Insert a scrap of filament and listen for the"click" of the switch. Filament should move freely without resistance all the way through the housing. If you have any resistance, use a 2mm drill bit or 2mm reamer by hand to expand the filament path. 
6. Tighten limit switch bolts carefully as you are screwing directly into the print material. 
7. Now is a good time to insert the 4 threaded heated inserts on the sides. 
8. Install onto extruder with original bolts and reassemble extruder. 
9. Remove limit switch (careful not to lose the ball bearing). 
10. You are going to be running two lengths of wire through the original wire loom. Color is up to you, but I went with classic white and black for signal and ground. Ground (-) is wired to the C post and Signal (+) is wired to NC. The middle NO post can be snipped off or ignored. 
11. For sake of modularity, I only wired about 1" of wire from the switch to a female JST connector. I then put a male connector on the end of the 2 wires before they go into the loom. If I ever need to remove the whole extruder, I don't have to re-run the wires through the loom. 
12. Once switch is soldered up, reinstall it on the extruder and set that to the side or reinstall it on the toolhead carriage.
13. Turning our attention to the wire loom, carefully un-wrap the factory tape. If it gets damaged, black gaffers tape or German-style textile electrical tape are good options. You might also try a nice wide bit of adhesive heat shrink tubing if you have some that is wide enough. Run the two wires through the loom being careful to keep them flat, untwisted, and on the same side of the ribbon cable. Be sure you've cut them a bit longer than the ribbon cable just for margin of error. 
14. Once you've gotten the wires through the other end of the loom, work out the distance to the endstop pin you want to use and trim the wires an appropriate length so that they won't be strained. 
15. Refer to your board's documentation to ensure you terminate wires in the right order with the correct connector.
16. Connect to board then connect ant connections at the toolhead. 
17. Refer to your printer OS (marlin/klipper) documentation for Amy config changes needed to enable the new sensor. 

Good use cases may be:

• Simple runout detection
• Filament break detection
• Newly inserted filament to prompt or execute macro
• Multi-material load/unload/Bowden length detection