The Creality CR-6 SE's auto bed levelling system impressed me greatly. Ever since testing that printer I've been scheming about creating a similar version for the Ender 3. In this video I've managed just that, with free, printable parts and the standard electronics reused. This is compatible with Ender 3, Ender 5 and the original CR-10 (not S Pro).
This is not as good as something established as the BLtouch or EZABL. After slowing the probing speed it seemed to be repeatable to about 0.05mm on Z. I'm hoping the community now takes this concepts and improves it greatly.
Complete video guide here: https://youtu.be/hs6IVfNrf5k
Includes step by step instructions for printing, post processing, assembly and firmware/slicer changes.
Update: I've fitted a Bigtreetech mixing hot end to this printer: https://s.click.aliexpress.com/e/_dYpgnlf
Two new parts have been added to suit, a new mounting shroud and extended fan duct.
Video here: