CrashProbe (Z auto leveling by Crash)

thingiverse

This experimental Z height probe employs a hall effect sensor, wired to an available analog input port. To use it, simply lower the retract bar and the probe will be positioned beneath the nozzle tip. Before printing, a G-code will measure bed height and adjust the Z axis accordingly. Post probing, as the nozzle nears regular print height, the magnet automatically pulls the probe into its storage position. Consequently, a Z end stop becomes unnecessary, with the probe capable of self-adjusting Z height to accommodate varying bed heights. For setup, follow this G-code sequence: G28 X0 Y0 (home X and Y only), G29 (perform 3-point bed probing), and G1 Z.5 F200 (force probe retraction). Instructions can be found at http://reprap.org/wiki/CrashProbe. The required components include a printed part, 3mm stub, and a magnet from an old CD drive. Kindly refer to the same source for detailed instructions and thanks to the author. I merely extracted STL files from a Sketchup file.