This is an experimental Z height probe using a hall effect sensor wired to a spare analog input port.
Sliding the retract bar drops the probe below the nozzle tip.
Before a print, a gcode will probe the bed and set the Z height.
After probing, when the nozzle approaches normal print height the magnet will automatically pull the probe up into it's storage position.
A Z end stop is no longer used and the probe will automatically adjust Z height to varying bed heights.
After configuring Marlin, just add this Gcode start sequence:
G28 X0 Y0 ;Home X and Y ONLY
G29 ;3 Point bed Probe
G1 Z.5 F200 ;Force probe retraction
All that you need is that printed part, 3mm stub and a magnet.
I used 3mm stub and a part of magnet from an old CD drive.
All instructions are under this page
Thanks for the author.
All that I done is extract STL files from Sketchup file... ;)