Which proximity sensor to use?
Tomas Sanladerer has produced a nice video on this topic: https://www.youtube.com/watch?v=il9bNWn66BY
@ 7:31 you see an overview of the precision of various sensors, including the ones you mention. It appears that the M12-4 and M18-8 sensors are more accurate than the M8-8.
It depends on your surface. Inductive works for Aluminium heat beds, but not when covered with glass. Capacitive sensors are inaccurate and should be avoided. The touch sensor types work with all surfaces.
I'm using the LJ18A3 - 8-Z / BX DC6-36V 8 mm Inductive Proximity Sensor NPN @ 12V using an OptoCoupler module.
My bed is mk3 Aluminium. What is an optocoupler???Why do we need it???Can you share the wiring diagram?
@JoseBen Actually, that recently became a question of its own: https://3dprinting.stackexchange.com/questions/6358/inductive-sensor-in-24v-machine