How to attach a LJC18A3-H-Z/BX capacitive probe via optocoupler to Creality Ender 3 (and other printers)

This is a quick description on mounting the LJC18A3-H-Z/BX on an Ender 3 as an alternative to ready made kits available on the market.
You need some soldering skills and a bit of electro-technical knowledge. If you don't know what you are doing, buy the kits!!

I have deliberately not attached a PCB layout because this is so easy to set up on a prototype board - no PCB needed! And it's more fun! ;)
Warning: you need to flash a boot-loader on your Creality Melzi board. You will loose your ori