This will explains the basic process of using a optical end stop on RAMPS 1.4 with Arduino 2560.

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Required Hardware:
Arduino 2560 with USB cable
RAMPS 1.4
Optical Endstop with cable

Required Software:

Arduino studios 1.8.2 or higher
https://www.arduino.cc/
Pronterface.exe
http://www.pronterface.com/
MarlinFW (Firmware)
http://marlinfw.org/meta/download/