If you are looking for the continuation of this great post, I have published a post on how to interface a Pixhawk with an Arduino and use it to control on-board lights. It is the explanation of my code above and how to modify it setp by step:
https://discuss.ardupilot.org/t/mavlink-and-arduino-step-by-step/25566
Specially dedicated to @LuisVale 
.