Hello there,
Currently I am doing a project using APM 2.8 and i wish to add a simple obstacle avoidance system to it using cheap sonar like HC-SR04, i just need the copter to back off when the sonar detects and object in front.
Directly connecting the sonar onto the APM would not be a choice as there is no more space in the memory to accommodate new commands. The only way to do it is to use an external processor/controller like Arduino or raspberry pi. currently i have an Arduino Mega 2560.
What I understand from many hours of reading forums, is that there are a few ways to do it. Either using I2C connection, UART or sending pwm signal to the input on APM like how the RC sends the signal to the APM. But i couldn’t find a guide on how to set up the connection and how to even send and read messages through MAVLink.
Please can anyone guide me on this? Many thanks in advance. 
Regards,
Jack Kee