Hi
can you point me to a link to the code which is running on raspberry pi to communicate with pixhawk to turn lights on etc
Hi @sibujacob, welcome to the forum 
The autopilot firmware that runs on the flight controller (Pixhawk) is called ArduSub, and it communicates via (and is controlled by) the MAVLink Protocol.
The onboard computer (Raspberry Pi) runs some software that acts as a proxy to pass MAVLink messages between the serial connection to the flight controller and the ethernet connection to the control station (topside) computer. In a default setup the Raspberry Pi is not actually doing any controlling of the autopilot, it’s just passing along the control messages it receives from the topside.
For systems running our old Companion Software, there’s a diagram showing that connection here - the proxy software we used to use is called MAVproxy, and the “Pilot Control” and “Telemetry” arrows shown in the diagram are both accomplished using the MAVLink protocol.
Our new BlueOS software currently uses mavlink-router as its proxy software, although that may change in future.
Feel free to ask follow-ups if there’s something in particular you’re aiming to do or understand
@Eliot…If there were few people like you in the Px4 community forum, the world of flight controllers would have been explained and structured beautifully ! thank you for the feedback … I am still learning certain information and will revert back … 
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