The particle accelerator at CERN is a very complex system. I worked as part of the group who designed the "electronic brains" controlling the behaviour of thousand of gigantic magnets.
The purpose of the electronics I worked on was simply to read information or measurements, and to make decisions according to them.
An example would be to shut down something if a short circuit or excessive heat is detected.
More precisely I worked on two big projects.
The biggest project was the design of a system that monitors if some measurements are within acceptable boundaries. If they ever go out of the normal boundaries, the system issues a signal to remove the particles from the accelerator.
This sounds simple, but the real world requirements made this a very complex system.
My role was to design all the hardware (as seen in the picture bellow), get it manufactured, troubleshoot the prototype, write sofware for the microcontrolelrs and FPGAs on it, and test it once deployed inside the LHC system.
Another project was to debug a system that wasn't working. This system was a radiation tolerant communication interface. Both the firmware and hardware had been tested individually, but when put together they didn't behave as expected.