First hopper passes through the light...
... which turns red and buzzes. Your hopper is diverted in a different direction from those in front.
Uh-oh! It was sensing for non-metallic material inside the hoppers! Obviously that would bugger up the recycling process a tad. Hopper A is now being taken to... well, whatever you think might happen to hoppers with non-metal inside, and Hopper B is now approaching the blue light!