Odele

Her two wires came in, left and right, each on or off as usual. But Odele didn't care how many were glowing. She cared whether they were the same or different. If both were on — same — she stayed dark. If both were off — also same — she stayed dark. It was only when the two DISAGREED, one glowing and one not, that Odele flared up bright, as if to say: these two don't match, and someone ought to know.

"That's a strange thing to get excited about," said a snail. "Agreement is peaceful. Why light up for a squabble?"

"Because a difference is news," said Odele. "When both my friends agree — both on, or both off — nothing needs saying; everything's settled. It's the moment they DISagree that something has changed, or something is wrong, or something interesting just happened. That's exactly when you want a light."

The circuit found a hundred uses for her. There was a pair of doors that were supposed to always be in the same state — both open or both shut. Wire them into Odele, and as long as they agreed she rested, dark and content. But the instant one door was open while the other was shut — different — Odele lit up at once: mismatch! someone left a door wrong! She was the little alarm that only ever rang when two things that ought to match, didn't.

She even turned out to be secretly essential to adding numbers — because when you add two bits, the "does-it-carry" part is really just the quiet question are these two different? But Odele didn't brag about that. She just did her one strange, precise job: sleeping through agreement, waking for difference.

"You're the opposite of a peacekeeper," the snail decided. "You go off when there's a disagreement."