Wire Bulb

Every other light in the game is on or off. This one is split into four quarters, each answering to its own wire color, so a single bulb can report the state of four separate circuits at once. Feed each color from a different sensor or switch and you have a compact status readout for a trap chain or a farm, without building a row of lamps and lever plates.

It floats wherever you put it with no block or wall behind it, which helps when the readout has to sit in the middle of an open room. Do not count it toward a house, though. The game will not accept it as the light an NPC room requires, so any build wrapped around one still needs a torch or a lamp tucked in somewhere.