Observer

Observer watches exactly one block and reports when it changes. The carved face is the sensor, and it sees the single block directly in front of it and nothing else — not a mob walking past, not a block two spaces further on. When that block is placed, broken or changes state, a short pulse leaves the small red output on the opposite side.

Orientation is what trips people up, because an Observer cannot be rotated once it is down: the face points where you aimed when you placed it, and getting it wrong means breaking it and placing again. Point the face at the crop, the piston head or the block you want watched, and take the signal off the back.

Crop farms are the standard job. An Observer beside the top of a sugar cane fires the instant the cane grows, driving a piston that cuts it, and the same pattern harvests bamboo, melons, pumpkins and cactus without a clock ticking all day. Two of them facing each other make a self-sustaining observer clock that runs until you break it.

Observers are also pushable, which is what makes flying machines possible — an Observer, a sticky piston and a slime block will carry themselves across a world. If yours seems dead, check what you are asking it to see: it reacts to block changes, not to items or players crossing in front of it, and its output is a single flick rather than a held signal, so a lamp wired straight to one blinks instead of staying lit.