Observer

An observer is a block that emits a redstone signal when an adjacent block is updated.

Breaking
An observer requires a pickaxe to be mined. When mined without a pickaxe, it will drop nothing.

Usage
An observer is placed similarly to a piston, and will observe the block that it is placed against.

In Pocket Edition the observer block texture has two holes which are at opposite ends when the block is placed. The smaller hole outputs a redstone signal whilst the bigger hole detects the block update. In the Computer Edition, the texture of the detection end is that of an 'observing' face.

Behavior
An observer will detect changes in its target's block ID or data value, or the breaking or placing of a block.

When it detects something, the observer powers redstone dust, redstone comparators and redstone repeaters, as well as activates mechanism components located at its opposite end for power level 15 for 1 redstone tick. (2 game ticks)

It also counts as a block update when the observer itself is moved by a piston. When this happens, an observer emits a pulse after being pushed or pulled, but not beforehand.

Observers behave as a transparent block even though they block light. This means they cannot be powered with an external power source, nor power themselves with their own output.

Observers can detect a wider range of phenomena than a block update detector (BUD) circuit in the computer edition can detect, though has different limitations.

Trivia

 * When moved by a piston, an observer sends a 1 tick pulse after being moved, which makes it useful for slime block flying machines.