Observer

An observer is a block that emits a redstone signal when the block or fluid it faces experiences a change.

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

Usage
An observer is placed similarly to a piston. It observes the block that it is placed against. The texture of the detecting side is that of an observing face. As observers can detect the state of other observers, placing two adjacent observers, each watching the other, can make a fast and compact redstone clock. They send out a pulse.

Behavior
$$, an observer detects changes in its target's block states, or the breaking or placing of a block (i.e. changes in its block state, but not its block entity data). This means that changes like the age of crops can be detected because they are part of the block states.

$$, an observer acts as a block update detector and detects anything that causes a block update.

The causes and propagation of block updates are different between Java Edition and Bedrock Edition. As a result, each can detect some kinds of changes that the other cannot. See the table below for a comparison.

When it detects something, the observer emits a redstone pulse of strong power at level 15 for 2 game ticks (1 redstone tick). The pulse can power redstone dust, a redstone comparator, a redstone repeater, or any mechanism component located at its opposite end.

$$, the pulse is emitted with a delay of 1 redstone tick. In Bedrock Edition, it is supposed to be delayed by 1 tick as well but is actually delayed 2 redstone ticks due to, a bug causing redstone delays to be incorrect when components are activated by world changes (which, in the case of the observer in Bedrock Edition, is the only way it can be activated), as opposed to pure redstone components ticking. Its timing can also be incorrect due to.

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.

Though they block light, observers behave as transparent blocks in one sense: they cannot be powered by an external power source, nor power themselves with their own output (they can be powered with their own output if it's connected to the observing face).

Because observers in Java Edition detect changes in the block state, and not block updates, they can detect a wider range of phenomena than a block update detector (BUD) circuit in Java Edition can detect (as some block state changes don't cause block updates). Observers in Bedrock Edition do detect block updates but not block state changes, and so they detect the exact same things that any other BUD would detect in that edition.

Note Blocks
The observer can be placed under note blocks to produce a "bass drum" sound.

ID




Metadata
$$, observers use the following data values:

Trivia

 * When moved by a piston, an observer sends a 2-game tick (1 redstone tick) pulse after being moved, which makes it useful for slime block flying machines.
 * The current Observer texture was created because Jeb kept confusing the front with the back. He said it was inspired by the "rejected texture" created by Tommaso Checchi.