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.

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

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 emits a 1-tick (2 game ticks) redstone pulse from its opposite end.

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.

Observers will not detect the following block ID or data value changes:


 * Powering / depowering of activator rails
 * Inverting / deinverting a daylight sensor
 * Activation of a dropper or dispenser
 * Spread of grass to dirt, or decay of grass into dirt
 * Activation / deactivation of redstone ore
 * Growth of nether wart
 * Aging of fire, or changes to the data value that signifies flammable surfaces surrounding it
 * Changes in the shape of walls, fences, iron bars, glass panes, stairs, redstone dust, melon stem or pumpkin stem
 * A grass block becoming snowy or not snowy
 * Farmland going through its stages of hydration
 * A piston (not the head) extending or contracting
 * Locking / unlocking a redstone repeater or hopper
 * Moving bottles to / from a brewing stand