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.

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.

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:
 * power level 15 in Pocket Edition, and power level 1 in computer edition
 * 1 redstone tick (2 game ticks) in Pocket Edition, and 0 redstone ticks (1 game tick) in the computer edition

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.

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
 * Activation / deactivation of redstone ore, or of a redstone torch
 * A piston (not the head) extending or contracting
 * Locking / unlocking a hopper or redstone repeater
 * Spread of grass to dirt, or decay of grass into dirt
 * Growth of nether wart, forced growth of cocoa pods by bone meal, or growth of vines into empty space.
 * Aging of saplings or sugar cane
 * 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, vines, melon stem or pumpkin stem(Some only in PE)
 * A grass block becoming snowy or not snowy
 * Farmland going through its stages of hydration
 * Moving bottles to / from a brewing stand
 * Anything comparators measure as containers

Trivia

 * When moved by a piston, an observer sends a 0 tick pulse on PC (a 1 tick pulse on Pocket Edition) when moving is finished, which makes it useful for slime block flying machines.