Observer

An observer is a block that emits a quick redstone pulse from its back when the block or fluid directly in front of its "face" experiences a change.
== Obtaining ==
=== Breaking ===
An observer requires a pickaxe to be mined. When mined without a pickaxe, it drops nothing.
=== Crafting ===
== 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. This enables them to be used in flying machines.
Observers behave as both opaque and transparent blocks: they block light and allow mob spawning on top, but do not block opening chests below, do not cut off redstone wire, and cannot conduct redstone power.
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 Bedrock 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.
=== Limitations ===
Change in block state 	Detected in 	Detected in
Change in block state 	 	
A block or a fluid being placed or broken in any way; either directly by player, enderman, or ravager or through:
* Placing/breaking two-block-tall/wide blocks
* Snow golem or snowfall creating snow
* Snow melting
* Dispenser placing carved pumpkin, wither skull, or shulker box
* Creating/removing fluid using player or dispenser
* Placing scaffolding on top/side of another scaffolding
* Entities that is on fire touching powder snow, causing it to melt
* Block being broken by fluid, (sticky) piston, explosion, opacity,For example, a button cling on a block, when break the block that a button are clinging, that button will break and cause block update which can be detect by observer.
* Inconsistent light levelFor example, a mushroom are on light level of 13, according to Minecraft Wiki, at level 13 and above, mushroom will uproot unless on mycelium, podzol or nylium. When update the mushroom, that mushroom will break and cause block update which can be detect by observer.Another example, a wheat block are on light level of 0, according to Minecraft Wiki, at level 0 and below, wheat block will uproot. When update the wheat, that wheat will break and cause block update which can be detect by observer.
* Flammable blocks getting burned up by fire or lava
* A falling block turn into a falling block entity and vice versa
* Cactus breaking either by placing a solid block next to it or by placing a fluid on top of it
* Villager planting and destroying crops
* Zombie breaking doors
* Leaves decaying
* Chorus plant break due to their invalid placementFor example, place 2 chorus plants on top of an end stone. When place a chorus plant on the side of the bottom one, that "bottom" chorus plant will break and cause block update which can be detect by observer.
* Using up a damaged anvil, causing the damaged anvil to break
* Eating all the cake
* Water touching a sponge, causing the area near the sponge to be removed and create a block update to the water
* Place the block into the replaceable block, replace a block by left-click and right-click at the same time;
* A wither rose block being planted after wither kills a mob if the ground is grass block, dirt, coarse dirt, moss block, mud, soul sand, soul soil, or netherrack
* Left-clicking the dragon egg, causing it to teleport and create a block update at the starting teleporting point and the teleporting destination of the dragon egg
* Boat and boat with chest breaking a lily pad
* Turtle laying turtle egg(s), trampling an eggs=1 turtle egg or turtle egg(s) hatching into baby turtle(s)
* Damaging a silverfish, causing a nearby infested block turn into silverfish and create a block update
* Frog laying frogspawn or frogspawn hatch into tadpole
* Thrown trident breaks pointed dripstone
* Sniffer egg hatch into snifflet
* The upper part of pitcher crop spawn when apply bone meal on the pitcher crop that have block state age is 2	 	
Water being inserted to or removed from a block that can be waterlogged. 	 	
A water (or lava, if lavaSourceConversion is set to true) source block being created by having 2 water source blocks unite to each other. 	 	
Fire being ignited or extinguished in any way; either directly by player or through:
* A dispenser
* Fire spreading to flammable blocks next to lava or fire
* Fireball from a dispenser, a ghast, or a blaze
* Lightning
* A bed or a respawn anchor exploding
* Place an end crystal on bedrock or obsidian in the End
* Being extinguished by fluid, splash water bottle, lingering water bottle, or naturally 	 	
A fire is turn into a soul fire (but not vice versa). 	 	
Fluid or powder snow being placed in or removed from a cauldron or the level of the water/lava/powder snow cauldron changing in any way, for example using bucket (empty, water, lava, powder snow), bottle, water bottle, rain, snowfall, pointed dripstone, burning entity on water/powder snow cauldron. 	 	
Block being replace by , , or commands.If a block get replaced by itself (with the same block states) by the command, the observer will still be able to detect that. This does not happen for and .If observer is placed using or , that observer will be powered. This don't work for . 	 	
Change block state of block using debug stick. 	 	
A nether portal being created. 	 	
A nether portal being destroyed. 	 	
An end portal being created. 	 	
An end portal being destroyed. 	 	
The creation of an iron golem, a snow golem, or a wither. 	 	
A (sticky) piston (head) extending or retracting. 	 	
A block being pushed or pulled by a (sticky) piston; either directly or stuck to slime block or honey block moved by the piston.If an observer being pushed/pulled by a (sticky) piston, that observer will also powered. 	 	
A monster spawner block activating. 	 	
The mob spawn of a monster spawner being changed using a spawn egg. 	 	
A (trapped) chest turns into a double (trapped) chest and vice versa. 	 	
A chest, a trapped chest, or an ender chest opening or closing. 	 	
A shulker box opening or closing. 	 	
A barrel opening or closing. 	 	
The inventory of a block that a comparator can measure as a container changing. 	 	
The crack stage of an anvil changing after use. 	 	
A snow layer being added into snow by player or by snowfall. the game rule snowAccumulationHeight must be larger than . 	 	
A beacon activating or deactivating. 	 	
A command block producing a success. 	 	
The unconditional/conditional stage of a command block changing. 	 	
The impulse/chain/repeat stage of a command block changing. 	 	
A piece of a cake or a cake with candles changing. 	 	
A hopper being activated or deactivated. (Turning the hopper on/off) 	 	
Dry farmland changing between its six dry-looking hydration stages. 	 	
Farmland changing between dry and humid or vice versa. 	 	
A dispenser or a dropper activating or deactivating. 	 	
A dispenser, dropper, piston or sticky piston changing quasi-connectivity state.The change of quasi-connected state usually coincides with some other block state change for these mechanisms, such as activating to dispense an item, or the piston head extending/retracting. Therefore, this effect is only noticeable when a dropper or dispenser "deactivates" by losing its quasi-connected status when the redstone power source is turned off. To demonstrate, use a lever to power a redstone repeater that quasi-connects to a dropper. Monitor the dropper with an observer. Turn the lever on. The dropper will drop an item and the observer will trigger. Turn the lever off. The dropper will not do anything, but the observer will trigger again because it detects the dropper's change in quasi-connected state.	 	
A (deepslate) redstone ore block activating or deactivating. 	 	
Grass block or mycelium turning to dirt or nylium to netherrack after being spread using bone meal or grass block or mycelium decaying to dirt or nylium into netherrack. 	 	
Coarse dirt or rooted dirt converting into dirt through the use of a hoe. 	 	
Grass block, dirt, or dirt path converting into farmland through the use of a hoe. 	 	
Grass block, podzol, mycelium, dirt, coarse dirt, or rooted dirt converting into dirt path through the use of a shovel. 	 	
Farmland or dirt path converting into dirt. 	 	
Concrete powder converting into concrete, either through direct contact with water, or entering water as a falling block. 	 	
A sea pickle being added into a sea pickle. 	 	
A pink petal being added into a pink petal. 	 	
A turtle egg being added into a turtle egg. 	 	
A turtle egg being removed from a turtle egg by breaking or trampling. 	 	
The hatch block state of a turtle egg and a sniffer egg changing. 	 	
A stone, cobblestone, deepslate, or any stone brick variant block being turned into an infested block by silverfish. 	 	
A double slab being created by adding a slab into another slab. 	 	
Ice melting into water or water freezing into ice. 	 	
Water freezing into frosted ice through the use of Frost Walker or frosted ice melting into water. 	 	
Turn pumpkin into carved pumpkin using shears. 	 	
Turn logs/woods/stems/hyphae/block of bamboo into stripped logs/woods/stems/hyphae/block of stripped bamboo using axes. 	 	
The stage of a sapling or bamboo changing. 	 	
The age of mangrove propagules, bamboo, wheat, carrots, potatoes, beetroots, melon stem, pumpkin stem, nether wart, cocoa, sweet berry bush, chorus flower, cactus, sugar cane, kelp, weeping vines, twisting vines, cave vines, fire, frosted ice, torchflower crop, or pitcher crop changing. 	 	
A torchflower crop turning into a torchflower. 	 	
Fluid flowing into the empty space to or draining from a block.When the fluid source block is removed, all of the fluid flowing from the source will also be removed and cause a block update. 	 	
Fluid, light block, or composter level changing. 	 	
(Flowing) water turning into stone through contact with lava. 	 	
Flowing lava turning into cobblestone through contact with water. 	 	
Lava turning into obsidian through contact with water. 	 	
(Flowing) lava turning into basalt through contact with soul soil and blue ice. 	 	
Water turning into bubble column or vice versa. 	 	
Upward bubble column turning into whirlpool bubble column or vice versa. 	 	
An eye of ender being inserted into an end portal frame. 	 	
Player or villager sleeping/waking up in a bed. 	 	
A grass block, a mycelium, or a podzol block becoming snowy or not snowy. 	 	
Text in a sign or a hanging sign being edited. 	 	
A dye or glow being applied to a sign or a hanging sign or removed from it. 	 	
Waxing a sign or a hanging sign using a honeycomb. 	 	
The mode of a structure block or a comparator changing. 	 	
A grass block, grass or fern being grazed by a sheep. 	 	
A carrot crop being eaten by a rabbit. 	, rabbits do not eat carrot crops; see and . 	
A button being pressed or returning to inactive state. 	 	
A pressure plate or the detector rail activating or returning to inactive state. 	 	
The power of a weighted pressure plate, redstone wire, daylight detector, or target changing. 	 	
A lever being flicked. 	 	
A redstone lamp or a redstone torch lighting up or turning off. 	 	
The mode of daylight detector being changed. 	 	
A TNT igniting. 	 	
Coral/coral block/coral fan converting into dead coral/coral block/coral fan. 	, the death of a coral fan cannot happen under normal circumstances — it happens the very next tick after the coral fan is placed, so the observer will already be reading the placement or water removal update, and coral fans placed using commands are waterlogged — but if lag is used to make the unwaterlogging and death events happen separately, the observer will detect both. For coral, the death also happens too fast for the observer to detect it separate from the placement or water removal, but a non-waterlogged coral can be placed using commands. Coral blocks take multiple seconds to die, and removing water next to the coral block does not activate the observer anyway, so they can be detected under normal circumstances.	
A sponge turning into a wet sponge. 	 	
A wet sponge drying into a sponge in the Nether.This event cannot normally happen, but if commands are used to place a wet sponge in the nether, the sponge will dry when it recieves a random tick. , sponges in the Nether always stay wet. 	 	
A door being opened or closed by a player, a villager, a piglin, a piglin brute, a vindicator, a wandering trader, or redstone. 	 	
A trapdoor being opened or closed by a player or redstone. 	 	
A fence gate being opened or closed by a player or redstone. 	 	
The in_wall block state of a fence gate changing. 	 	
A furnace, a blast furnace, or a smoker lighting up or turning off. 	 	
The distance of leaves or a scaffolding changing. 	 	
The bottom block state of a scaffolding changing. 	 	
A potion being moved in or out of a brewing stand. 	 	
Book and quill or written book being moved in or out of a lectern. 	 	
The note of a note block being changed. 	 	
A note block being left-clicked in Survival mode. 	 	
The instrument block state of a note block changing. 	 	
A music disc being inserted to or removed from a jukebox by player, hopper, or minecart with hopper. 	 	
A redstone repeater locking or unlocking. 	 	
The delay of a redstone repeater being changed. 	 	
A redstone repeater, a redstone comparator, an observer, a note block, a lectern, a lightning rod, an activator rail, a powered rail, a sculk sensorWhen deactivated, the sculk sensor's and the calibrated sculk sensor's "sculk_sensor_phase" block stage is converted from "active"->"cooldown"->"inactive". However, this happens so fast that observer can't detect it, so that isn't counted., a calibrated sculk sensor, a tripwire, or a tripwire hook activating or deactivating. 	 	
A tripwire or a tripwire hook attaching or detaching. 	 	
The disarmed or disarmed_bit block state of a tripwire changing.The disarmed or disarmed_bit block state cannot change without also detaching the tripwire, so this event cannot happen in isolation. 	 	
Sculk shrieker starting shrieking. 	, observers detect when a sculk shrieker stops shrieking, but not when it starts shrieking. 	
Sculk shrieker stopping shrieking. 	 	
Sculk catalyst blooming or unblooming. 	 	
A dragon head or a piglin head activating or deactivating. 	 	
A conduit activating or deactivating. 	 	
An item frame being placed or an item being added to, rotated in or removed from an item frame. 	 	
Something being teleported by an end gateway. 	 	
An item being inserted to or removed from a flower pot. 	 	
The shape or rotation of walls, fences, iron bars, glass panes, stairs, tripwire, redstone, vines, mushroom blocks, mushroom stem, fire, all type of rails, glow lichen, sculk vein, pointed dripstone, or chorus plant changing. 	 	
The top block state of the walls changing. 	 	
A melon/pumpkin creates from a melon stem/pumpkin stem. 	 	
The stem of a melon or pumpkin attaching or detaching. 	, placing a pumpkin or melon next to a corresponding stem does not cause the stem to attach. 	
Kelp/weeping vines/twisting vines/cave vines turning into a kelp/weeping vines/twisting vines/cave vines plant or vice versa. 	 	
The berries block state of the cave vines (plant) changing. 	 	
A campfire being lit or extinguished. 	 	
Food being cooked or popping out of a campfire. 	 	
A hay bale being placed or removed below a campfire. 	 	
Glowstone being added into a respawn anchor. 	 	
A respawn anchor being used. 	 	
A lodestone being used. 	 	
Oxidation level of unwaxed copper or cut copper block, slab, or stairs changing. 	 	
A copper or cut copper block, slab, or stairs being waxed or unwaxed. 	 	
Grass/fern/seagrass turning into tall grass/large fern/tall seagrass after bone meal is used on it. 	 	
Cactus, sugar cane, bamboo, bamboo shoot, vines, weeping vines, twisting vines, kelp, cave vines, pointed dripstone, or chorus flower growing. 	 	
Chorus flower growing or leaving a chorus plant behind and causing a block update to that. 	 	
An amethyst cluster growing. 	 	
A bamboo shoot turning into bamboo. 	 	
The leaves block state of bamboo changing. 	 	
Mushroom spreading. 	 	
1-block-tall flowers spreading after bone meal is used on one. 	 	
Glow lichen spreading after bone meal is used on one. 	 	
Moss blocks spreading after bone meal is used on one. 	 	
A big dripleaf increasing or decreasing its tilt. 	 	
A big dripleaf turning into a big dripleaf stem. 	 	
A big dripleaf growing after bone meal is used on it. 	 	
A small dripleaf turning into a big dripleaf after bone meal is used on it. 	 	
Hanging roots being created underneath rooted dirt after bone meal is used on it. 	 	
A hanging mangrove propagule being created underneath mangrove leaves after bone meal is used on it. 	 	
The number of sea pickle(s) changing after bone meal is used on a coral block. 	 	
A sea pickle being created after bone meal is used on a coral block.{{only 	, using bone meal on a coral block does not create sea pickles, due to . 	
Using bone meal on pink petals that do not already have 4 petals. 	 	
VegetationGrass, tall grass, 1-block-tall flower, seagrass, tall seagrass, coral, (wall) coral fan, moss carpet, azalea, flowering azalea, roots, fungus, nether sprouts, twisting vine, and pink petals. growing on top of or inside grass block, water, moss block, or nylium after bone meal is used on one. 	 	
A sapling or a propagule turning into a tree. 	 	
A mushroom turning into a huge mushroom after bone meal is used on it. 	 	
An azalea turning into a azalea tree after bone meal is used on it. 	 	
A fungi turning into a huge fungi after bone meal is used on it. 	 	
A grass block turning into dirt when a sapling grows into a tree on that grass block. 	 	
A grass block/dirt turning into podzolBoth under the sapling and the area around the sapling. when a 2×2 spruce sapling grows into a tree on that grass block/dirt. 	 	
A grass block/dirt or any other blocks turning into rooted dirt by when an azalea or a flowering azalea grows on that grass block/dirt. 	 	
Mud turning into muddy mangrove roots when a mangrove tree grows. 	 	
A block that has sculk_replaceable tag turning into sculk by sculk catalyst. 	 	
A sculk vein/Sculk veins being created by sculk catalyst. 	 	
The shape of sculk vein(s) changing by sculk catalyst. 	 	
A sculk sensor or a sculk shrieker being created by sculk catalyst. 	 	
A bell ringing. 	 	
A bee leaving a bee nest or a beehive. 	 	
A bee entering a bee nest or a beehive (thus increasing the honey level). 	 	
The honey being collected from a bee nest or a beehive using shears or a glass bottle. 	 	
A candle being added into a candle. 	 	
A candle being added into a cake. 	 	
A candle or candle cake being lit or extinguished. 	 	
Dirt/coarse dirt/rooted dirt turning into mud through the use of a water bottle. 	 	
Mud turning into clay through the use of pointed dripstone. 	 	
A book being add or remove from a chiseled bookshelf by player, hopper, minecart with hopper, or dropper. 	 	
A suspicious block becoming less dusted as the player uses a brush, or more dusted if the player stops using a brush. 	 	
A suspicious block turning into sand or gravel. 	 	
All other block/block state changes. 	 	
=== Note blocks ===
The observer can be placed under note blocks to produce bass drum sounds.
== Sounds ==
:
:
== Data values ==
=== ID ===
:
:
=== Block states ===
== History ==
== Issues ==
== Trivia ==
* 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.
== Gallery ==
Jeb Observer.png|Development image of an observer from Jeb.
Frowny Observers.png|The first image released of the new observer texture for Java Edition.
== See also ==
* Tutorials/Block update detector
* Redstone Comparator
== References ==
== External Links ==
*Block of the Week: Observer – Minecraft.net on November 10, 2017
Category:Mechanisms
Category:Manufactured blocks
Category:Redstone mechanics
Category:Mechanics
de:Beobachter
fr:Observateur
it:Osservatore
ja:オブザーバー
ko:관측기
nl:Observeerder
pl:Detektor
pt:Observador
ru:Наблюдатель
zh:侦测器