Redstone Comparator

Redstone Comparator

Transparency	Yes
Luminance	No (7 when powered, in Pocket Edition)
Blast resistance	0
Tool	Any tool
Renewable	No
Stackable	Yes (64)
Flammable	No
Drops	Redstone Comparator (1)
Data values	See Data values
Name	See Data values

A redstone comparator is a block used in redstone circuits to maintain, compare, or subtract signal strength, or to measure certain block states (primarily the fullness of containers).

Obtaining

To remove a redstone comparator, mine it. A redstone comparator can be broken instantly with anything, and drops itself as an item.

A redstone comparator will also be removed and drop itself as an item:

• if its attachment block is moved, removed, or destroyed.
• if water flows into its space.
• if a piston tries to push it or moves a block into its space.

If lava flows into a redstone comparator's space, the redstone comparator will be destroyed without dropping itself as an item.

Crafting

Name	Ingredients	Crafting recipe	Description
Redstone Comparator	Redstone Torch + Nether Quartz + Stone		Can use stone only, no variants.[Java and Console editions only]
Redstone Comparator	Redstone Torch + Nether Quartz + Any Stone		Can use stone and its variants interchangeably.[Pocket Edition only]

Usage

A redstone comparator can be placed on the top of any opaque block with a solid full-height top surface (including upside-down slabs and upside-down stairs), as well as on hoppers. To place a redstone comparator, use the Place Block control.

The redstone comparator has a front and a back — the arrow on the top of the comparator points to the front. When placed, the comparator will face away from the player. The comparator has two miniature redstone torches at the back and one at the front. The back torches turn on when the comparator's output is greater than zero (the arrow on top also turns red). The front torch has two states which can be toggled with the Use Item control:

• Down and unpowered (indicating the comparator is in "comparison mode")
• Up and powered (indicating the comparator is in "subtraction mode")

The redstone comparator can take a signal strength input from its rear as well as from both sides. Side inputs are only accepted from redstone blocks, redstone dust, redstone repeaters, and other comparators. The redstone comparator's front is its output.

It takes 1 redstone tick (2 game ticks, or 0.1 seconds barring lag) for signals to move through a redstone comparator, either from the rear or from the sides. This applies to changing signal strengths as well as simply to turning on and off. Redstone comparators usually will not respond to 1-tick fluctuations of power or signal strength — for example, a 1-clock input will be treated as always off from the side, and always on from the rear.

The redstone comparator has four functions: maintain signal strength, compare signal strength, subtract signal strength, and measure certain block states (primarily the fullness of containers).

Maintain signal strength

A redstone comparator with no powered side inputs will simply output the same signal strength as its rear input.

Compare signal strength

Comparators in comparison mode.

A redstone comparator in comparison mode (front torch down and unpowered) will compare its rear input to its two side inputs. If either side input is greater than the rear input, the comparator output turns off. If neither side input is greater than the rear input, the comparator simply outputs the same signal strength as its rear input.

Subtract signal strength

A redstone comparator in subtraction mode (front torch up and powered) will subtract the signal strength of the highest side input from the signal strength of the rear input (minimum 0 signal strength).

For example, if the rear input signal strength is 7, the left side is 2, and the right side is 4, then the output will be a signal strength of 3, because 7 - MAX(2,4) = 3.

Measure block state

A redstone comparator can measure the fullness of a chest, as well as other block states, even through a block.

A redstone comparator will treat certain blocks behind it as power sources and output a signal strength proportional to the block's state. The comparator may be separated from the measured block by a solid block. But in Java Edition, if the solid block is powered to signal strength 15 the comparator will output 15 no matter the fullness of the container.

Containers

Minimum Items for Container Signal Strength

Power Level	Containers and Slots
	3	5	9	27	54	—
0	0i	0i	0i	0i	0i	—
1	1i	1i	1i	1i	1i	"13"
2	14i	23i	42i	1s 60i	3s 55i	"cat"
3	28i	46i	1s 19i	3s 55i	7s 46i	"blocks"
4	42i	1s 5i	1s 60i	5s 51i	11s 37i	"chirp"
5	55i	1s 28i	2s 37i	7s 46i	15s 28i	"far"
6	1s 5i	1s 51i	3s 14i	9s 42i	19s 19i	"mall"
7	1s 19i	2s 10i	3s 55i	11s 37i	23s 10i	"melohi"
8	1s 32i	2s 32i	4s 32i	13s 32i	27s	"stal"
9	1s 46i	2s 55i	5s 10i	15s 28i	30s 55i	"strad"
10	1s 60i	3s 14i	5s 51i	17s 23i	34s 46i	"ward"
11	2s 10i	3s 37i	6s 28i	19s 19i	38s 37i	"11"
12	2s 23i	3s 60i	7s 5i	21s 14i	42s 28i	"wait"
13	2s 37i	4s 19i	7s 46i	23s 10i	46s 19i	—
14	2s 51i	4s 42i	8s 23i	25s 5i	50s 10i	—
15	3s	5s	9s	27s	54s	—

A redstone comparator used to measure the state of a container outputs a signal strength in proportion to how full the container is (0 for empty, 15 for full, etc.).

Containers which can be measured by a comparator include:

• 
  Furnace
• 
  Brewing Stand
• 
  Hopper
• 
  Minecart with Hopper on top of a detector rail
• 
  Dispenser
• 
  Dropper
• 
  Chest
• 
  Trapped Chest
• 
  Minecart with Chest on top of a detector rail
• 
  
  Large chest
• 
  
  Large trapped chest
• 
  Shulker Box (any color)

When a comparator measures a large chest or large trapped chest, it measures the entire large chest (54 slots), not just the half directly behind the comparator. A chest or trapped chest which cannot be opened (either because it has an opaque block or ocelot above it) will always produce an output of 0 no matter how many items are in the container.

The Minimum Items for Container Signal Strength table (right) shows the minimum number of 64-stackable items required to produce specific signal strengths from various containers. A number followed by an "s" indicates the number of full item stacks required (not displayed if a full stack is not required), and a number followed by an "i" indicates the number of additional items required (not displayed if no items are required beyond some number of item stacks). For items which stack up to a maximum of 16 (snowballs, signs, ender pearls, etc.), divide the "i" number by 4 and round up. For non-stackable items, count any "i" number as one additional item.

For example, to produce a signal strength of 10 from a hopper requires a minimum of 3 full stacks plus 14 more items (or 4 16-stackable items).

Calculating signal strength from items

When a container is empty, the output is off.

When it is not empty, the output signal strength is calculated as follows:

signal strength = truncate(1 + ((sum of all slots' fullnesses) / (number of slots in container)) * 14)

fullness of a slot = (number of items in slot) / (max stack size for this type of item)

Example: 300 blocks in a dispenser (which has 9 slots), where each block stacks to a maximum of 64, produces output with a signal strength of 8:

1 + ((300 items / 64 items per slot) / 9 slots) * 14 = 8.292, truncated is 8

Note that a non-stackable item is counted as a full slot (1 item in a slot, with a max stack size of 1: 1 / 1 = 1.0), and items which stack up to 16 (such as ender pearls and snowballs and eggs) are similarly considered a full slot at 16.

Calculating items from signal strength

It can be useful in redstone circuits to use containers with comparators to create signals of a specific strength. The number of items required in a container to produce a signal of desired strength is calculated as follows:

items required = max(desired signal strength, roundup((total slots in container * 64 / 14) * (desired signal strength - 1) ) )

Example: To use a furnace (which has 3 slots) to create a strength 9 signal, you need 110 items:

max(9, (3*64/14)*(9-1)) = 109.714, rounded up is 110

Miscellaneous

Comparators used to measure containers.

Some non-container blocks can also be measured by a redstone comparator:

Cake

A cake outputs a signal strength relative to the amount of cake remaining. Each slice is worth 2 signal strength, with 7 total slices, for an output of 14 for a full cake.

Cauldron signal strength

Cauldron

A cauldron outputs different signal strengths depending on how much water is inside. From completely empty to completely full, the output values are 0, 1, 2, and 3.

Command Block

A command block stores the "success count" of the last command executed, which represents the number of times the most recently used command of this command block succeeded. A "success" is defined by the command's success conditions: if a red error message is returned in the chat, the command was not successful.

Most commands can only succeed once per execution, but certain commands (such as those which accept players as arguments) can succeed multiple times, and the comparator will output the number of times it succeeded (maximum 15).

A command block continues to store the success count of the last command executed until it executes its command again, thus the comparator will continue to output the same signal strength even after the command block is no longer being activated (it doesn't turn off when the signal to the command block turns off).

End Portal Frame

An end portal frame outputs a full signal of 15 if it contains an eye of ender and zero otherwise.

A comparator can measure the presence and rotation of an item frame's contents.

Item Frame

A comparator can measure the state of an item frame's contents. In order for a comparator to measure an item frame's contents, it must be placed behind the block the item frame is attached to, facing away from the item frame. An item frame comparator will output 0 if the item frame is empty, or 1 to 8 for any item depending on its rotation (1 at initial placement, plus 1 for each right-click rotation to a maximum of 8, then wrapping back to 1).

Jukebox

A jukebox outputs a signal strength which indicates which record is currently playing. For which records produce which signal strengths, see the Minimum Items for Container Signal Strength table above.

Data values

A redstone comparator is defined by its ID, block data, and block entity. A redstone comparator also has a block state which is expected to replace the functionality of block data in a future version.

ID

Redstone Comparator	ID Name	Numerical ID
Block (inactive)	unpowered_comparator	149
Block (active)	powered_comparator	150
Item	comparator	404

Block data

See also: Data values

A redstone comparator's block data specifies its orientation, mode, and powered status.

Bits	Values
0x1 0x2	A two-bit field storing a value from 0 to 3 specifying the redstone comparator's orientation: 0: Facing north. 1: Facing east. 2: Facing south. 3: Facing west.
0x4	Set if in subtraction mode (front torch up and powered).
0x8	Set if powered (at any power level).

Block entity

See also: Block entity format

A redstone comparator has a block entity associated with it that holds additional data about the block. The redstone comparator's block entity ID is comparator.

• 
  Block entity data
  • Tags common to all block entities see Template:Nbt inherit/blockentity/template
  • 
     OutputSignal: Represents the strength of the analog signal output of this redstone comparator.

Block state

See also: Block states

Name	Value	Description
facing	north south east west	The direction from the output side to the input side of the comparator, or the opposite from the direction the player faces while placing the comparator.
mode	compare subtract	Specifies the current mode of the redstone comparator.
powered	true false	True if the redstone comparator is being powered.

History

Official release
24 November 2012					Jeb stated that there may be a "capacitor" in Minecraft.
27 December 2012					Dinnerbone released pictures of the first version of the "comparator", stating it was a replacement for the "capacitor" idea which has variable, alternate inputs.
2 January 2013					Dinnerbone released one more picture of the comparator. The picture itself showing a digital-to-analogue converter, using the comparator as the main block.
1.5			13w01a		Added redstone comparator, with 0 delay.
			13w01b		Added delay of 1 game tick (1/2 redstone tick) to comparator to fix bugs.
					Added ability to measure containers to redstone comparator.
			13w02a		Updated texture to show quartz in the middle.
					Changed algorithm for measuring containers so that comparators output a signal with as few as 1 item in the container.
			13w02b		Comparators now treat large chests as a single container.
			13w03a		Comparators now output success count of command blocks.
					Comparators now measure container minecarts on detector rails.
			13w04a		Comparators now measure jukeboxes.
			13w05a		Comparators no longer cause constant block updates, the delay is made consistent, and side input no longer causes a pulse output.
					Block 150 (powered_comparator) is no longer used; powered state is now represented by the 8s bit on block 149 (unpowered_comparator).
			13w05b		Comparator delay changed from 1 game tick (1/2 redstone tick) to 2 game ticks (1 redstone tick).
			13w09c		The redstone signal strength from a comparator next to a brewing stand with 3 water bottles in it is the same as one with 3 water bottles and 1 ingredient in it.
1.6.1			13w18a		Comparators now measure cauldrons and end portal frames.
1.7.2			13w37a		Blocks 149 (unpowered_comparator) and 150 (powered_comparator) were removed from the /give command.
1.8			14w04a		Comparators now measure item frames.
			14w28a		Comparators now measure cakes.
1.9			15w42a		With the addition of the blaze powder fuel slot, brewing stands now have 5 slots instead of 4. Their original comparative power values are listed below: Original values 0: 0 1: 1i 2: 19i 3: 37i 4: 55i 5: 1s 10i 6: 1s 28i 7: 1s 46i 8: 2s 9: 2s 19i 10: 2s 37i 11: 2s 55i 12: 3s 10i 13: 3s 28i 14: 3s 46i 15: 4s
			15w47a		Comparators' side inputs now take power from redstone blocks.
Pocket Edition Alpha
0.14.0			build 1		Added redstone comparators.
Console Edition
TU19	CU7	1.12	Patch 1		Added redstone comparators.
TU31	CU19	1.22	Patch 3		Can now measure item frames.

Issues

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