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The Redstone Comparator (pronounced kəmˈparətər) is a block for use with redstone circuitry. It has two inputs: one from the back ('signal A') and one from the side ('signal B'). If signal B is stronger than signal A, there will be no output. Otherwise, it can be set to either repeat signal A at its current strength, or output the difference between both inputs in subtraction mode.

It provides a "subtraction" feature which results in the output being the strength of the back ('signal A') minus the strength of the side ('signal B'), however for this to work the player must make sure that the front redstone torch is lit. Signal B has to be less powerful than signal A for it to work. An example is if the back has a strength of 15, and the side has a strength of 1 the output is 14. Right-clicking the comparator will toggle the torch.

The comparator also provides a feature which is activated when the rear of the comparator (the side with two redstone torches) is placed against a storage block. Doing this causes the comparator to emit a signal that is equal to the percentage the storage block is full. Some people call this fill-checking.

The comparator looks similar to a repeater, except with an additional redstone torch.

Crafting

Ingredients Input » Output
Redstone Torches,

Stone, Nether Quartz

Template:Grid/Crafting Table

Uses

As a low-delay wire/diode

Like a repeater, the comparator acts as a diode. On its own, a comparator can be used as a diode (with 1 redstone tick delay), providing output of the same strength as its input, but not allowing current to flow against the direction of its arrow.

As a comparator

The comparator's side inputs may be used to set a threshold for the input signal. The input, signal A, will only be output if it is greater than or equal to signal B. Otherwise, there is no output. Signal A is the signal input to the back of the comparator. Signal B is considered the greater of the two side inputs; the weaker side input is ignored. This is similar to a real-life op-amp without a feedback circuit, with signal A to its non-inverting input and signal B to its inverting input.

As a diminisher

If a comparator is right-clicked, its third redstone torch becomes active. In this state, the output will be signal A minus signal B: the input signal will be reduced by the comparison signal. If signal B is greater than or equal to signal A, of course, there will be no output. This is similar to a real-life op-amp with a feedback wire from its output directly to its negative input (hence no amplifying effect but subtracts inputs), with signal A to its non-inverting input and signal B to its inverting input.

As a clock

If a Redstone Torch is connected directly to signal A and the output is forwarded to signal B and the comparator toggle is set to on, the device will act as a clock.

As an inventory contents checker

If a comparator is placed next to a container, it will provide an output based on the percentage of used space in the container. This includes: regular/large/trapped Chest, Furnace, Dispenser, Hopper, Brewing Stand, Dropper, Jukebox, or a Detector Rail with a Minecart with Chest / Hopper on it. The comparator may be separated from the input by one solid block if the 3 blocks form a line, however redstone current from below the separator may interfere. See below for more info.

Inventory checking output

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 = (amount 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) are similarly considered a full slot at 16.

Specific output signal strength creation

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(8, (3*64/14)*(9-1)) = 109.714, rounded up is 110

Power listings of a chest

This chart shows the redstone output of a regular chest. (Note this does not include Large Chests.)

1 Blocks/items = 1 SignalStr

124 Blocks/items = 2 SignalStr

247 Blocks/items = 3 SignalStr

371 Blocks/items = 4 SignalStr

494 Blocks/items = 5 SignalStr

618 Blocks/items = 6 SignalStr

741 Blocks/items = 7 SignalStr

864 Blocks/items = 8 SignalStr

988 Blocks/items = 9 SignalStr

1111 Blocks/items = 10 SignalStr

1235 Blocks/items = 11 SignalStr

1358 Blocks/items = 12 SignalStr

1482 Blocks/items = 13 SignalStr

1605 Blocks/items = 14 SignalStr

1728 Blocks/items = 15 SignalStr

Jukebox record identification

Unlike most containers, the signal output by a jukebox represents the record it contains. It outputs:

Signal Record
0 none
1 13
2 cat
3 blocks
4 chirp
5 far
6 mall
7 mellohi
8 stal
9 strad
10 ward
11 11
12 wait
13 unused
14 unused
15 unused

Video

Redstone Comparator/video

History

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Issues

Issues relating to "Redstone Comparator" are maintained on the bug tracker. Report issues there.

Trivia

  • The comparator does not re-strengthen a redstone signal. This means it can not be used for super-long instant circuits.
  • If placed every second block between redstone, they can endlessly carry a signal, at same speed than repeaters on first setting, without it losing power strength. Placing a non-transparent block at every corner allows it to bend without losing power. It is also possible to reduce the needed amount of comparators further (and thus improve the signal speed) by facing the comparators into solid blocks behind which redstone is placed
  • A redstone comparator outputs a strong signal. This does not mean it resets the signal strength like repeaters, but it means the signal can be picked up through a block without using a repeater (Like a repeater)
  • If you join up the redstone torches, an arrow is created which points to the redstone output. This arrow can also be seen as a red triangle on the Comparator, if one looks hard enough.

Gallery

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