Redstone Comparator

The Redstone Comparator, originally announced as the Capacitor, is a block for use with redstone circuitry, planned to be added in the Redstone Update. 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') being subtracted by 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 torch must be on for comparator subtraction to work.

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.

As a delay-free wire/diode
Like a repeater, the comparator acts as a diode. On its own, a comparator can be used as a diode (With only 1/2 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 signal B. Otherwise, there is no output. Signal A is the signal inputted into the back of the comparator. Signal B is considered the greater of the two side inputs; the weaker side input is ignored.

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 signal A, of course, there will be no output.

As an inventory capacity checker
If a comparator is provided a container ((non-trapped) chest, furnace, dispenser, hopper, or brewing stand) as an input, it will provide an output based on the percentage of used space in the container. 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
Items that cannot be stacked (like Minecarts) are considered as a full stack. That means that a dispenser filled with 9 minecarts will output a full signal.

Trivia

 * The comparator does not, however, re-strengthen a redstone signal. This means it can not be used for super-long instant circuits.
 * If placed every block, they can carry a signal for a long time, and at a higher speed than repeaters, without it losing power. It can turn 15 times.
 * 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)
 * When a redstone comparator is placed to power a dispenser, then powered up, the comparator will act as a rapid pulsar. It will not blink, but still power the dispenser to shoot at a rate of 300 items per minute (or 5 items per second), which can be useful to making extremely compact machine guns. Also due to the comparator's rapid activation rate (1/2 tick) the machine gun can turn on in a moment's notice, useful for making traps. Note: The dispenser will run out of ammunition (If it is filled with 64 in all 9 slots) in two minutes. (This is without a hopper adding extra ammunition)

Gallery
Komparator Comparador de Redstone Comparateur Компаратор 红石比较器