The redstone comparator is the block most players install a hopper next to, right-click twice, and then never think about again. It is also the only vanilla block that can read the contents of a container and turn it into a number, which makes it the foundation of nearly every useful automatic system in the game.
Two modes, one block
A comparator has a torch at the front that is either off or lit. Off is comparison mode. Lit is subtraction mode. You switch between them by right-clicking the block.
In comparison mode, the comparator outputs its rear input unless a side input is stronger, in which case it outputs nothing. In subtraction mode, it outputs the rear input minus the strongest side input, floored at zero. That is the whole behaviour. Everything else people do with comparators is a consequence of those two rules plus the fact that the rear input can come from a container.
Reading a container
Place a comparator so its back faces a chest, hopper, furnace, barrel or brewing stand, and it outputs a signal based on how full that container is. The scale is not linear in the way people expect. An empty container outputs zero. Anything at all in it outputs at least one. From there the signal climbs towards fifteen as the container fills.
The important consequence: a single item in a double chest gives you signal strength one, and so does a stack of thirty. Comparators are far better at answering “is there anything in here” and “is this nearly full” than they are at counting.
The signal strength scale in practice
- 0 – completely empty.
- 1 – anything from one item up to roughly one slot’s worth of a full container.
- 15 – every slot full of full stacks.
Because the scale is fullness-relative, the same number of items gives a different reading in a hopper than in a double chest. Hoppers have five slots, so each item shifts the reading much further. This is why almost every item-detection circuit uses a hopper rather than a chest as the sensor: you get useful resolution from small quantities.
If you are still getting comfortable with what signal strength means at all, the redstone basics walkthrough covers the fifteen-block decay rule that everything here sits on top of.
Three circuits worth memorising
The item filter. A hopper pointing into a container, with a comparator reading the hopper and a set of named items locking the unwanted slots. This is the backbone of every sorting system, and it works because a hopper’s five-slot capacity makes the comparator reading precise enough to distinguish “one item arrived” from “nothing arrived”.
The full-chest alarm. Comparator on a storage chest, signal into a repeater chain, output into a lamp or a note block. You will know a farm has backed up before the hoppers start spilling. Anyone who has built a multi-item sorting wall learns this one after the first overflow.
The clock stopper. Comparator reading a furnace or brewing stand, used to cut a hopper clock once the process finishes. Cheaper than a timer and it never drifts.
Non-container readings
Comparators also read a handful of blocks that are not storage. A cake outputs a signal proportional to slices remaining. A cauldron reads its fill level. An item frame reads the rotation of the item inside it, which gives you sixteen distinct states in a single block and is the classic hidden-door lever. A jukebox reads which disc is playing.
The item frame trick is the one worth remembering, because it gives you a compact analogue input with no redstone dust at all. Rotate the item, get a different number, feed that into a comparator in subtraction mode, and you have a combination lock.
Common mistakes
Facing the comparator the wrong way is the usual one. The two small torches at the back are the input side; the single torch at the front is the output side and the mode indicator. If your circuit reads nothing, rotate the comparator before you rebuild anything.
The second mistake is expecting a comparator to update instantly. It has a one-tick delay, and chaining several in a row adds up. In a fast clock that matters; in a storage alarm it does not.
The third is trying to count items precisely. If you need an exact count, you need a chain of comparators reading successive hoppers, and at that point you are building a hardware counter rather than reading a number. Most designs that look like they need counting actually need thresholds, and thresholds are what comparators are good at.
Where to go next
Once the comparator makes sense, most automation stops feeling like memorised layouts and starts feeling like plumbing. A farm that shuts itself off when storage is full, a furnace array that only burns fuel while it has work, a door that opens on a hidden state – all of them are the same block doing the same two things.
Pair it with an observer and you have the other half of the toolkit: comparators read state, observers detect change. The observer and piston farm layouts show what that combination looks like once it is doing real work.
Images: Supplementaries and Twigs project galleries.