Redstone Comparators Explained

The comparator is the redstone component most players skip, and it is the one that unlocks the genuinely useful contraptions. It does two jobs, and once you can tell them apart the rest follows.

Which way it points

A comparator has a front and a back, and getting them the wrong way round is the classic beginner mistake.

• The two torches at the back are the input side.
• The single torch at the front is the output side, and the signal comes out there.

Place it so the arrow points away from your input and toward whatever you are powering.

The two modes

Right-click the comparator to toggle between them. Watch the front torch:

• Front torch off — comparison mode. The comparator outputs its rear signal, unless a side input is stronger, in which case it outputs nothing.
• Front torch on — subtraction mode. The comparator outputs the rear signal minus the strongest side signal.

Comparison mode answers "is my side input weaker than my back input?". Subtraction mode does actual arithmetic on signal strengths.

If you are new to signal strength at all, redstone basics covers how a signal weakens over 15 blocks first.

Reading containers

This is the feature worth learning comparators for. Put a comparator against a chest, barrel, hopper, dropper, furnace or brewing stand and it outputs a signal proportional to how full that container is.

An empty container gives 0. A completely full one gives 15. Anything in between scales with how full it is, counting stack sizes — so a chest holding 64 items that stack to 64 reads lower than a chest holding 64 items that do not stack at all.

The practical uses:

• Storage full alarms. Comparator into a redstone lamp tells you at a glance when a farm's collection chest needs emptying.
• Auto-shutoff. Feed the comparator output into a redstone torch to invert it, and stop a farm when its chest is full.
• Item detection. A comparator on a hopper fires the moment anything passes through it, which is how most automatic doors and dispensers get triggered by an item.

Other things comparators can read

Less obvious, all genuinely useful:

• Cauldrons — signal by water or lava level.
• Composters — signal by fill level, which is how automatic bone meal farms shut themselves off.
• Item frames — rotating the item in a frame steps the signal from 1 to 8. This is the standard way to build a hidden door switch.
• Cake — one signal step per slice eaten.
• Jukeboxes — a different signal per music disc.
• Beehives — honey level, so you can automate harvesting without breaking the hive.

Two patterns worth memorising

The full-chest cutoff. Comparator on the chest, output into a redstone torch mounted on a block, torch powers the farm. When the chest fills, the comparator powers the block, the torch goes out, the farm stops. Empty the chest and it starts again with no input from you.

The item-frame combination lock. Put item frames on the wall in front of a comparator each, rotate each to a specific position, and only one combination produces the signal pattern that opens the door. Pairs nicely with a piston door.

Common mistakes

• Facing it backwards. The signal comes out of the single-torch end.
• Expecting it to work through blocks. Comparators read the container they are directly touching, not one a block away.
• Confusing it with a repeater. Repeaters boost and delay a signal. Comparators compare, subtract and read. They look similar and do nothing alike.
• Forgetting hoppers face somewhere. A comparator on a hopper reads what is in it, but the hopper is also busy pushing items out, so the reading can flicker. Lock the hopper if you need a stable reading.

Once comparators make sense, redstone clocks and timers becomes far easier — most compact clocks use one.