You’ve seen it. It’s that weird, three-pronged stone slab sitting in your creative menu or gathering dust in a chest because, honestly, the crafting recipe looks more intimidating than a Wither fight. Most players just stick to repeaters and torches. But if you actually want to build anything more complex than a basic iron door, you’ve got to figure out how do you use a redstone comparator without blowing up your circuit or your brain.
It’s the smartest block in Minecraft.
Unlike a repeater, which just boosts a signal or adds a delay, the comparator actually "thinks." It makes decisions based on the signals it gets. It’s essentially a tiny, logic-based computer chip carved out of quartz and stone.
The Basics: What the Heck Am I Looking At?
First off, look at the physical block. It has a front and a back. The "front" has a single torch that can be toggled on or off. The "back" has two torches.
Direction matters here. A lot.
Signals go into the back (the side with two torches) or the sides. The signal comes out of the front (the side with the single torch). If that front torch is off, you’re in Comparison Mode. If you click it and it turns red, you’ve entered Subtraction Mode.
Comparison Mode: The "Is This Big Enough?" Test
By default, the comparator is in Comparison Mode.
Think of it as a gatekeeper. It looks at the signal coming into the back (we'll call this the Main Signal) and compares it to the signals coming into its sides. If either side signal is stronger than the back signal, the comparator just stops. It shuts down. No power for you.
However, if the back signal is equal to or stronger than the side signals, the comparator passes the signal through at its original strength.
It doesn't boost the signal like a repeater. If a signal of strength 7 goes in, a signal of strength 7 comes out. This is huge for maintaining specific signal lengths in complex builds where you don't want a repeater resetting everything to a full 15.
How Do You Use a Redstone Comparator to Read Containers?
This is arguably the most common use case for the average survival player. You place a comparator directly against a chest, hopper, or barrel. Suddenly, the comparator outputs a signal based on how "full" that container is.
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Empty chest? Zero signal. One single piece of dirt? Usually still a zero or a tiny flicker. A double chest full of diamond blocks? That’s a signal strength of 15.
This works for almost everything:
- Brewing Stands: It knows how many bottles are inside.
- Lecterns: It sends a signal based on which page you’re currently reading. This is how players make "secret" doors that open when you turn to page 5 of a book.
- Cauldrons: It measures the water level.
- Composters: It knows how much "compost" is in the pile.
Imagine an automated storage system. You use the comparator to detect when a hopper is full, which triggers a light or moves a minecart. It’s the backbone of every "professional" Minecraft farm. Without this specific "inventory checking" feature, we’d all be manually sorting our cobblestone like it’s 2011.
Subtraction Mode: Minecraft Math for Nerds
Right-click the comparator. That front torch glows. Now you’re playing with Subtraction Mode.
This is where people usually get frustrated and quit, but it’s actually pretty simple. It takes the signal from the back and subtracts the strongest signal from either side.
If you have a signal of 10 coming into the back and a signal of 4 coming into the side, the output is 6. Simple subtraction.
Why would you ever need this?
Well, it’s the key to the fastest "clock" in the game. If you loop a comparator back into its own side while in subtraction mode, it creates a rapid-fire pulse. It’s so fast it can actually lag out a weak server if you aren’t careful. This "comparator clock" is what people use to power rapid-fire dispensers or high-speed item elevators. It’s jerky, loud, and incredibly efficient.
The Secret World of Signal Strengths
Most casual players think of redstone as "on" or "off." Digital.
But redstone is actually analog. It has 16 levels (0 to 15). The comparator is the only block that lets you manipulate these specific numbers reliably.
For example, if you use a Redstone Dust line that stretches 10 blocks, the strength at the end is 5. If you feed that into a comparator, you can use side signals to "filter" that 5. You can create locks that only open if a specific amount of items are in a chest, or doors that require a specific signal strength from a weighted pressure plate.
It adds a layer of nuance that a standard torch or repeater can't touch.
Real-World (In-Game) Examples of Mastery
Let’s look at the Item Filter.
In an auto-sorter, you have a hopper filled with 41 of the item you want to sort and 4 "filler" items. A comparator sits behind that hopper. Because the hopper has exactly 45 items, it outputs a signal strength of exactly 2. This signal reaches two pieces of redstone dust but isn't strong enough to reach the third.
When one more item enters the hopper (making it 46 items), the signal strength jumps to 3.
This extra "kick" of power triggers a repeater, which unlocks the hopper below, letting the item drop through. As soon as the item drops, the signal goes back to 2, and the system locks again. It’s elegant. It’s precise. And it’s only possible because of how a comparator calculates inventory space.
The Lectern Secret Door
You’ve seen these on YouTube. Someone flips through a book on a pedestal, and a wall opens.
A comparator sits behind the lectern. A Book and Quill with 15 pages is placed on the lectern. Each page corresponds to one level of redstone signal. Page 1 is signal 1. Page 15 is signal 15.
By running a redstone line from that comparator and placing a repeater at exactly the 13th block, you’ve created a lock. The signal only reaches that repeater (and opens the door) when the player turns to page 13. It’s literally "reading" the book.
Common Mistakes and Pitfalls
"Why isn't my comparator working?"
Usually, it’s one of three things.
- Wrong side. You’re trying to input the "main" signal into the side instead of the back.
- Solid blocks. Comparators can "read" through one solid block. If you put a chest, then a block of stone, then a comparator, it will still see the chest. But it won't work if there are two blocks.
- Subtraction Mode confusion. You accidentally right-clicked the front torch, and now your signal is being subtracted into oblivion.
Also, remember that comparators don't have a "tick" delay like repeaters do—sort of. They have a 1-tick delay (0.1 seconds), which is the same as a repeater set to its first notch. However, unlike repeaters, they don't "clean up" a messy signal. If your signal is flickering, the comparator will flicker too.
Technical Nuance: The Transparent Block Problem
One thing the wiki doesn't always make clear is that comparators are finicky about what they can "power."
A comparator can power a block of Redstone Dust, another comparator, or a repeater. It can also power a solid block, which then powers things adjacent to it. But it cannot power "transparent" blocks like glass, leaves, or slabs. If you're building a compact vertical circuit and your comparator seems "dead," check the block it's pointing into. If you can see through that block, your signal is stopping there.
Actionable Next Steps for Redstone Success
Stop just reading about it and actually go build a Pulse Sustainer. This is a great beginner-to-intermediate project.
- Place two comparators side-by-side, facing opposite directions.
- Connect their ends with redstone dust.
- Hit one end with a button pulse.
Because the signal keeps looping through the comparators, it slowly decays. The signal "fades" out instead of just turning off instantly. It's how you make a wooden button stay "active" for 30 seconds instead of just one.
Once you master the decay of a signal in a comparator loop, you’ve officially graduated from "Redstone Noob" to someone who actually understands how the game's logic works.
Experiment with the subtraction mode by placing a lever on the side and seeing how many dust pieces turn off as you move the lever closer or further away. It’s the best way to visualize the math happening under the hood. Now, get into your world and start replacing those clunky repeater chains with something a bit more sophisticated.