Redstone Repeaters: Delay, Locking and Signal Strength

A repeater does three jobs in one block: it lifts a fading signal back to full strength, it holds that signal back by a set number of ticks, and it refuses to let power travel backwards through it. Get those three behaviours straight and most of your "it works sometimes" circuits start working every time.

Craft it and point it the right way

The recipe is three stone across the bottom row, then a redstone torch, redstone dust and a second redstone torch across the middle row. That is stone, not cobblestone — smelt the cobble first. One craft gives one repeater. If you are still shaky on dust, torches and power sources, start with redstone basics and come back.

Place a repeater on any block with a solid top face. It orients itself so the signal travels away from you, and the end it faces is the output end. There is no arrow printed on the top face — you read the direction from the delay torches, which sit toward the output end and step back toward the input as you raise the delay. Power enters at the flat back end and leaves at the front.

• Back — the only input face. Dust, another repeater, a comparator or any powered block behind it will drive it.
• Front — the only output face. It strongly powers the block it faces, so dust on top of or beside that block reads 15.
• Sides — ignored completely, with one exception covered further down.

The four delay settings

Right-click cycles the delay through four settings: 1, 2, 3 and 4 redstone ticks. One redstone tick is two game ticks, or 0.1 seconds, so the range is 0.1 to 0.4 seconds per repeater. The movable torch on the repeater slides further back with each step, so you can read the setting at a glance across a room.

Two things people get wrong here:

• The minimum is one tick, not zero. Every repeater in a line costs you at least 0.1 seconds, so ten repeaters in a row is a full second of latency before the far end even hears about it.
• The delay applies to switching off as well as switching on. A repeater is not a "delay the start" block, it is a fixed offset on both edges.

That symmetry is what makes repeaters the standard tool for syncing parallel lines. If one branch of a piston door runs three blocks further than the other, add ticks to the short branch until both pistons fire together.

"Repeating": why the output is always 15

Redstone dust starts at strength 15 next to a lever or torch and loses one level per block travelled, so it dies after fifteen blocks. Put a repeater where the sixteenth dust would go — the fifteenth dust still carries strength 1, which is plenty to drive it — and the signal leaves the repeater's front at 15 again, ready for another fifteen blocks.

The important part: the output is 15 no matter what the input was. A repeater fed a signal of strength 1 outputs 15. A repeater fed 15 outputs 15. It is a boost and a level-clamp at the same time.

That is exactly what you want on a long transmission line, and exactly what you do not want on a data line. Item sorters, hopper fullness readouts and anything driven by comparators carry meaning in the signal strength itself. Drop a repeater into that line and every value collapses to 15, and the circuit downstream loses the information it was reading. Route data on plain dust, or through a comparator, and save repeaters for lines where "on" is the only thing that matters.

Diode behaviour: one-way power

Redstone dust is promiscuous. A dust line powers everything it touches, in every direction, including back down the branch you fed it from. That backflow is the cause of a huge share of piston doors that open once and then jam.

A repeater is a diode. Power goes in the back and out the front, and nothing goes the other way. Two practical consequences:

• Junction protection. Where two triggers meet a shared line, put a repeater on each incoming branch pointing into the junction. Pressing one button no longer back-feeds the other button's circuit. This is the fix for most of the strange double-firing in a piston door.
• Parallel lanes. Two repeaters side by side, facing opposite ways, carry two independent signals through the same one-block corridor without either leaking into the other. Dust cannot do that.

There is a related point on the input side that most guides state backwards. Strong power and weak power differ in exactly one respect: whether redstone dust next to the block lights up. Repeaters, comparators, pistons, lamps and doors do not care either way. So a block with dust lying on top of it is only weakly powered, and dust on the far side of it stays dark — but a repeater with its back against that block reads it perfectly well. Pulling a signal out of a weakly powered block with a repeater is a standard trick, not a bug. What a repeater will not do is read power arriving at its sides or its front. If a repeater stays dark, check that the dust or powered block is behind it, not beside it.

Locking a repeater

The one exception to "sides are ignored": if a powered repeater or comparator faces directly into the side of a repeater, that repeater locks. The movable delay torch is replaced by a plain grey bar across the top, so a locked repeater is obvious at a glance.

A locked repeater freezes its output in whatever state it was already in. Locked while on, it stays on. Locked while off, it stays off. Its input can change as much as it likes and nothing happens at the output. When the lock releases, the repeater re-reads its input and updates after its normal delay.

Two rules worth memorising:

• Only a repeater or a comparator can lock. Dust running into the side, a lever on the side or an observer facing the side will do nothing.
• Comparators cannot be locked. Locking is a repeater-only feature.

That gives you a one-bit memory cell with two blocks and no torches:

1. 1.Run your data line into the back of repeater A.
2. 2.Place repeater B directly beside A, pointing into A's flank.
3. 3.Feed B from a lever, button or clock — this is your "write" line.
4. 4.Power B and A locks, holding whatever it was outputting at that instant.
5. 5.Unpower B and A resumes tracking its input.

Use it to sample and hold a comparator reading, to freeze a display while the storage behind it churns, or to hold a door state after the trigger has gone.

Repeater or comparator?

They look similar and behave nothing alike.

• Delay. Repeater is adjustable, 1 to 4 ticks. Comparator is fixed at 1 tick.
• Output strength. Repeater always outputs 15. Comparator passes the strength through, or subtracts, depending on mode.
• Sides. Repeater ignores them unless the neighbour is a repeater or comparator, in which case it locks. Comparator actively reads both sides as inputs.
• Containers. Repeaters read nothing. Comparators read the fill level of the block behind them, which is the backbone of anything built on hopper mechanics.
• Locking. Repeaters can be locked. Comparators cannot.

If you need a diode that does not wipe signal strength, use a comparator in comparison mode with both sides clear. You get one tick of delay and the value passes through untouched.

Build a tunable clock

A ring of dust and repeaters cannot oscillate on its own — it needs an inversion somewhere. Add a torch and you have the classic adjustable clock:

1. 1.Place a solid block and put a redstone torch on its side. The torch is on.
2. 2.Run dust from the torch into the back of a repeater.
3. 3.Run the repeater's output back into the block the torch is mounted on.
4. 4.The block gets powered, the torch cuts out, the line dies, the repeater releases, the torch comes back. It oscillates.

The delay around the loop is the torch's own tick plus the repeater's setting, and one full on-off cycle takes twice that. Add repeaters or crank the delay setting to slow it down; chain several 4-tick repeaters for slow, lamp-friendly periods. For much longer intervals, hoppers beat repeaters outright — see clocks and timers.

To switch this clock off cleanly, do not cut its power — lock it. Point a lever-driven repeater into the side of the loop's repeater and the clock halts in a defined state instead of stopping wherever it happened to be.

Build a monostable: one pulse, fixed length

Repeaters can lengthen a pulse but never shorten one. Feed a one-tick pulse into a repeater set to 4 and you get a four-tick pulse out, because the repeater commits to switching on and only schedules its switch-off afterwards. Feed a long pulse into any repeater and it comes out the same length. So if you need "however long the button is held, emit exactly N ticks", a repeater alone cannot do it — pair it with a comparator:

1. 1.Split your input into two paths.
2. 2.Path one goes straight into the back of a comparator.
3. 3.Right-click that comparator once so its front torch lights and rises. It is now in subtraction mode.
4. 4.Path two goes through one or more repeaters and into the side of the same comparator.
5. 5.Take your output from the comparator's front.

When the input arrives, the back is powered and the side is not, so the comparator outputs. A few ticks later the delayed copy reaches the side, cancels the back signal down to zero, and the output drops. The pulse length is whatever delay you dialled into the repeaters, regardless of how long the input is held. This is the standard pulse shortener, and it is the piece that lets a held lever fire a dispenser exactly once.

The failures you will actually hit

• Dust running alongside a repeater does nothing. Sides are not inputs. Route into the back.
• A repeater sits dark next to a lit line. The power is reaching its side or its front, not its back face. Turn it, or reroute the dust into the back.
• Dust dies at a block instead of carrying on past it. Dust only weakly powers the block it runs into, and weak power will not light dust on the far side. A repeater there reads it fine; if you need dust to continue, strongly power that block with a repeater, torch, lever or button.
• Two branches arrive out of sync. Count blocks, then add ticks to the short branch — one repeater tick is 0.1 seconds.
• A short pulse comes out fat. The repeater lengthened it to its own delay. Use the comparator subtractor above.
• An item sorter stopped sorting after you "fixed" a wire. You put a repeater in a signal-strength line and flattened every value to 15.
• A repeater looks stuck with a bar across it. It is locked. Find the repeater or comparator pointing into its flank.