Redstone circuits/Clock

A clock circuit is a redstone circuit that produces a clock signal: a pattern of pulses that repeats itself.
== Introduction ==
Clock generators are devices where the output is toggling between on and off constantly. The customary name x-clock is derived from half of the period length, which is also usually the pulse width. For example, a classic 5-clock produces the sequence ...11111000001111100000... on the output.
Using only redstone torches and wire, it is possible to create clocks as short as a 3-clock. Using repeaters or pistons allows easy construction of any clock down to 1-clocks, and other devices can also be pressed into service. There are also special circuits called "rapid pulsers", which produce rapid pulses like a 1 tick clock, but inconsistently due to torches burning out. Indeed, torch based rapid pulses can be too fast for repeaters. Even with repeaters in use, 1-clock signals are difficult to handle in other circuits, as many components and circuits do not respond in a timely fashion.
Creating long clocks (more than a few ticks) can be more difficult, as adding repeaters eventually gets unwieldy. However, there are a number of approaches here, which are discussed in a separate section.
Clocks without an explicit toggle can often have one retrofitted, by wiring a lever or other switch to the controlling block of an inverter, or even to a redstone loop. In general, forcing the delay loop high eventually stops the clock, but the output may not respond until the current pulse has made its way through the loop. Whether the output gets stopped high or low depends on the clock and where in the loop players force it. Another option is to use a lever-controlled piston to open or close one of those loops, using either a solid block to transmit power, or a block of redstone to supply it.
While it isn't much discussed in the circuit builds below, there is one additional concept that is occasionally important: Phase. The phase of a running clock is the point it has reached in its cycle. For example, at one moment a 5 clock might be 3 ticks into its ON phase, 4 ticks later, it is 2 ticks into its OFF phase. A long-period clock might be noted as 2 minutes past the start of its ON phase. The exact beginning of a cycle depends on the clock, but it is usually the start of either the OFF phase or the ON phase. For most cases, phase doesn't matter because they just need pulses every 7 ticks or so. However, in-game computing circuits are more demanding, and if they are doing a daily clock, they should care whether the on phase is day or night.
== Torch clock ==
=== Rapid pulsar ===
Redundancy can be used to maintain a 1-clock, even as the torches burn out; the result is the so-called "Rapid Pulsar" (designs X, Y and (vertical) Z). However, the signal may not be consistent.
Device R creates energy in an irregular sequence. It is a variant of the "Rapid Pulsar" design shown above, except that each torch pulses in an irregular pseudo-random pattern as each torch coming on turns the other three (and itself) off. Occasionally torches burn out for a few seconds (until reset by a block update), during which time other torches blink. As of version 1.5.1, this is likely to favor one pair of torches, such as the east and west torches, which blink while the others stay dark. Output can be taken anywhere on the circuit.
Although "pulser" is the correct spelling for any general circuit that produces pulses, the traditional spelling of a clock circuit created from short-circuited redstone torches is "rapid pulsar".
File:Flash device.png|Random Short Generator
File:Flash device2.png|Random Short Generator from top
=== Torch loop ===
The basic torch pulser is the oldest clock circuit in Minecraft, simply an odd number of inverters (NOT gates) joined in a loop. The design has been mostly replaced by repeater-based clocks, but still works. Design A shows a 5-clock, which had historically been the shortest clock that can easily be made this way. Its pulse length can be extended by adding pairs of torches, while the inclusion of repeaters would turn it into a torch-repeater clock. Designs B and C show torch 5-clocks with a smaller footprint, while design G is a vertical version of the 5-clock.
Using this principle, it is possible to create 1-clocks and 3-clocks, but the first one is unstable and erratic as the torch would "burn out" and require a block update to reactivate it. The same used to apply to 3-clocks before an unknown Minecraft version.
Design D uses a different method to produce a torch-only 4-clock. Here, each torch's output is fed into two other torches, instead of only one as in a basic torch loop. It resembles a pair of interconnected RS-NOR latches.
== Repeater clock ==
A clock signal can be generated by introducing a pulse into a loop of repeaters.
;Repeater Loop 1-Clock
::2×3×2 (12 block volume)
::flat, silent
::clock output: 1 tick on, 1 tick off
:The simplest repeater clock is simply two repeaters connected with redstone dust in a loop.
:The tricky part is introducing a 1-tick pulse into the loop. If the pulse is too long, the repeaters are powered permanently and the only way to fix it is to break and then fix the circuit.
: A simple solution to this is to use a lever; flipping it on and then off 1 tick later. The most common method seems to be to place a redstone torch next to the clock, then quickly break it. This may take several attempts to do correctly, requiring the clock be broken and fixed between attempts. A more reliable method (shown right) is to place the torch on a powered block (a block of redstone, or any block powered by another torch or other power source) – the torch is on when placed, but turns off 1 tick later because it's attached to a powered block. The torch and powered block can then be removed, but stopping the clock later still requires breaking it.
:Variations: The dust in front of the repeaters can be replaced with blocks to save on redstone.
:Additional repeaters can be added to the loop, increasing the clock period. As long as all the repeaters are kept to a 1-tick delay, the pulse remains only 1 tick long no matter how many repeaters are added. If the delay is increased on any of the repeaters, the pulse length increases to match the longest repeater delay.
;Switchable Repeater Loop 1-Clock
::3×4×2 (24 block volume)
::flat, silent (while running)
::clock output: 1 tick on, 1 tick off
:This repeater loop can be switched on and off, by moving a block to complete or break the circuit loop.
:How it works: When the lever turns on (t = 0 redstone ticks), the sticky piston begins to extend. At t=1, the torch turns off, but the left repeater stays powered for 1 more tick. At t=1.5, the piston finishes extending and the moved block gets powered by the left repeater. At t=2, the left repeater turns off. At t=2.5, the right repeater begins to output the power passed to it by the block. From here on, it just continues as a 1-clock until the lever is turned off, breaking the loop.
;Repeater Loop 10 Hz Clock
::3×4×2 (24 block volume)
::flat, silent
::clock output: 1 tick on, 0 ticks off
: This clock produces a 10 Hz clock signal (10 activations per second) consisting of 1-tick on-pulses separated by 0-tick off-pulses (the off-pulse exists, but it is replaced by an on-pulse in the same game tick).
: Start the clock with a 1-tick pulse (for example, by placing a torch on a powered block). Stop the clock by breaking a piece of redstone dust. Alternatively, the switchable method described above may be used.
: A 10 Hz clock runs too fast for some redstone components to respond to. Command blocks and note blocks can handle the rapid activation. Doors, trapdoors, and fence gates produce sounds as if being activated and deactivated that quickly, but appear and act as if constantly activated. Pistons act as if constantly activated, but the 0-tick off-pulses produce the flickering appearance of a deactivated piston overlapping the activated piston. Other redstone components simply act as if constantly powered.
== Torch-repeater clock ==
Since the introduction of the repeater, the torch-loop clocks have been generally replaced with torch-repeater loops. In these clocks, most of the delay comes from repeaters, with a single torch to provide oscillation. Such clocks can't be shorter than a 3-clock (or the torch burns out), but they can be extended almost indefinitely (subject to space and material limits). However, once the loop reaches 9-16 repeaters (delays of 36-64 ticks), a TFF or clock multiplier can increase the period more cheaply (and compactly) than adding huge numbers of repeaters.) These examples are all (R+1)-clocks where R is the total repeater delay (that is, they spend R+1 ticks OFF, then the same time ON. All have at least one potential input that turns the clock OFF within half a cycle (after any current ON-phase passes the output). (Feeding an ON signal into the output also stops the clock, but of course then the output is high.) When the power turns off, the clock automatically restarts.
Design A shows a basic loop clock. The repeaters must have a total delay of at least 2 ticks, or the torch burns out. Powering the block turns off the clock. As many repeaters as needed can be added, and the loop can be expanded as needed with dust for cornering. The circuit as shown is flat, but large loops can be run onto multiple levels, to cut down on sprawl.
Design E is an extensible vertical clock. Its minimum size is 1×5×4, but it can be extended indefinitely, adding 2 repeaters (up to 8 ticks delay) for each block of extension. As shown, it has a minimum delay of 5 ticks. (This can be reduced to 3 or 4 by replacing repeaters with dust, or by using D instead.) A lever or redstone signal behind the torch stops the clock with output OFF (once any current ON-phase passes the output).
Design D is a tiny vertical clock, a compressed form of E, that can output a 3, 4, or 5-tick cycle.
Earliest Known Publication: June 30, 2011"ZirumsHeroTWR" (June 30, 2011). "Cobblestone Factory" (Video). YouTube.
The period is the repeater's delay plus 1, but the repeater must be set to at least 2 ticks or the torch burns out. This circuit is formally 1×3×3, but is most commonly built as a "V" on the ground, and can easily be buried entirely.
* A lever on, or redstone signal to, any of the four solid blocks can stop the clock. The torch is forced "off" while the dust is lit.
* Output can be taken almost anywhere, with a few exceptions:
** The blocks "crosswise" from the redstone dust (pistons work, but dust or a repeater is likely to jam the clock).
** The block under the repeater (a repeater or piston next to it is out-of-phase, and dust does not light). 
** Output from the dust side is reverse phase.
== Comparator clock ==
Comparators can be used to make fast clocks and slow pulsers.
=== Subtraction clock ===
;Subtraction 1-Clock
: 2×2×2 (8 block volume)
: flat, silent
: clock output: 1 tick on, 1 tick off
: A subtraction 1-clock toggles on and off every tick. It uses a redstone comparator in subtraction mode, with the output feeding to the comparator's side input.
: When the comparator first receives full power, it outputs strength 15 to the block in front of it, which passes the same signal strength to the dust next to it. The signal strength then declines by 1 (to 14) as it moves to the dust next to the comparator. In the next tick, the comparator subtracts 14 from its 15 input to output only signal strength 1. This is enough to barely power the block and the dust next to the block, but isn't strong enough to reach back to the dust next to the comparator, so on the next tick the comparator subtracts 0 from its input and the cycle starts again.
: Only the redstone dust next to the comparator actually toggles between on and off — the comparator, the block in front of it, and the dust next to the block toggles between signal strength 15 and 1. Add additional dust lines to these points to take output from them and allow the signal strength to decline to at least 14 and 0.
: A subtraction clock doesn't require full power for input — it works even with an input strength as small as 2.
: Variations: Players can use any full container as the "input" if a power source would be inconvenient in that location (such as right next to the output).
: Earliest Known Publication: February 9, 2013"plzent3r" (February 9, 2013) 
;Subtraction N-Clock
: 2×3×2 (12 block volume)
: flat, silent
: clock output: 2-5 ticks on, 2-5 ticks off
: With the repeater set to a 1-tick delay, this is a 2-clock (2 ticks on, 2 ticks off). Increase the repeater delay to slow the clock down, or even add additional repeaters. If the input strength is higher than 1, the block behind the repeater can be replaced with redstone dust; if higher than 2, the block in front of the comparator can also be replaced with redstone dust. Output can be taken from anywhere as long as the dot of redstone dust can power the block behind the repeater.
=== Fader pulser ===
A fader pulser is useful for making small clocks with periods less than 15 seconds (for longer periods, hopper clocks can be smaller), but they are difficult to adjust to a precise period. They use a fader circuit (aka "fader loop" – a comparator loop where the signal strength declines with every pass through the loop because it travels through at least one length of two or more redstone dust), renewed by a redstone torch every time it fades out.
;Fader 9-Pulser
::1×4×4, 1-wide, silent
::clock output: 1 tick on, 8 ticks off
:When the input turns off, the redstone torch initially "charges" the fader loop at signal strength 15. There's only one comparator in the loop so each cycle through the loop takes only 1 tick, and the signal strength declines by 2 each time through the loop, so the fader loop remains charged for 8 ticks. The redstone torch then turns on for only one tick because it short-circuits itself (the torch does not burn out because it's held off most of the time by the fader circuit).
;Fader 29-Pulser
::2×4×2, flat, silent
::clock output: 2 ticks on, 27 ticks off
:When the input turns off, the redstone torch initially "charges" the fader loop at signal strength 14 at the dust next to the block (the signal strength declined by 1 getting there from the torch). There are two comparators in the loop so each cycle takes 2 ticks, and the signal strength declines by 1 each time through the loop, so the fader loop remains charged for 28 ticks. One tick later, the redstone torch turns back on, re-powering the fader loop (it stays on for 2 ticks so it overlaps the fader loop's on time by one tick).
:Variations: 
:* Add more comparators to increase the clock's period.
::* Add redstone between the first set of comparators to limit the on ticks to 2
:* Skip the redstone torch for a non-repeating version (pulse extender). 
:* 1.16+ vertical version doesn't work because of redstone changes 
=== Alternating clock ===
Alternates between two different signal strengths every other tick.
Can be used to compact circuits that require lockstep timing.
== Hopper clocks ==
A hopper clock (a.k.a. "hopper timer") uses the movement of items between at least two hoppers to create a clock signal.
General aspects:
* flat/tileable
* noisy/silent
* clock output: from 4 ticks on, 4 ticks off to hours and days or short impulses
* clock period: from 8 ticks to hours and days
Go to #Hopper clock schematics for details.
File:Hopper-loop clock.png|Hopper-Loop Clock
File:N-hopper-loop clock.png|N-Hopper-Loop Clock Shown: 4-Hopper-Loop Clock.
File:Ethonian hopper clock.png|Ethonian Hopper Clock (EHC) – Both pistons are sticky.
File:Ethonian hopper clock (1-wide compact).png|1-Wide Compact EHC
File:Ethonian hopper clock (1-wide tileable).png|1-Wide Tileable EHC
File:Ethonian hopper clock (1-wide upside-down).png|1-Wide Upside-Down EHC
File:Modified ehc no sticky.png|Non-stick EHC – doesn't require sticky pistons
File:Clock Circuit - Triggered, Self-Holding EHC.png|Hopper Timer (self-locking EHC with trigger input)
File:Rs nor latch hopper clock.png|RS NOR Latch Hopper Clock
File:1-wide rs nor latch hopper clock.png|1-Wide RS NOR Latch Hopper Clock
File:Hopper-latch hopper clock.png|Hopper-Latch Hopper Clock
File:Sethbling's hopper clock.png|Sethbling's Hopper Clock
File:Sethbling's hopper clock (simplified).png|SethBling's Hopper Clock (Simplified)
File:Sethbling's hopper clock (amputated).png|SethBling's Hopper Clock (Amputated)
File:2-stage mhdc.png|Multiplicative Hopper-Dropper Clock
File:3-stage vertical mhdc.png|3-Stage Vertical MHDC — 72 block volume, clock period up to 10.7 years
=== Hopper clock schematics ===
This sub-page contains ~24 schematics. Open it only if needed.
Or open the same page on its own: Hopper clocks
== Dropper-Dropper clock ==
::7×4×2 (56 block volume)
::clock period: 4 ticks/item (up to 230 seconds)
::Earliest known publication: Apr 24, 2018"SapioStevey" (Apr 24, 2018). "Minecraft Redstone - Dropper-Dropper clock" (Video). YouTube.
Simple design that does not require iron, because it uses no hoppers or pistons. However, it does require nether quartz. Pulsing output can be taken from the long dust trails in the top-right and bottom-left corners, while stable output can be taken from 1-tile dusts at top-left and bottom-right. The repeaters at the top and bottom are set to 3 ticks. The clock may need resetting after reloading the world.
== Despawn clock ==
A despawn clock uses item despawn timing to create a clock signal.
Simply approaching a despawn clock can interfere with its timing, because any player might accidentally pick up the despawning item.
;Dropper Despawn Clock
::3×3×2 (18 block volume)
::clock output: 5 minutes off, 3-7 ticks on
:Start the clock by turning off the input. The torch turns on, the dropper drops an item on the pressure plate turning the torch off. After 5 minutes, the item despawns (disappear) and the pressure plate deactivates, allowing the torch to turn on, causing the dropper to eject another item onto the pressure plate.
:If completely filled with items, the dropper must be re-filled every 48 hours, or continually supplied with items from a hopper pipe. Two chickens constrained above a hopper can keep a dropper despawn clock supplied with eggs indefinitely.
:Variations: Longer clock periods can be achieved by chaining multiple despawn clocks together, so that each torch triggers the next dropper instead of its own. When chaining multiple despawn clocks, the dropper must be placed so that it is activated only by the previous torch and not the previous pressure plate.
:A dispenser can also be used, instead of a dropper, but is slightly more resource-expensive (and not advised with use of eggs).
;Summon Despawn Clock
::1×2×2 (4 block volume)
::clock output: up to 32 minutes off, 1.5 ticks on
:The command block executes a command to summon an item onto the pressure plate. The exact command can vary, but looks something like this:
::}}
:The command above summons an item entity (an item in the world, rather than in a player or container inventory), one block in the +x direction (west) from the command block, and specifies that the item is a stick and has an "age" of X.
:Replace X with a value that determines how long the item should last before despawning: 6000 - 20 × <seconds> (for example, 5940 for a 3-second despawn). Every game tick, this value increases by 1, and the item despawns when the value reaches 6,000. Normally, items start at 0 and last 5 minutes (6000 game ticks = 300 seconds = 5 minutes), but setting the item entity's initial Age changes that.
:When calculating X for a specific clock period, note that pressure plates stay active for a short period after the item despawns. A wooden pressure plate takes 10 ticks (1 second) to deactivate after the item despawns and a weighted pressure plate takes 5 ticks (0.5 seconds). This also limits how fast a summon despawn clock can be made to run.
:X can be negative for clock periods greater than 5 minutes (for example, -6000 for a 10-minute despawn). The maximum time possible is a little over 32 minutes, with X = -32768 (-32768 = 27.3 minutes, plus another 5 minutes to get to +6000).
:Start the clock by turning off the input.
== Command block clock ==
A setblock clock works by replacing a block of redstone or a redstone torch repeatedly with a command block activated by the block of redstone it places. A command takes 0.5 ticks to place a block, so these clocks are capable of producing 20 0-tick pulse per second. Only redstone dust, note blocks, and other command blocks can activate that rapidly – other mechanism components and repeaters powered by a setblock clock usually pulse only 5 times per second (like a 1-clock), while comparators may activate once and then stay on or not activate at all.
To prevent the destroyed blocks from dropping items use . To prevent the clock from spamming the chat use . To prevent the clock from spamming the server log use .
Both of these clocks begin running as soon as they're built. To turn them off, activate the command block setting the block of redstone from a secondary source. To turn them back on, remove the source of secondary activation and replace the block of redstone.
;Setblock Clock
::1×1×2 (2 block volume)
::1-wide
::clock output: 0-tick pulse every 0.5 ticks.
:The command block should have the following command: setblock ~ ~1 ~ minecraft:redstone_block 0 destroy.
:Variations: The command block and block of redstone can be configured in any direction.
;Silent Setblock Clock
::1×1×2 (4 block volume)
::1-wide, silent
::clock output: 0-tick pulse every 0.5 ticks.
: Command block "R" should have the following command: setblock ~ ~-1 ~ redstone_block. Command block "S" should have the following command: setblock ~ ~1 ~ stone (or any other solid opaque block that doesn't cause light updates when replacing the block of redstone).
: Variations: The command blocks and block of redstone can be configured in any way that the block of redstone can power both command blocks simultaneously, but command block "S" executes before command block "R" (command blocks that are powered simultaneously activate from lowest coordinate to highest coordinate on each axis).
;Fill Clock
: A fill clock works just like either version of the setblock clock, except it uses the command to setblock an entire volume with blocks of redstone. This allows the clock to activate or power many locations at once without lines of redstone dust requiring support blocks.
: Command block "R" should have the following command: fill ~ ~-1 ~ ~4 ~-1 ~ redstone_block. Command block "S" should have the following command: fill ~ ~1 ~ ~4 ~1 ~ stone (or any other solid opaque block that doesn't cause light updates when replacing the block of redstone). Adjust the commands for the number of blocks of redstone required and the direction they are oriented. 
:Positions "a" could be command blocks, note blocks, etc.
== Piston clock ==
Pistons can be used to create clocks with a modifiable pulse delay without the use of pulse generators. pistons can be clocked in a fashion that leaves the arm extended only for the time required to push an adjacent block. However, note that if sticky pistons are regularly used this way (that is, as a 1-clock), they can occasionally "drop" (fail to retract) their block, which usually stops the clock. (Specifically, if the setup allows for a pulse less than 1 tick long, that causes a sticky piston drop its block. This can be useful, notably for toggles.) Piston clocks in general can be easily turned off or on by a "toggle" input T.
==== Minimal Piston Clock (A) ====
Design A requires only a sticky piston and redstone wire, and is controllable. It runs as long as the toggle line (its power source) is on, and turns off when the toggle line is off. Repeaters can be added to increase its delay. If the repeater is replaced with wire, it can be used as a 1-tick clock, but it is prone to "dropping" its block.
==== Minimal Dual-Piston Clock (B) ====
Design B shows how to counter block dropping with an optional, non-sticky, piston. The non sticky piston (the bottom one) is needed for the 1 tick clock as a self repair mechanism. It prevents detaching of the moving block from the sticky piston. If using it only for a 1-tick cycle, the repeater (under the extended piston) can be replaced with redstone wire. The toggle line stops the clock on a high signal. 
==== Dual Block Piston Clock (C) ====
Design C requires two sticky pistons, and can be easily stopped by just setting one side of the redstone high. The repeaters can be indefinitely extended to make a long delay clock.
==== Compact Sticky Piston Clock (D) ====
Design D needs just one sticky piston, but at the repeater must be set to 2 or more ticks. If it is set to one tick, the torch burns out. The output signal can be taken from any part of the circuit. This design can also be controlled; a high input on the toggle line stops the clock.
==== Advanced 1-tick Piston Clock (F) ====
Design F is an unusual, stable, 1-tick piston clock. Unlike most repeater-based 1-clocks, its signal is fast enough to make a sticky piston reliably toggle its block, dropping and picking it up on alternate pulses. For the clock to work, the block the piston moves must be placed last. The piston extends and retracts quickly. The output wire appears to stay off, because it's changing state faster than the game visually updates. However, attaching a redstone lamp, dispenser, dropper, piston, etc. to the output shows that it is working. The clock can be turned off by a redstone signal (e.g. the lever shown on the block below it) to the piston.
This clock is 2 blocks high, 3 wide, and 5 long. There is a solid block under the piston. The redstone torch between the repeater and the output line is at ground level.
==== Simple 3-gametick Piston Clock (G) ====
Design G is the simplest design and can be used to create rapid clocks. However, it is not controllable, so the only way to stop such a circuit, without adding additional parts, is to break one component (one redstone wire is recommended). Place a block of redstone on a sticky piston, then lay down redstone so that the block powers the piston. Then, once the piston is powered and moves the block, the redstone current stops, pulling the block back to the original position, which causes the block to power the wire again, and so on. This clock creates a 0-tick pulse every 3-gameticks.
==== Self-Powered Piston Clock (H) ====
Design H is the simplest and the only one used vertically.
To make this design, place a sticky piston facing up with a redstone wire next to it on one edge. Next to the redstone wire but still 1 block away from the piston, place a solid block and place redstone wire on top of it. Then, next to that block, but still 1 block away from the piston, place obsidian two blocks up with a redstone wire on top of it. Above the sticky piston place a slime block. Finally, on top of that, place a redstone block. The clock activates immediately. It works on the principle of quasi-connectivity, and the wire directly next to the piston is used to update it.
Players can also add on to this design and make it toggleable. To do this simply make a sticky piston push a solid block blocking the path from the redstone block to the piston. Because solid blocks stop redstone from connecting with a block diagonally, this stops the piston from powering on again and starting the clock again. Players can connect a lever to finish this addition.
== 0 Tick Piston Clocks ==
0-tick clocks make use of 0-tick pulses and 0-tick chaining to create 0-tick pulses on a regular basis.
=== 1-output 3-gt clock ===
Every 3 gameticks, the redstone block is 0-ticked to left and then 0-ticked back, creating a 0-tick pulse. The clock can be toggled by cutting the redstone line on the right
=== 2-output 3-gt clock===
This redstone clock create two 0-tick pulses every 3 gameticks. The 0-tick pulses are timed with the correct block event delay to allow the pulses to reliably chain two 0-tick pulses.
When the repeater is powered, the back sticky piston starts extending. This un-cuts a redstone wire below the block, causing the front sticky piston to be powered and extend, causing the back piston to 0-tick tick block. This then causes the top block to get 0-ticked back, cutting the bottom wire again, and outputing a 0-tick pulse on the left. This causes the front piston to get 0-ticked, which then gets 0-ticked back, creating the second 0-tick output on the right.
=== 1-gt clock ===
This clock makes use of three modules that output a signal every 3 gameticks, offset from each other by 0 gameticks, 2 gameticks, and 4 gameticks respectively. This outputs a pulse every gameticks, or 20 times a second.
== Minecart clock ==
Minecart clocks are simple, easy to build and modify, but are somewhat unreliable. A minecart clock is made by creating a small track rails with one or more powered and detector rails, arranged so that a minecart can run forever either around the track (A), or back and forth from end to end (B, C). (These need not be sloped—properly placed powered rails lets a minecart "bounce" off solid blocks — but the player get some extra time as the cart slows down.) The redstone torch can also be placed in the center of the rails, making it more compact. A larger vertical track (design C) is claimed to produce an exceptionally stable clock. Note that the minecart never quite hits the top of the track.
When running an empty minecart on the loop or back-and-forth, the cart generates redstone signals as it passes over the detector rail(s). Minecart clocks can be extended or shortened easily by adding and removing track, to adjust the delay between signals. On the flip side, they are easily disrupted by wandering players or mobs, and a long clock can take a fair bit of space. Also, the exact period is generally not apparent from the design. The need for gold in the booster rails can also be a problem for some players.
==Observer clocks==
Two observers watching each other makes for a compact rapid clock. creating a 1 redstone tick clock (2gt). A single observer could also be used as output, creating a 2 redstone tick (4gt) clock. One observer with redstone to let it watch itself can also be used. Wrap the redstone from the observing point over the top and around one side of the observer to the input. Break and replace the redstone being observed. You can add repeaters for longer periods. To switch the clock on of off, a lever can force the circuit high as usual, or a piston can be used to move one of the observers.
== Long-period clocks ==
Creating long repeater loops can be expensive. However, there are several sorts of clocks that are naturally quite long, or can easily be made so, and some are described above:
* Devices can send item entities through the world: Items flowing on a stream, falling through cobwebs, or just waiting to despawn (that's a 5-minute timer provided by the game). Droppers or dispensers, and hoppers with comparators, can be quite useful here.
** Additional stages added to the multiplicative hopper-dropper clock each multiplies the previous clock period by up to 1,152, quickly increasing the clock period beyond any reasonable use.
** A simple despawn clock is shown above. These do have a couple of liabilities:
*** If the pressure plates are not fully enclosed, the trigger item may fall to one side, stopping the clock. 
*** The droppers eventually run out of items. A droppers full of (e.g.) seeds serve for 48 hours, that is 2 days of real time. If this is insufficient, hoppers and chests can be added to refill the dropper (12 days per chest's worth). Alternately, a pair of chickens can provide enough eggs to keep the clock going indefinitely. A small full-auto melon or pumpkin farm can also serve to fill the hoppers.
* Boats and minecarts can be used with pressure plates or tripwires.
* Pseudoclocks can even be based on plant growth. For these, timing isn't exact, but they can still be useful for getting occasional signals over long periods.
* "Factorial stacking" of clocks: Precise clocks (that is, repeater or repeater-torch loops) with different periods may be connected to an AND gate in order to generate larger periods with much less expense. One way to make a 60-second (600 ticks) would be to use 150 repeaters set on 4-ticks of delay, or players could connect two clocks with the periods of 24 and 25 ticks (that's 13 repeaters) to an AND gate. Note that if the input clocks' on state is longer than 1 tick, they must filter them with an Edge Detector or Long Pulse Detector, to prevent overlapping on imperfect syncs. The disadvantages here are:
** The circuitry can be fairly finicky, and players may need a circuit just to start all the clocks simultaneously.
** The lengths of the sub-clocks need to be chosen to avoid common factors in their periods. This list of the first few prime numbers may be useful: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103. Any one of the clocks can be an integer power of a different prime, and they do not share factors or they occasionally "beat" together, causing an extra or missed pulse.
** A cycle of 1 Minecraft day (24000 game ticks, but 12000 redstone ticks) can be produced by stacking clocks of periods 125, 32, and 3. A multiplier (as described below) may be helpful for the longest of these.
* Then there's the obvious: the daylight detector acts as a clock with a period of one in-game day. The duty cycle can be adjusted by using comparators at different threshold values. Keep in mind that weather may interfere with this, and of course the phase is fixed. The daylight sensor does offer a unique feature: Since it responds to the actual progress of the game day, it does not lose time if its chunk is unloaded. Naturally if its chunk is not loaded, it can't actually activate any circuitry, but when a player comes by later, the clock remains in sync with the daily cycle. By comparison, suppose that (say) an MHDC with TFFs extending it to 20 minutes is started at dawn, but the chunk is then unloaded. When the player comes back to reload the chunk (say, at dusk), the clock continues counting its 20 minutes from wherever it left off.
There are also a couple of extension techniques that apply to any clock whatsoever, including irregular pseudoclocks:
* A T flip-flop can be used to double the period of any clock. This also converts the pulse to have the same length ON and OFF, if it didn't before. (Pseudoclocks are not completely regularized, but they get smoothed out.)
* Latched repeaters allow production of a general clock multiplier, detailed below. This can be used to multiply the period of any clock, and they can be used in series.
=== Clock multiplier ===
This nearly-flat circuit (also known as a ring counter) takes a clock input of period P and any pulse length, and outputs as a clock of period N×P, where N is the number of latches used; the output is on for a pulse length of P, and off for the remaining (N-1)×P. N is limited to 12 or so by redstone signal attenuation; however, the design can simply be repeated to multiply the period again, e.g. a 21-multiplier can be made by chaining a 7-multiplier and a 3-multiplier. This can be continued indefinitely, and unlike factorial stacking there is no restriction on the multipliers.
The build is somewhat tricky: The multiplier loop is in fact a torchless repeater-loop clock. This needs to be started separately, before the latches are engaged. The easiest way to start it is probably to add a temporary "startup circuit" starting 4 blocks from the dust part of the loop: Place a power source, then dust and a block for it to power. Finally place a redstone torch on the block, positioned to power the redstone loop. The torch flashes on for one tick before "realizing" it is powered, and this starts the loop as a clock, which cycles until the latches are powered. This startup rig can then be removed.
The latches are driven by an edge detector, which takes a rising edge and produces an OFF pulse; the pulse length must match the delays of the latched repeaters, so that the multiplier's pulse advances one repeater per edge. The delay/pulse length must also be no longer than the input clock, so it's probably best to keep them both at 1. Note that the delays of the latched repeaters are not actually part of the output period; the latches count off input edges. The circuit's output is ON while the last repeater is lit and lighting the dust loop.
This circuit need not be fed with a regular clock. With any varying input, it counts N rising edges and output HIGH between the (N-1)th and Nth rising edge.
Variations:
* A T flip-flop can be used to "normalize" the pulse to half on/half-off, while doubling the output period. Design L5 from that page is suitable and compact.
* By separating the latched repeaters with redstone dust (to read their signals individually), this circuit could be generalized into a "state cycler", which can activate a series of other circuits or devices in order, as triggered by input pulses.
* The return line can be run underneath the clock, making the build higher but narrower, or the entire repeater-latch loop can be extended to run backward on a lower level, similar to Torch-Repeater Clock design E. If used as a state cycler, this also makes the dust between the steps more accessible.
Efficiency: An efficient approach to making long-period clocks is to start with a repeater loop of 9 to 16 repeaters (up to 64 ticks), then add multiplier banks with N of 7, 5, and 3 (bigger is more efficient). Doublings should be done with T flipflops, as 2 of those are cheaper and perhaps shorter than a 4-multiplier. A couple of notes:
* Using two 7-multipliers (×49) is slightly more expensive, but shorter, than getting ×50 with 5×5×2, or getting ×48 with 3×4×4 or 6×8;.
* An 8-multiplier is slightly more expensive, but shorter, than separate 2- and 4-multipliers. However, two of them are both longer and more expensive than three 4-multipliers.
Earliest Known Publication: October 22, 2012"ftheriachab" (October 22, 2012) — 
=== Redstone Repeaters with Feedback ===
By using a ring of redstone repeaters tapped at specific intervals and an OR gate set in a feedback loop extremely long durations can be created. Durations of minutes, hours, even days can be created using a minimal amount of parts.
Clock cycle time = 0.4 × ( - 1) seconds.
Hence each time the player add a single redstone repeater, they can effectively double the cycle time. The same circuit can be used to create long duration clocks and delays of any duration in 0.4s increments. 
Super Delay on YouTube 
Copy of working minecraft save game 
Below is an example of a free running 10 element clock that takes 409.2 seconds (6.82 minutes) to cycle. It outputs from the XOR Gate a unique stream of 0's and 1's that repeats every 409.2 seconds. 
 
To turn it into a clock all we need to do is add a 10-Input Decoder that looks for one of those unique sequences. A NAND gate goes low when all redstone repeaters are outputting high.
By adding a RS flip-flop, we can reset our clock.
Here is a version where the decoder resets the clock at the 3 minute mark.
 
In electronics this device is commonly known as a "Linear Feedback Shift Register" (LFSR), players can make them count up, count down, create psudo-random binary sequences for testing logic circuits. In TCP/IP a 32-bit 'Linear Feedback Shift Register' is used to perform data integrity checks ie CRC-32. LFSR's also create the codes for CDMA phones and GPS (Global Positioning System). 
Note that the XOR gate takes it inputs (Taps) from redstone repeater 7 and 10. For simplicity sake, these have been listed 2 tap LFSR sequences. In Minecraft, one could make a 1-many delay line structure to create more complicated clocks. 
 
 
 
 
== References ==
C
de::technik:Taktgeber
es:Circuitos de redstone/Reloj
ja:クロック回路
zh:时钟电路