Minecraft wiki entry for Clock, associated text: 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).