Minecraft wiki entry for Clock, associated text: Two observers watching each other makes for a compact rapid clock. creating a 1-tick 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.