Minecraft wiki entry for Clock, associated text: 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.