Minecraft wiki entry for Clock, associated text: 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.