Minecraft wiki entry for Clock, associated text: 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.