Minecraft wiki entry for Cobblestone_farming, associated text: The upward facing piston is on level 0. The hole east of it contains redstone and is covered by any transparent block (glass shown for visibility). The redstone torch in the main section is temporary; once you've built the clock, place a lever on either input block (green wool), and flip the lever, then replace the torch with redstone dust. (This lever is turning the clock, and generator, off, so turning the lever on will stop the generator.) After building, flipping the lever again will run the generator, or do a bit of tidying-up: The rest of level 2 can be filled with any block, and you may want to cover the lava (and even the exposed repeater), with a slab.