Minecraft wiki entry for Mob_farm, associated text: This cascades down through all of the layers (make as many as you like, but anywhere between 3 and 10 should be plenty). To start this cascade, the topmost dispenser needs to be activated. It is recommended to do so using an Ethonian hopper clock with about ten items in it. You can trigger the topmost observer using the redstone output from one of the emplacement the redstone block could be in the clock. It is also recommended to make the layer on which you place the clock bigger than the other the hide the layers under from the sun, and to place torchs on it to prevent mob spawn.