Minecraft wiki entry for Egg_farming, associated text: The despawn timer (upper edge of diagram) is a dropper facing down over pressure plate. It works by dropping an item onto the pressure plate, which will turn off the torch and enable the clock until the item despawns. The block in front of the pressure plate helps avoid accidentally picking up the item as you pass near, but if you go close enough you can still pick it up and cut off the timer. Once you've built and connected the despawn timer, you can turn the lever back off, as the inactive timer will keep the clock disabled. The despawn timer's dropper can be loaded with any disposable item, such as surplus seeds or eggs. The block in front of the pressure plate is just to make it a little harder to accidentally pick up the item—glass will let you see if the item is on target, or has missed the pressure plate.