Minecraft wiki entry for Airlock, associated text: Since the introduction of pistons, they can be used to make much more compact airlocks using the same basic techniques. Piston-based airlocks are not restricted to using sand or gravel, but they may require Redstone circuitry which tends to make them costlier and more complex than torch/sand or torch/gravel airlocks. A simple way to build one would be to make a piston system that is retracted when the current is stopped and then placing Redstone across the passage.