How To Make A Minecraft Piston Powered Retractable Bridge
Sticky pistons enable you to create blocks that slide in and out (and up and down.) With a little bit of redstone work that means you can create retractable bridges. One moment you're walking across a safe as houses wooden bridge, the next moment a switch is flipped and the bridge is gone. This article will help you work out the redstone wiring gremlins so that you can create safe houses and nefarious traps on your own terms.
The major problem with a bridge of extendable pistons is that it can be difficult for the redstone novice to work out how to get the pistons to sit side by side and all be powered at the same time. If you simply put six pistons side by side and attempt to run redstone wires to them all, you'll find that the wiring simply makes a closed circuit and powers approximately nothing.
Fortunately, it is possible to power pistons from the top and the bottom. So what you do is you place your pistons neatly in a row, then place another block directly behind alternating pistons. (So place a block behind the 1st piston, then the 3rd piston, so on and do on) The redstone wiring goes over this block.
Then you dig out a single block hole behind each of the remaining pistons. You'll place your redstone wiring in this 'hole'. Now when you place your redstone wiring, there will be a two block gap where the redstone meets the piston on adjacent pistons. This prevents the redstone from forming a circuit without contacting the piston.
The wiring will still 'join up' in a line on the next row of blocks back, which is what you want. You can then run a redstone wire back from this line and attach a switch, or if you want to be fancy and hide the wiring, a block of air above the wiring with another block with a switch on it will also operate the bridge.
In this example, I've made the bridge a quite obviously different material from the rest of the surrounding terrain, but it isn't hard to create bridges that blend seamlessly with the terrain, bridges that others would never know were there. It's also possible to create 'bridges' on multiple layers, so instead of having a single row of blocks sliding out, you have several rows sliding out one after the other. This is more advanced of course, but once you've mastered the art of building relatively simple redstone creations, you'll be well on your way to designing and building more complex creations.