Redstone Tutorial: Mob-Proof Door
This door is easy to build, is zombie-proof (zombies are able to bash down wooden doors if the difficulty level is set to Hard) yet convenient for the player to get in and out.
The door is operated by a push-button on the outside:
And a pressure plate on the inside:
On the way out, you just run over the pressure plate and the door opens and closes behind you.
On the way back in, you push the button and the door will stay open until you pass through.
How does it work?
The mechanism under the door is a simple circuit called an "RS NOR latch", which is useful in many situations. It is useful for turning a pulse (such as a button press or a minecart passing over a detector rail) into a sustained signal that stays on until it is switched off again.
The simplest form of RS NOR latch looks like this:
As you can see. it's very similar to the circuit under the door. The only function of the repeater under the door is to help squeeze the circuit into a smaller space.
In the case of our mob-proof door, pushing the button changes the state of the latch, and the pressure plate changes it back.
On the way out of the door, the pressure plate just opens the door because it's next to it, and the latch isn't involved.