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In order to protect a portal in the nether I have an obsidian room with an iron door, which works off a switch in the room. The problem is that if I leave the room and someone else on the server closes the door from the inside I can't get back to that portal.

Powering red stone on the outside of the door doesn't seem to work, as the switch inside is already powering it, so I'm guessing something like an XOR gate will be the solution, however I am finding it difficult to see how to make this work in practice.

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3 Answers

up vote 5 down vote accepted

I would suggest using buttons instead of switches. You push a button, the door opens, then the button deactivates, closing the door.
It's true that the delay is really short, maybe not even enough for you to get through. This problem can be solved using repeaters to build circuit delaying the closing signal. I currently don't have enough time to try and build a working solution, but if you won't figure it out on your own by then, I'll do my best to update this post later in the day.

Edit: Just for for the sake completeness: instead of buttons, you could also use pressure plates. Just place one in front of the door from both sides. Be wary though, because mobs can also trigger them. (I almost lost my mind once, when I had a door wired like this and a damn chicken kept stepping on the plate and banging with the door. I had to replace the plate with a button.)

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Buttons directly adjacent to a door give you just enough time to get through. –  Dan Puzey Nov 29 '11 at 9:21
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Oh, maybe I'm just lame, then. :D I often had problems getting through on time. (Well, it's also true I often had problems with lags, so that might be related. :) –  ver Nov 29 '11 at 9:23
    
Buttons might also be the easiest to have - I'm never sure about running redstone through obsidian walls. –  Rory Alsop Nov 29 '11 at 12:02
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This worked perfectly, and in terms of testing I didn't even need to remove the old lever, just added a button on each side of the obsidian wall adjacent to the iron door. The lever will leave the door open or shut, whereas if it is shut the buttons will open it long enough to get through - from either side. –  Rory Alsop Nov 29 '11 at 23:45
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You can use a XOR or XNOR gate which will make the output change to either switch.

They look like this in Minecraft:

XOR gate:

XOR gates

XNOR gate:

XNOR gates

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thinking the F form of the XOR or the D form of the XNOR might be nicely small. –  Rory Alsop Nov 29 '11 at 12:01
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Rory, I use an adjacent open block with a lever on the top to solve this:

    @@@
    @xL
    @x@

Where 'x' is door, '@' is a solid block, and 'L' is a lever. So the top of this block can be reached from both sides.

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