Minecraft: Redstone OR Gate
So what does the or gate do?
While no signal applied send a signal,
and if we supply a signal send no signal.
So now we can have a door for example with pressure plates to send a signal thru the or gate and opens the door.
So standing on the plates on either side will open the door...
Now obviously you are not going to use wool for your house... but you can see how the OR gate is used to make a automatic door.
In the example you need the following: (Excluding building material used in constuction, in the example I used wool)