Minecraft command block door using a key [duplicate]

First of all, summon an invisible ArmorStand at your door, replacing X, Y and Z with the coordinates of the bottom part of your door:

/summon ArmorStand X Y Z {CustomName:"keyDetector",Invisible:1,Invulnerable:1}

Next, run the following commands in Always Active mode in command blocks, running the first command in a repeating command block (repeating_command_block) and the others in chain command blocks (chain_command_block), following the repeating command block:

/scoreboard players tag @a add hasKey {Inventory:[{tag:{display:{Name:"Small Key"}}}]}

/execute @a[tag=hasKey] ~ ~ ~ /execute @e[name=keyDetector,type=ArmorStand] ~ ~ ~ /setblock ~ ~ ~ wooden_door 4

/execute @a[tag=hasKey] ~ ~ ~ /execute @e[name=keyDetector,type=ArmorStand] ~ ~ ~ /setblock ~ ~1 ~ wooden_door 9

So in the end it should look something like this:

The command blocks, containing the command blocks in order from left to right:

The door with the ArmorStand (for display purposes you can see the ArmorStand and it's name on this image, but you shouldn't be able to see it on your map):

Instead of using command blocks to detect the key and open the door, you can just use a hopper sorter to get the key from the person (which also doubles as removing the key from their inventory since they don't need it anymore) and then have it trigger command blocks which open the door.