Well there are several techniques to make doors with passwords, but I think this is by far the easiest method. Although this tutorial is relatively long, it is still relatively easy - trust me).

STEP 1: BUILDING A KEY PAD

If you want to do a key pad password to a door(or anything else) the following is required. Note, that the colors of the keys relate to the key position and order they must be pressed to unlock the door. In this example the code is 5734:

For consistency, I have colored all keys in this tutorial, and the colours are assigned thus: FIRST DIGIT , SECOND DIGIT , THIRD DIGIT , FOURTH DIGIT

In the map editor align the buttons of your key pad like this:

B1,B2,B3
B4,B5,B6
B7,B8,B9
BR,B0,BA

Make sure all buttons have wait=0.1

Buttons B0-B9 can have the "port 0#" texture. Where you make it bigger and make it so only the last number is shown, the port 0# textures are in SIGNS category in Qoole 99.

To make the 0-button, take port01, but use the zero in the beginning.

BR can have the empty round, gray button that turns orange when pressed.

BA can have elevator button pointing UP.

STEP 2: MAKING THE LOCK CODE

The first code number must have a trigger referring to the first trigger_changetarget

As stated above, the code is set to 5734

B1:
targetname: B1
target: but_reset

B2:
targetname: B2
target: but_reset

B3:
targetname: B3
target: but_reset

B4:
targetname: B4
target: but_reset

B5:
targetname: B5
target: but_open1

B6:
targetname: B6
target: but_reset

B7:
targetname: B7
target: but_reset

B8:
targetname: B8
target: but_reset

B9:
targetname: B9
target: but_reset

B0:
targetname: B0
target: but_reset

BR:
targetname: (nothing)
target: but_reset

BA:
targetname: BA
target: sound_denied

STEP 3: CODING THE TRIGGER_CHANGETARGETS

11 trigger_changetarget are required to make the key pad work. Note that all these triggers here are trigger_changetarget. Set the trigger_changetargets to:

targetname: but_reset
target: B0
newtarget: but_reset

targetname: but_reset
target: B1
newtarget: but_reset

targetname: but_reset
target: B2
newtarget: but_reset

targetname: but_reset
target: B3
newtarget: but_reset

targetname: but_reset
target: B4
newtarget: but_reset

NOTE: The first code digit must be reset correctly, or else the code lock will "lock" after entering a wrong code digit!! (I learnt that the hard way)

targetname: but_reset
target: B5
newtarget: but_open1

targetname: but_reset
target: B6
newtarget: but_reset

targetname: but_reset
target: B7
newtarget: but_reset

targetname: but_reset
target: B8
newtarget: but_reset

targetname: but_reset
target: B9
newtarget: but_reset

STEP 4: SETTING THE RESET FOR THE ENTER BUTTON

The reset for "enter" button by:

targetname: but_reset
target: BA
newtarget: sound_denied

STEP 5: BUILDING THE CODE CHECK

Now we create an 8 trigger_changetarget to update the codelock, if the correct code digit has been entered.

GROUP 1

targetname: but_open1
target: B5
newtarget: but_reset

targetname: but_open1
target: B7
newtarget: but_open2

GROUP 2

targetname: but_open2
target: B7
newtarget: but_reset

targetname: but_open2
target: B3
newtarget: but_open3

GROUP 3

targetname: but_open3
target: B3
newtarget: but_reset

targetname: but_open3
target: B4
newtarget: but_open4

GROUP 4

targetname: but_open4
target: B4
newtarget: but_reset

targetname: but_open4
target: BA
newtarget: sound_granted

STEP 6: CONNECTING THE CODE LOCK TO THE DOOR

Connecting the code lock to the door is essential, else the codelock is completely useless. To do this you need to create two small button OUTSIDE the map.

The first button:
targetname: sound_denied
target: but_reset
sound: Access Denied
wait: 0.1

The second button:
targetname: sound_granted
target: but_door
sound: Access Granted
wait: 0.1

STEP 7: TARGETING THE DOOR

Now we target but_door to what we want when the correct code is entered. We target it to a single func_door:

targetname: but_door
target: but_reset

<<< NOTE!! The but_reset is vital to reset the code when the door has been opened. For example, in multiplayer maps this means the next play can not open the door, without re-entering the code

CONCLUSION

The codelock can be used again and again. I believe this will be cool in multiplayer maps, where the developer can have a "code" to enter a protect-room, or full with weapons. Imagine a button to open a door with lava under all the other players. Why not a turret? use your imagination!

Finally I have included an flow diagram of all the triggers and buttons required to make this work. It doesn't display to well here, so click and download: Have fun with your code locks!