Just wanted to put forward my design for a PROPER button combination lock, there are already ways to do it using RS NOR latches and AND gates but i've cracked the combination aspect. Whereas an RS NOR system will let you in what ever order you press the buttons in, mine requires the exact sequence of buttons to be pressed.
 Key:

B=Button(Input).
RSB=Reset Button.
R
R
R=RS NOR.
I=Inverter.
W=Wire.
O=Output.
Heres the design outline:

So to start off we have to make each button push a constant input, not just the 2 seconds of signal the button provides, for this we use RS NOR latches.

BwwwwR
              R
RSBww R

x 4 or however many input buttons you want.

You then take the output of each RS NOR latch and chain it Via an inverter to the next RS NOR latch's reset line.

Bwwwwwwwwwwwwww R
                                          R
O FROM RSNOR1wI wwR

The output of the last RS NOR latch goes to the object you want to switch a door ect.
The Reset of the first RSNOR then goes to all the reset buttons you want to fully reset the circuit.

Then to open the door ect. The player must press the buttons in order, because if they press a button before the one that goes before it the previous RSNOR's output will still be reseting the next RS NOR along.

Here's the best drawing of the finished design i can do.

B1wwwRww      B2wwRww          B3wwwRww         B4wwRww    
             R  w                 R  w                       R  w                    R   www O
RSBwwRwwwwwwwwRwwwwwwwwwwRwwwwwwwwwRww

Hope thats not too confusing =P
Here's a picture of the finished lot.

Finn.png

Hope you enjoyed.
Propoganda.

Files: 

Finn.png