I've seen a few combination safes on youtube, but they all used an at least 5 times bigger mechanism than this one. So if you build this safe you have less chance to run out of lego bricks before you're finished.
IMPORTANT: Before you decide to build this, look at the second picture below! Those are the fancy pieces needed. The first, the nine curved bricks, are absolutely required, but not necessarily in red. The two springs, are not extremely important; without it, the door doesn't 'jump' open as you can see in the video below. All other pieces I used, are common lego pieces which belong in an average lego collection, or are well replaceable by a surrogate.
So if you decide to build it, proceed to the next step!
Step 1: The Rotors
2: Place three curved bricks around it, leaving a gap which will be the rotor's notch.
3: A 4x4 plate on top of it.
4: Another round brick, on top of the plate.
5: Cover the rest with tiles.
6: Another plate, on top of the round brick, only for rotor 1 and 2.
7: Now place a 1x1 round brick on the top plate, the place of it will influence the number sequence of your safe. See the next image, an animated gif, to learn where you can put it.
8: Rotor 2 should be exactly like this. Rotor 3 aswell, but without the plate plus 1x1 round brick at the end. For rotor 1 you only need to add the two round plates at the beginning, without the stuff sticked in them.