Kind of a different approach, Unbreakable...
But what if the player stands on the wrong button? Then things have to be reset... and that will require another technique.
Believe me, I've did this once. I made a small keyboard, where the player had to enter a 4-digit code. Just looked at it and it took me about 35 entities. The keyboard also featured a cancel and an OK key BTW...
But I explained my way a few posts earlier... It does require some experience with entities though.
Missmaul, take a look here:
http://collective.valve-erc.com/index.php?area=entity&game=hland search for multimanager and trigger_changetarget, the main entities you'll need for this trick.