I was tinkering with red-stone, and came up with this lock. I'm not sure if it hasn't already been
made in some way; It is a pretty simple device so I don't see how it couldn't.

Anyway, It is a 3 by 3 floor piston combination lock. you place sand blocks in the holes in any
combination you choose, then you flick the switch. If your guess is correct, then the door will
open, and you will be free to open the chest. If you are incorrect, nothing will happen, and you will
need to reset by flicking the switch back, and change your sand placement.

As I said, this is a fairly simple device; it uses no advanced red-stone. With tinkering, the combo
can be changed as well. This is only a proof-of-concept design, as the amount of red-stone and area
used to make it is quite large.

The room itself has a hint as to the combination hidden within itself. An astute eye should spot it quickly.

Enjoy!