The Concealed codelock (CC for short) is a design of mine, which can be used in a similar fashion as a safe lock, such as that on the picture above.
The rotation sensor used here is just a simple 10k potentiometer. The concealability in this is that you can use a double potentiometer. One part of the potentiometer will be used for the lock and the other one for something else, such as a amplifier, or any other normal home apliance that has a potentiometer for something.
The combination to open the lock consists of a sequence like ABCDCBCB. This combination means you'd need to first set the dial to B, then C, B, C, D, C, B and finally A. After this the LEDs will begin to blink and the relay will switch itself on.
The more complete version of this can be found on http://daqq.eu/index.php?show=pr... (including nice pictures and a sort of manual).
The schematic is in the zip file included.