From alan-parekh.com: Single Button Code Entry System

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Forget keypads and RFID tags, the guys over at alan-parekh.com have come up with the simplest possible user interface for a code entry system: a single button. Sold in kit form via their online store and incorperating a PIC 16F628A microcontroler and relay, this kit allows you to program and input a four-digit code using only one button.

Details and a video explaining how the kit works can be found here.