I'm sure you have all heard of the trick in movies, where someone goes into a hidden room , which can only be opened by pulling out the correct book from a bookshelf.
I decided to change this movie cliche a bit. I took a sharp IR sensor and mounted on a bookshelf. It measured the distance to any book that was pulled out, and checked to see if that was the "key" book. After getting it to work with one book pulled out. I made it a combination of books. It has to be 7-9-5 in that order, any other books are ignored. It resets after 15 seconds.
Pay attention to the characters on the LCD screen in the video!