This is an amazing Secret Door project that uses a Firgelli linear actuator and an Arduino to open and close a secret door either by pulling on a book or using a secret knock!
Basically the bookcase slides into a false wall that was built to create a hidden space in an attic. The linear actuator pushes and pulls the bookcase in and out and is activated by the Arduino. A microswitch was wired to the Arduino that tells the actuator to open or close when the secret book is pulled. The Arduino was also programmed with a secret knock detection algorithm so that the door can be opened with a secret knock. Take a look at this video to see the door in action.