It has double protection. The first is a combination of four switches, each with 4 gears, thereby resulting 265 possible switch positions. If the right one is found, the circuit is closed and the PICAXE starts to work. Now the second security system kicks in. The correct sequence of knocks must be applied!
If this code is entered incorrectly a fitting melody is played and an 80 second countdown begins. Only after this countdown is a new attempt is permitted. When the right combination is found a different tune will be heard, and the book will open. If you want to close the book you have to simply press a small hidden button.