Step 1: Get a Book!
If it's anything like the book I found in the used book store, you will want to read it cover to cover. After that, using a ruler mark off a decent sized square area and cut it out all the way to the back cover using an X-acto knife or razor blade(s)
Step 2: Finish Compartment One
To seal the pages and the back, I used hot glue as it seemed to be a better option than all the messy glue mixtures I have seen on some projects. I also used a piece of self-adhesive red felt to line the main compartment.
Step 3: Compartment Two
Now on to the second compartment which will house our electronics. Once you have the components you will be able to determine the best size for the final fit. Cut out the compartment like you did the first one but not nearly as deep. You only have to cut it out as deep as a 9 volt battery. Notice I also cut back one page and left a 'door' that will cover the 2nd hidden compartment.
Step 4: Build the 'secret' Feature...
What you need...
1. Radio Shack (or other brand) voice recording module.
2. 9 volt battery
3. SPDT Lever Switch
4. 1/8" Phone Jack
Remove the button attached to the circuit board by peeling back the three clips that hold it in place. Then solder two wires to the contacts on the circuit board and solder the lever switch to the other ends. Be sure to use the N.C. or Normally Closed contact.
Step 5: Finalize the Module...
The recording module comes with it's own microphone so you can record your voice, but I wanted to take it a step further, so I added a 1/8" phone jack to the microphone connections so I could record anything onto the module up to 20 seconds long. J4 and MIC are the microphone connections shown in the image.
Step 6: Install the Module...
I installed the module in the 2nd compartment and used hot glue after double checking the location and lever operation when closing and opening the book.
Step 7: Record Your Audio...
You can record whatever you want on the module as long as it's no longer than 20 seconds. It sounds remarkably well considering the size of the speaker. The compartment serves as a basic enclosure. When you are done having fun with it, use it as a real secret book safe and add an alarm sound effect! Hope you enjoyed this Instructable!