# Secret Hollow Book Intrusion Detector ## Overview ![](https://cdn-learn.adafruit.com/assets/assets/000/050/933/medium800/projects_IMG_1192_2k.jpg?1518549007) https://youtu.be/q3l6by2vNyg The bad guys will always go for your safe to discover your secrets, so why not store them in plain sight with a hollowed out? Or, perhaps you need to hand off an item to one of your operatives -- use a hollow book as a dead drop, perhaps hidden amongst the stacks at a library or used book store... This guide will show you how to build the classic secret hollow book safe. You'll also learn how to make a motion detection alarm using a Circuit Playground Express, and get a primer on the ultimate physical security app for Android -- Haven. ![](https://cdn-learn.adafruit.com/assets/assets/000/050/934/medium800/projects_IMG_1041_2k.jpg?1518549022) ## Materials & Tools You'll need an old book that is no longer wanted. A great source for these can be a thrift shop or the used book store at your local library. Hardcover books are nice to work with, but you can use a soft cover just as well. Be sure the book is thick enough to hold your items. In hollow and seal the book, you'll need: - Utility knife with fresh, sharp blade - Straight edge - Speed square - Pencil - Small paintbrush - Mod Podge or white glue - Sheet of Mylar or plastic cling wrap - Small clamps (optional, you can stack heavy items instead) ## Optional Android If you want to try out the Haven app for capturing whomever is snooping in your book safe, you'll need an Android phone. 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