Details This book discusses the basic components of any alarm system. All alarm systems have two basic functions. First, they monitor their environment looking for a change such as a door or window opening or someone moving about in the room. Second, they alert the legal owner or user to this change. The system described in this book uses a scanning type software to detect intruders. It behaves like a guard dog, pacing up and down the fence line on the lookout for either an intruder or a familiar person. If you have an alarm key, you can disarm the system and enter. With the scanning method, the software is easy to write and explain. It can scan eight alarm zones plus two special fire zones in about one second. You don't have to be an electrical engineer to install an alarm system, just a decent carpenter, painter, and plasterer! Because this alarm system runs on 12 volts, you don't have to be a licensed electrician either to install it. The alarm system presented here uses Python software on the Raspberry Pi combined with some elementary electronic circuits. The code described in the book, as well as CAD files and a bill of materials for the alarm panel, are available for free downloading. The book provides the reader with examples of typical configurations coming straight from the author's experience. After reviewing the hardware components typically used in common alarm systems, the author shows how to plan one yourself. To implement a modular alarm, no matter if it is for a single house or for a business or restaurant, the book shows how to skillfully combine a Raspberry Pi with small auxiliary electronic circuits. These are not installation instructions but food for thought that will enable readers to find a solution to their needs.