PrivateEyePi is an open, configurable, automatable home alarm system that you can build and program yourself. Its maker (identified only as “Gadjet Nut”) has documented the whole system minutely and provides parts lists and pricing; wiring schematics; and all the code you’ll need. You can use motion detectors, or switches attached to doors, or a mixture of the two. There are instructions on adding cameras to the setup, and you can even add a temperature gauge to check on whether your central heating’s working when you’re away. You’ll be able to monitor everything via your computer or smart phone.