We've looked at a few security-related projects here in the past, ranging from RFID-tag unlocking for your PC to using a pepper mill to unlock a door. The Home Guarduino from Allan Murray, though, really takes the cake in terms of the scope, power, and flexibility of the finished project.
Designed as a drop-in driver for Allan's existing wired passive infrared (PIR) motion sensors and smoke detectors, the Arduino-powered Home Guarduino is a considerable upgrade from the stock system. Replacing an original alarm system to which nobody knew the unlock code, the Home Guarduino offers a list of features you'd be hard pushed to find on an off-the-shelf alarm: it includes support for eight programmable tamper-proof zones, can be armed and disarmed through an RFID or Near-Field Communication (NFC) tag, connects to the internet via Wi-Fi and mobile data for uninterruptable monitoring and alerting, and even includes a further five zones to which wireless sensors can be added.
Impressively, the system is driven using low-cost hardware: the keypad unit features the popular Arduino Nano installed into a custom-build PCB, while the main board uses another Arduino Nano coupled with an ESP8266 module for wireless connectivity. A lot of the more expensive parts - such as a 7Ah lead-acid battery backup, lockable anti-tamper chassis, siren, and the sensors - were salvaged from Allan's previous alarm system, rendered useless when the code was forgotten and the manufacturer proved of little help.
Allan has gone into considerable detail in his Instructable, and if you're thinking about upgrading your own home security system it's a must-read.
We moved into a new house a couple of years ago, and it already had a security system installed. Problem was that nobody knew the password! The previous owners never used it, and the manufacturer had gone out of business. Despite extensive internet searches, and even consulting a security expert, the system remained a 'brick on the wall'. SOLUTION: A perfect opportunity for DIY.