Now it is very easy to build your own siren sound alarm, small enough to be carried in a garment pocket. This powerful audio-visual alert device is realized using the renowned Arduino platform. The 9V ...

Here is an ultra simple Arduino Night Alarm project wired around a handful of inexpensive and easily available components. One advantage of using Arduino as a security system is the flexibility!

He’s using an Arduino Uno and an Ethernet breakout board to hook it up to the network. This allows him to send text messages to himself when the alarm system is armed, disarmed, or triggered.

Making your own security system isn't too hard, but if you'd like a super cheap and simple method, Instructables user deba168 shows how to make a wireless motion detector with an Arduino.