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!

Once you get the alarm set up, it’ll sense movement near the Arduino, snap a picture, then send it over to you using If This Then That. All you’ll need is an Arduino Yún, PIR sensor, a USB ...

Put aside all of the projects that use an Arduino to blink a few LEDs or drive one servo motor. [IngGaro]’s latest project uses the full range of features available in this versatile microcontroller ...

As photons (light) land on the detector, the resistance will decrease. The more light is we will have a lower resistance. By reading different values from the sensor, we can detect if is it light , ...

The alarm/target works using a piezoelectric sensor attached to the clock’s housing. When the alarm sounds, if it senses a dart hit by counting the signal pulses generated, it turns off.

You'll need an Arduino along with a bunch of various sensors to get the project rolling, and once it's all set up you'll have a simple, but cheap system to monitor your home.