A Raspberry Pi with motion detection and SMS.

I've built one using an old nokia that supports AT commands.

Pretty fun stuff and no soldering required. Next up I'll try adding a camera and send MMS messages through the mobile phone with photos when motion is detected.

Helpful links:

http://www.amazon.co.uk/Emgreat-Detecting-Pyroelectric-HC-SR...

http://www.amazon.co.uk/Pyroelectric-Infrared-Motion-Sensor-...

http://www.raspberrypi-spy.co.uk/2013/01/cheap-pir-sensors-a...

http://www.adafruit.com/products/189

https://www.modmypi.com/blog/raspberry-pi-gpio-sensing-motio...

http://raspberrypi.stackexchange.com/questions/13334/how-can...