Here’s an easy open source motion sensing project by Matt Williamson which sends you an SMS message when motion is detected. The hardware consists of an Arduino and Parallax PIR sensor (Part No. 555-28027). This arrangement communicates via serial with a Python server on an attached computer, which then uses the TelAPI service to accomplish the SMS transmission.
You can find the Arduino and Python source code and related docs on the project’s Github page.
This is a great little project!
… designed by a cellular consortium to boost text messaging revenue among hackers.