Project Type: Arduino
Software: Arduino IDE
OS: I did this project in Ubuntu but it can be done in Windows or OS X
Project Source: http://makezine.com/projects/pir-sensor-arduino-alarm/

*Note: I actually did this project last Sunday.

This is a super simple motion detector project that anyone can do.
Just follow along with the project instructions at the Make website here.
It took me maybe 10 minutes to do.

The youtube video from Make has a brief description on what the PIR sensor is and how it works.

There is very basic breadboarding involved.
The project includes a sample code to get the project up and running.
The sketch is simple and perfect to studying input/output.

The wires for the speaker I used were too thin to connect into the arduino directly.
I got around this by connecting the wires to the bread board then routing to the arduino with jumper wires.

I may try shrinking this project down with a ATtiny45.