Arduino Proximity Alarm

Lucas Reed @ instructables.com has build a proximity alarm using Arduino and an ultrasonic sensor from Parallax. If a nearby object is approaching then the alarm will sound and an RGB LED will light up. He writes:

An ultrasonic rangefinder and Arduino microcontroller will check for nearby objects and if need be, alert you with a piezo-buzzer and RGB LED. Learn about these components using electronics lab simulations and eventually design, code, simulate, and build the proximity alarm itself! This is great if you are looking for a quick project to learn about digital electronics.

Arduino Proximity Alarm – [Link]