A Beginner's Arduino Guide/Projects/Sensors

You can make use of sensors to detect events and changes in the real world, and have your Arduino respond to them.

Common sensors include:
 * Photosensors - Measures the level of light in an area.
 * Microphones - Often used for detecting loud noises. (Most Arduino computers aren't suitable for voice recognition)
 * Ultrasonic rangefinders - Bounces inaudible sounds off of objects and listens for their echo to determine distance from objects.