Using motion detectors and an Arduino you can trigger lights, fans, or just about anything when the sensor detects a movement (by a person, animal etc..).
There are different types of motion detectors. This Passive Infared Motion Detector or PIR senses the infared radiation emitted by all objects above absolute zero. When something passes in front of a PIR sensor it detects the change in background temperature and then triggers an output voltage. This voltage can be read by an Arduino digital input and then the Arduino can act upon it.
- Motion Sensor