The HC-SR04 ultrasonic sensor uses sonar to determine distance to an object - just like bats or dolphins do.

This module offers excellent range accuracy and stable readings in an easy-to-use package. It operation is not affected by sunlight or black material (although acoustically soft materials like cloth can be difficult to detect). Similar in performance to the SRF005 but with the low-price of a Sharp infrared sensor.
Perfect for collision avoidance on your next robotics or multirotor project!