The HC-SR04 Ultrasonic Sensor is a very affordable proximity/distance sensor that has been used mainly for object avoidance in various robotics projects . It essentially gives your Arduino eyes / spacial awareness and can prevent your robot from crashing or falling off a table. It has also been used in blind stick applications, water level sensing, and even as a parking sensor.
1. Arduino Board
2. HC-SR04 Ultrasonic Distance Sensor
3. Connecting Wires
See output on terminal