Arduino distance meter
HC-SR04 is a very popular ultrasonic sensor for short distance measurements in DIY projects. This Instructable describes how to use it with an Arduino board to build an inexpensive distance meter with range 2-400cm. The measured distance is displayed on a Nokia 5110 LCD screen using a large size font for improved readability. The Instructable explains the details of how the HC-SR04 sensor works and can be interfaced with the Arduino for ranging with an accuracy of 3mm.
Arduino distance meter
In simpler words, the sensor, sends an ultrasound, then it detects wheather this ultrasound has bounced off of an object or not. If the ultrasound has bounced off, it measures the time that it took it to bounce. Since we know the speed of sound, we can easily measure the distance that the ultrasound traveled!
The following video tutorial from the author explains all these details: