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The HC SR04 Ultrasonic Distance Sensor sensor is the most cost-effective solution for applications where precision range-finding, 5V operation and low-cost are needed. The SR04 Range Finder has only four pins and can be used with two Digital I/O pins.
The SR04 Distance Sensor is very easy to use. Ranging is started by applying a 10us wide (or longer) pulse on the "Trigger" pin. The "Echo" pin goes high the SR04 has detected the return echo from the reflecting surface.
The distance is determined by the speed of sound and the time between the Trigger and Echo pulses. The distance travelled is twice the distace from the sensor to the reflecting surface, once to the surface and once from the reflacting surface back to the receiver on the SR04. So the distance can be calculated as
Distance = 0.5 * EchoTime * SpeedOfSound
The speed of sound is about 330 meters/s or 33000 cm/s in metric units and 12870 Inch/s in Imperial units. See http://en.wikipedia.org/wiki/Speed_of_sound for more details and better formulas.