The Parallax Inc PING((( ultrasonic sensor provides precise, non-contact distance measurements from about 2cm to 3meters (0.8 inches to 3.3 yards). This is a very easy method for distance measurement using sonar. This sensor is perfect for any number of applications that require you to perform measurements between moving or stationary objects. An ultrasonic (well above human hearing) pulse is transmitted from the unit and distance-to-target is determined by measuring the time required for the echo return. The output from the PING))) sensor is a variable-width pulse that corresponds to the distance to the target.
Note:
PING is ideal for use with Raspberry Pi and Arduino based robot projects.
Parallax offers an extensive range of sensor, communication and interface modules for the experimenter working on robotic systems. Most of them are supplied with extensive documentation, making them ideal for use in education.

Simple pulse in/pulse out communication on single GPIO pin
Burst indicator LED shows measurement in progress
Narrow acceptance angle
3-pin header makes it easy to connect, using a servo extension cable, if necessary
Power requirement: 20mA at 5VDC
Communication: TTL pulse
Dimensions: 22 x 46 x 16 mm
Operating temperature ranges between 0 and 70°C
Ultrasonic measurements work in any lighting condition, making it a good choice to supplement infrared object detectors

Security systems
Interactive animated exhibits
Parking assistant systems
Robotic navigation