The PING))) Ultrasonic Sensor, from the Maker Shed, is one of my favorite sensors. It has a range from .75 inches to 10 feet and is surprisingly accurate and easy to use with an Arduino. This versatile sensor can be used for motion sensing, distance measuring, and object detection making it perfect for robotics. Since the PING))) sensor is well documented and popular you will have no problem finding information on how to integrate it into your next project!
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This is an awesome little sensor. I bought a Tonka 3209 toy bulldozer for $3.00 at Goodwill and connected the PING))) sensor up to my Arduino and an H-bridge, and a couple hours later had an autonomous, object-avoiding trackbot cruising around the house!