Arduino HC-SR04
The Arduino HC-SR04 is different from most of those commonly seen on the market, which have 4 pins compared to 3 pins. But the operation principle is the same, transmit signal, and depends on when the signal is received to estimate the distance.
Arduino HC-SR04 not only inexpensive but also practical, use a sample AVR development boar –Arduino , with a LCD screen , you can make a easy rangefinder device , which can ranging 2cm to 400cm . (although i couldn’t get it work more than 2 meters.
Please don’t ask me how to use LCD, as it’s been explained really well on the Arduino official site, please check here:
