HC-SR04 ultrasonic ranging sensor. This economical sensor provides 2cm to 400cm of non-contact measurement functionality with a ranging accuracy that can reach up to 3mm. Each HC-SR04 module includes an ultrasonic transmitter, a receiver and a control circuit. There are only four pins on the HC-SR04: VCC (Power), Trig (Trigger), Echo (Receive), and GND (Ground). <a rel="nofollow" class="external autonumber" href="https://web.archive.org/web/20170916020957/https://cdn.sparkfun.com/datasheets/Sensors/Proximity/HCSR04.pdf">[1]</a> Sold for $4 by SparkFun in 2019.
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