Ultrasonic
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Ultrasonic Sensor
An ultrasonic sensor is an excellent way to detect obstacles.
Example: HC-SR04
Distance 2 to 450 cm
Current 15 mA
Wiring
Ultrasonic Module Trigger — Arduino Digital Pin
Ultrasonic Module Echo — Arduino Digital Pin
Ultrasonic Module VCC (+5V) — Arduino VCC (+5V)
Ultrasonic Module GND — Arduino GND
An internal schematics of the sensor can be found here.[1]