The ultrasonic sensor works by sending out sound waves and counting how long it takes for them to come back ( having been reflected from the nearest object.
In this circuit the ultrasonic sensor HC-SR04 plugs straight in to the Arduin, with pins 7 and 4 being used and GND and Vcc. The distance is calculated and every time the distance is less than a given threshold an alarm is raised, thus turning on an LED and a buzzer
•Ultrasonic Sensor HC-SR04
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