The HC-SR04 Ultrasonic Sensor is a very affordable proximity/distance sensor that has been used mainly for object avoidance in various robotics projects. It essentially gives your Arduino eyes / special awareness and can prevent your robot from crashing or falling off a table. It has also been used in blind stick applications, water level sensing, and even as a parking sensor.

HC-SR04 Ultrasonic Sensor

Circuit Connections of Ultrasonic Distance Sensor with Arduino

Arduino Code for Ultrasonic Distance Sensor

/*==========================================
Ultrasonic Distance Measurement 
 www.circuits4you.com
     
  Ultrasonic sensor Pins:
        VCC: +5VDC
        Trig : Trigger (INPUT) - Pin 4
        Echo: Echo (OUTPUT) - Pin 2
        GND: GND
========================================== */
 
int trigPin = 4;    //Trig - green Jumper
int echoPin = 2;    //Echo - yellow Jumper
long duration, cm, inches;
 
void setup() {
  //Serial Port begin
  Serial.begin (9600);
  //Define inputs and outputs
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
}
 
void loop()
{
 
  // The sensor is triggered by a HIGH pulse of 10 or more microseconds.
  // Give a short LOW pulse beforehand to ensure a clean HIGH pulse:
  digitalWrite(trigPin, LOW);
  delayMicroseconds(5);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
 
  // Read the signal from the sensor: a HIGH pulse whose
  // duration is the time (in microseconds) from the sending
  // of the ping to the reception of its echo off of an object.
  pinMode(echoPin, INPUT);
  duration = pulseIn(echoPin, HIGH);
 
  // convert the time into a distance
  cm = (duration/2) / 29.1;
  inches = (duration/2) / 74; 
  Serial.print("Distance: ");
  Serial.print(inches);
  Serial.print("in, ");
  Serial.print(cm);
  Serial.print("cm");
  Serial.println();
  
  delay(250);
}

Result of Ultrasonic Distance Sensor

                Open serial monitor and move hand in front of ultrasonic sensor to see the changes.