This is the HC-SRO4 Ultrasonic Distance Sensor. It's a common choice for measuring distance in all sorts of electronics projects.

On a basic level, this sensor operates much like a sonar on a submarine. It emits pulse of ultrasonic sound (at 40 kHz) and measures the amount of time it takes for that sound to bounce off an object and return to the sensor. Since both time and velocity are known, the distance from the sensor to the object can be easily calculated.

This particular model has a range of 2-500 cm and a maximum resolution of 0.3 cm. It measures outwards in a cone with 15° spread.

That spread angle is important to understand. 20cm from the sensor, the sensing area is a ~10cm wide circle. But 200cm from the sensor, the sensing area is a ~106cm wide circle. The sensor does not measure along a single laser-like ray; small objects will not be detected reliably.

Objective: This is a simple guide on how to make a distance detector using an Arduino, a HC-SRO4 Ultrasonic Sensor, a Buzzer, and some LED's.  The ultimate goal of this tutorial is to use the buzzer and LED's to display how far the object is from the ultrasonic sensor.

Materials Needed:

• (1x) Arduino Uno
• (1x) Breadboard
• (1x) HC-SRO4 Ultrasonic Sensor
• (1x) Buzzer
• (2x) Green LEDs
• (2x) Yellow LEDs
• (2x) Red LEDs
• (7x) 330 ohm Resistors
• A lot of jumper wires

Instructions:

• First things first, let's connect the 5 volt and ground pin to the breadboard.  Again, the wire attached to the 5 volt pin should be connected to the bottom channel of the breadboard, while the wire attached to the ground pin should be connected to the upper channel of the breadboard.
• Now it's time to attach the HC-SRO4 ultrasonic sensor.  It is easiest to place the ultrasonic sensor as far right to the breadboard as possible.  Referring back to the setup picture, you should connect the ground pin on the ultrasonic sensor to the ground channel on the breadboard.  Next connect the Echo pin on the sensor to pin 6 on the Arduino.  Now connect the Trig pin on the sensor to pin 7 on the Arduino, and lastly connect the VCC pin on the sensor to the 5 volt channel on the breadboard.  If you did that all correctly, your assembly should look like the picture above.
• Next is connecting the LED's to the breadboard and Arduino.  Once again referring back to the setup picture, attaching the LEDs is pretty basic, just with a lot of repetition.  The way to connect them is to connect the anode, or the longer leg, or the one on the right, to a pin on the Arduino with a jumper wire, and to connect the cathode, or the shorter leg, or the one on the left, to the ground channel on the breadboard using a 330 ohm resistor.  Then just repeat that step for all six of the LEDs, with the red LED all the way on the right being connected to pin 8 on the Arduino, the anode of the red LED to the left of that one being connected to pin 9 on the Arduino, and so on.  The last LED, that being the green LED all the way on the left, should have it's anode, or right leg, connected to pin 13 on the Arduino.  Once you have done that, your setup should look something like this.  

*DISCLAIMER* Resistors are not absolutely necessary for the build, however the are highly recommended to be used.  The only reason I am not using them is because I don't have enough resistors.  

• The last part of the physical setup for this build is attaching the buzzer to the breadboard and the Arduino.  This part is probably the easiest part of the setup.  All you have to do is attach the longer leg of the buzzer to pin 3 of the Arduino and attach the shorter leg of the buzzer to the ground channel of the breadboard.  

*DISCLAIMER* It is HIGHLY recommended to use a resistor in connecting the shorter leg of the buzzer to the ground channel of the breadboard.  This greatly reduces the volume of the buzzer.  You don't have to use a resistor, but if you don't, the buzzer will be very loud and quite frankly annoying.  

CODE

Now that you have finished the physical setup of the build, now its time for the code.  I assume that you already have the Arduino program on your computer, so now all you have to do is copy and paste the code from below.  

#define trigPin 7

#define echoPin 6

#define led 13

#define led2 12

#define led3 11

#define led4 10

#define led5 9

#define led6 8

#define buzzer 3

int sound = 250;

void setup() {

  Serial.begin (9600);

  pinMode(trigPin, OUTPUT);

  pinMode(echoPin, INPUT);

  pinMode(led, OUTPUT);

  pinMode(led2, OUTPUT);

  pinMode(led3, OUTPUT);

  pinMode(led4, OUTPUT);

  pinMode(led5, OUTPUT);

  pinMode(led6, OUTPUT);

  pinMode(buzzer, OUTPUT);

}

void loop() {

  long duration, distance;

  digitalWrite(trigPin, LOW); 

  delayMicroseconds(2);

  digitalWrite(trigPin, HIGH);

  delayMicroseconds(10);

  digitalWrite(trigPin, LOW);

  duration = pulseIn(echoPin, HIGH);

  distance = (duration/2) / 29.1;

  if (distance <= 30) {

    digitalWrite(led, HIGH);

    sound = 250;

}

  else {

    digitalWrite(led,LOW);

  }

  if (distance < 25) {

      digitalWrite(led2, HIGH);

      sound = 260;

}

  else {

      digitalWrite(led2, LOW);

  }

  if (distance < 20) {

      digitalWrite(led3, HIGH);

      sound = 270;

} 

  else {

    digitalWrite(led3, LOW);

  }

  if (distance < 15) {

    digitalWrite(led4, HIGH);

    sound = 280;

}

  else {

    digitalWrite(led4,LOW);

  }

  if (distance < 10) {

    digitalWrite(led5, HIGH);

    sound = 290;

}

  else {

    digitalWrite(led5,LOW);

  }

  if (distance < 5) {

    digitalWrite(led6, HIGH);

    sound = 300;

}

  else {

    digitalWrite(led6,LOW);

  }

  if (distance > 30 || distance <= 0){

    Serial.println("Out of range");

    noTone(buzzer);

  }

  else {

    Serial.print(distance);

    Serial.println(" cm");

    tone(buzzer, sound);

  }

  delay(500);

}

Ultrasonic sensors are distance sensors that use sound waves to detect how far away an object is. They send out high frequency bursts of sound and listen for its echo. They then determine how far away the object is based on how long it takes for the sound to return to the sensor. This variety requires an Arduino library to operate.

NewPing Library

#include <NewPing.h>

NewPing mysensor(5, 6, 200);

void setup() {
  Serial.begin(9600);
}

void loop() {
  int pingTime = mysensor.ping();

  int distance = mysensor.ping_in();

  int distance_cm = mysensor.ping_cm();

  Serial.println(distance);
}