How HC-SR04 Ultrasonic Sensor Works & Interface It With Arduino

Give your next Arduino project bat-powers with the HC-SR04 Ultrasonic Distance Sensor that can report the range of objects up to 13 feet away. This is a good thing to know when you’re trying to save your robot from hitting a wall.

They are low power (suitable for battery operated devices), affordable, easy to interface and extremely popular with hobbyists.

What is Ultrasound?

Ultrasound is a high-pitched sound wave whose frequency exceeds the audible range of human hearing.

Ultrasonic Frequency Range Spectrum

Humans can hear sound waves that vibrate in the range of about 20 times a second (a deep rumbling noise) to 20,000 times a second (a high-pitched whistle). However, ultrasound has a frequency of more than 20,000 Hz and is therefore inaudible to humans.

HC-SR04 Hardware Overview

An HC-SR04 ultrasonic distance sensor actually consists of two ultrasonic transducers.

One acts as a transmitter that converts the electrical signal into 40 KHz ultrasonic sound pulses. The other acts as a receiver and listens for the transmitted pulses.

When the receiver receives these pulses, it produces an output pulse whose width is proportional to the distance of the object in front.

This sensor provides excellent non-contact range detection between 2 cm to 400 cm (~13 feet) with an accuracy of 3 mm.

Since it operates on 5 volts, it can be connected directly to an Arduino or any other 5V logic microcontroller.

Technical Specifications

Here are the specifications:

Operating VoltageDC 5V
Operating Current15mA
Operating Frequency40KHz
Max Range4m
Min Range2cm
Ranging Accuracy3mm
Measuring Angle15 degree
Trigger Input Signal10µS TTL pulse
Dimension45 x 20 x 15mm

HC-SR04 Ultrasonic Sensor Pinout

Let’s take a look at its pinout.

HC-SR04 Ultrasonic Distance Sensor Pinout

VCC supplies power to the HC-SR04 ultrasonic sensor. You can connect it to the 5V output from your Arduino.

Trig (Trigger) pin is used to trigger ultrasonic sound pulses. By setting this pin to HIGH for 10µs, the sensor initiates an ultrasonic burst.

Echo pin goes high when the ultrasonic burst is transmitted and remains high until the sensor receives an echo, after which it goes low. By measuring the time the Echo pin stays high, the distance can be calculated.

GND is the ground pin. Connect it to the ground of the Arduino.

How Does HC-SR04 Ultrasonic Distance Sensor Work?

It all starts when the trigger pin is set HIGH for 10µs. In response, the sensor transmits an ultrasonic burst of eight pulses at 40 kHz. This 8-pulse pattern is specially designed so that the receiver can distinguish the transmitted pulses from ambient ultrasonic noise.

These eight ultrasonic pulses travel through the air away from the transmitter. Meanwhile the echo pin goes HIGH to initiate the echo-back signal.

If those pulses are not reflected back, the echo signal times out and goes low after 38ms (38 milliseconds). Thus a pulse of 38ms indicates no obstruction within the range of the sensor.

HC-SR04 Ultrasonic Sensor Working - Echo when no Obstacle

If those pulses are reflected back, the echo pin goes low as soon as the signal is received. This generates a pulse on the echo pin whose width varies from 150 µs to 25 ms depending on the time taken to receive the signal.

HC-SR04 Ultrasonic Sensor Working - Echo reflected from Obstacle

Calculating the Distance

The width of the received pulse is used to calculate the distance from the reflected object. This can be worked out using the simple distance-speed-time equation we learned in high school. An easy way to remember the equation is to put the letters in a triangle.

Distance Speed Time Formula Triangle

Let us take an example to make it more clear. Suppose we have an object in front of the sensor at an unknown distance and we receive a pulse of 500µs width on the echo pin. Now let’s calculate how far the object is from the sensor. For this we will use the below equation.

Distance = Speed x Time

Here we have the value of time i.e. 500 µs and we know the speed. Of course it’s the speed of sound! It is 340 m/s. To calculate the distance we need to convert the speed of sound into cm/µs. It is 0.034 cm/μs. With that information we can now calculate the distance!

Distance = 0.034 cm/µs x 500 µs

But we’re not done yet! Remember that the echo pulse indicates the time it takes for the signal to be sent and reflected back. So to get the distance, you have to divide your result by two.

Distance = (0.034 cm/µs x 500 µs) / 2

Distance = 8.5 cm

Now we know that the object is 8.5 cm away from the sensor.

Wiring an HC-SR04 Sensor to an Arduino

Now that we have a complete understanding of how the HC-SR04 ultrasonic sensor works we can start connecting it to our Arduino!

Connecting the HC-SR04 to Arduino is very easy. Start by placing the sensor on your breadboard. Connect the VCC pin to the 5V pin on the Arduino and the GND pin to the ground pin. Now connect the trig and echo pins to digital pins #9 and #10 respectively.

The following table lists the pin connections:

HC-SR04 SensorArduino

When you are done you should have something that looks similar to the image shown below.

Arduino Wiring Fritzing Normal Mode Connections with HC-SR04 Ultrasonic Sensor
Wiring HC-SR04 Ultrasonic Sensor to Arduino UNO – Normal Mode

Library Installation

Triggering the ultrasonic sensor and measuring the received signal pulse width manually is a lot of work but luckily there are many libraries available to us. One of the popular libraries is the NewPing library. This is the library we will use in our examples.

The NewPing library is quite advanced. It supports up to 15 ultrasonic sensors at once and can output directly in centimeters, inches, or time periods.

This library is not included in the Arduino IDE, so you will need to install it first.

To install the library navigate to Sketch > Include Libraries > Manage Libraries… Wait for Library Manager to download the library index and update the list of installed libraries.

manage libraries

Filter your search by typing ‘newping’. Click on the first entry and then select Install.

newping library installation

Arduino Example Code

Here is a simple sketch that uses the serial monitor to display a distance measured in centimeters. Give this sketch a try before we start a detailed analysis of it.

// Include NewPing Library
#include "NewPing.h"

// Hook up HC-SR04 with Trig to Arduino Pin 9, Echo to Arduino pin 10
#define TRIGGER_PIN 9
#define ECHO_PIN 10

// Maximum distance we want to ping for (in centimeters).
#define MAX_DISTANCE 400	

// NewPing setup of pins and maximum distance.

void setup() {

void loop() {
	Serial.print("Distance = ");
	Serial.println(" cm");

Once the sketch is uploaded, open your serial monitor, set the baud rate to 9600 bps. Try pointing the sensor at objects lying around you. You should see the measured distance begin to stream by.

HC-SR04 Ultrasonic Sensor Arduino Distance Measurement Sketch Output on Serial Monitor
Output on Serial Monitor

Code Explanation:

The sketch starts by including the newly installed NewPing library.

#include "NewPing.h"

First the Arduino pins are defined to which the Trig and Echo pins of the HC-SR04 are connected. We have also defined a constant called MAX_DISTANCE. It will set a maximum distance where pings beyond that distance are read as no ping “clear”. MAX_DISTANCE is currently set to 400 [default = 500cm].

#define TRIGGER_PIN 9
#define ECHO_PIN 10
#define MAX_DISTANCE 400

After this, an instance of NewPing library named sonar is created.


In the setup, we initialize the serial communication with PC.

void setup() {

In the loop, we simply call the ping_cm() function and print the result on the serial monitor. This function sends a ping and returns the distance in centimeters.

void loop() {
	Serial.print("Distance = ");
	Serial.println(" cm");

Other useful functions in NewPing Library

There are a few useful functions you can use with NewPing object.

  • Above sketch returns the distance in centimeters. If you want result to be in inches, use sonar.ping_in() function.

  • The sketch above only has a resolution of one centimeter. If you want to get the result in decimal form you can use NewPing in duration mode instead of distance mode. You need to change this line:


    with below line

    Serial.print(( / 2) * 0.0343);
  • There is a method called ping_median(iterations) in the NewPing library to improve the accuracy of your HC-SR04. This method takes multiple measurements instead of just one, discards out-of-range readings, and then averages the remaining readings. By default it only takes 5 readings but you can specify as many as you want.

    int iterations = 5;
    Serial.print((sonar.ping_median(iterations) / 2) * 0.0343);

Arduino Project – Contactless Distance Finder

Let’s create a quick project to demonstrate how a simple ultrasonic sensor can be turned into a sophisticated contactless distance finder. In this project we will be using a 16×2 Character LCD which displays a horizontal bar to represent the distance from the object.

If you’re not familiar with 16×2 character LCDs, consider reading the tutorial below.


Next we need to make the connection to the LCD as shown below.

Arduino Wiring Fritzing Connections with HC-SR04 Ultrasonic Sensor and 16x2 LCD
Wiring HC-SR04 Ultrasonic Sensor and 16×2 LCD to Arduino UNO

Library Installation

Before we upload the code and start playing with the sensor we need to install a library called LCDBarGraph. This library will help in drawing a horizontal bar on the LCD, where the length of the bar will represent the distance to the object.

To install the library navigate to Sketch > Include Libraries > Manage Libraries… Wait for Library Manager to download the library index and update the list of installed libraries. Filter your search by typing ‘lcdbargraph’. Click on the first entry and then select Install.

lcdbargraph library installation

Arduino Code

Once you have installed the library, try the below sketch.

// includes the LiquidCrystal Library
#include <LiquidCrystal.h> 

// includes the LcdBarGraph Library
#include <LcdBarGraph.h>

// Maximum distance we want to ping for (in centimeters).
#define max_distance 200

// Creates an LCD object. Parameters: (rs, enable, d4, d5, d6, d7)
LiquidCrystal lcd(12, 11, 5, 4, 3, 2); 

LcdBarGraph lbg(&lcd, 16, 0, 1); // Creates an LCD Bargraph object.

const int trigPin = 9;
const int echoPin = 10;
long duration;
int distance;

void setup() 
	lcd.begin(16,2); // Initializes the interface to the LCD screen
	pinMode(trigPin, OUTPUT);
	pinMode(echoPin, INPUT);

void loop() 
	// Write a pulse to the HC-SR04 Trigger Pin
	digitalWrite(trigPin, LOW);
	digitalWrite(trigPin, HIGH);
	digitalWrite(trigPin, LOW);
	// Measure the response from the HC-SR04 Echo Pin
	duration = pulseIn(echoPin, HIGH);
	// Determine distance from duration
	// Use 343 metres per second as speed of sound
	distance= duration*0.034/2;
	// Prints "Distance: <value>" on the first line of the LCD
	lcd.print("Distance: "); 
	lcd.print(" cm");

	// Draws bargraph on the second line of the LCD
	lbg.drawValue(distance, max_distance);

The output looks like this.

HC-SR04 Ultrasonic Sensor Arduino Distance Measurement Bargraph Output on 16x2 LCD
Bargraph Output on 16×2 Character LCD

Code Explanation:

First of all, you have to set up the Liquid Crystal Library as usual. Next, create an LcdBarGraph instance with the LiquidCrystal instance you just created. You should reference LiquidCrystal to the constructor of LcdBarGraph.

The constructor of LcdBarGraph takes three more parameters. The second is the number of the Character column in the LCD (in our case it is 16). The last two parameters are optional and allow custom positioning of the bar.

// creating bargraph instance
LcdBarGraph lbg(&lcd, 16, 0, 1);

After calculating the distance from the sensor we use drawValue(value, maxValue) function to display the bargraph. It draws a bargraph with a value between 0 and maxValue.

//display bargraph
lbg.drawValue(distance, max_distance);

Interfacing HC-SR04 with 3-Wire Mode

When you have a limited number of digital I/O pins on your Arduino, you can take advantage of 3-pin mode. Normally when you connect HC-SR04 sensor to Arduino you need two I/O pins. However in 3-wire mode you only need a single I/O pin instead of two.

In this mode a single I/O pin is used as both input and output. This is possible because both Trig and Echo are not used at the same time.

The following table lists the pin connections:

HC-SR04 SensorArduino
Trig + Echo9

Here is how you can hook up the HC-SR04 sensor to Arduino using 3-wire mode.

Arduino Wiring Fritzing 3 Wire Mode Connections with HC-SR04 Ultrasonic Sensor
Wiring HC-SR04 Ultrasonic Sensor to Arduino UNO – 3 Wire Mode

All you have to do is connect both the TRIG and ECHO pins to digital pin #9 and define pin 9 for both pin values in the code. Rest of the code is same.

#define TRIGGER_PIN 9 // Trigger and Echo both on pin 9
#define ECHO_PIN 9

What are the limitations?

The HC-SR04 ultrasonic sensor is really good in terms of accuracy and overall usability especially compared to other low cost ultrasonic sensors. This does not mean that the HC-SR04 sensor will work all the time. The following pictures show some of the limitations of the HC-SR04:

  • The distance between the sensor and the object/obstacle is greater than 13 feet.

    HC-SR04 Limitation - cannot measure distance more than 13 feet
  • The object has its reflective surface at a shallow angle so that the sound is not reflected back to the sensor.

    HC-SR04 Limitation - cannot detect object at a shallow angle
  • The object is too small to reflect enough sound back to the sensor. Also, if your HC-SR04 sensor is mounted low on your device, you are likely to get sound reflecting off the floor.

    HC-SR04 Limitation - cannot detect small objects
  • Some objects with soft, irregular surfaces (such as stuffed animals) absorb sound rather than reflect it, so the HC-SR04 sensor may find it difficult to detect such objects.

    HC-SR04 Limitation - cannot detect soft irregular surface object