---
title: "Raspberry Pi & Ultrasonic Module – Calculating Distance in C"
date: 2014-05-23T00:00:00
slug: raspberry-pi-ultrasonic-module-calculating-distance-in-c

---
In this brief article I will be posting the C code I used to measure the distance between the Raspberry Pi and any object in front of the HC-SR04 module. I used this article to show me how to wire up the ultrasonic module using a breadboard. You will also need to download and install wiringPi from the wiringPi website, full details [here](http://wiringpi.com/download-and-install/).

The code is based on the Python script from [this article](http://www.raspberrypi-spy.co.uk/2012/12/ultrasonic-distance-measurement-using-python-part-1/) written by Matt Hawkins.

```c
/*
         Measures the distance between the Pi and an object using an ultrasonic module
         Original source by Matt Hawkins (09/01/2013, http://www.raspberrypi-spy.co.uk/2012/12/ultrasonic-distance-measurement-using-python-part-1/) in Python
         Written in C by Lewis Dale (09/04/2014, http://blog.lewisdale.co.uk)

        WiringPi Library by Gordon Henderson (http://wiringpi.com)
 */
 #include <stdio.h>
 #include <stdlib.h>
 #include <time.h>
 #include <wiringPi.h>
/*Define the GPIO pins using WiringPi Pin Numbers*/
 #define GPIO_TRIGGER  4
 #define GPIO_ECHO  5

 int main()
 {
         /*Set up the Pi for GIPO*/
         wiringPiSetup ();
         printf("Ultrasonic Measurement ");
         while(1)
         {
                 double distance;
                 int i;
                 distance = 0;

                 /*Loop 5 times for greater accuracy*/
                 for(i=0;i<5;i++)
                 {
                         unsigned int start, end;
                         double total;

                         /*Set the trig pin to output, and the echo pin to input*/
                         pinMode(GPIO_TRIGGER,OUTPUT);
                         pinMode(GPIO_ECHO,INPUT);

                         /*Make sure the module is not outputting*/
                         digitalWrite(GPIO_TRIGGER, LOW);
                         /*500ms delay to allow the module to settle*/
                         delay(500);
                         /*Send a 10us pulse to trigger*/
                         digitalWrite(GPIO_TRIGGER,HIGH);
                         delayMicroseconds (10);
                         digitalWrite(GPIO_TRIGGER,LOW);

                         start = millis();

                         while(digitalRead(GPIO_ECHO) == 0)
                         {
                                 start = millis();
                         }
                         while(digitalRead(GPIO_ECHO) == 1)
                         {
                                 end = millis();
                         }

                         /*Calculate pulse length and convert to seconds*/
                         total = (double) (end-start) / 1000;

                         /*Distance travelled is time * speed of sound (cm/s)
                          Halved as we only want distance to object*/
                         total = total * 34300;
                         total = total / 2;
                         /*Append to total distance*/
                         distance += (double) total;
                 }

                 /*Calculate mean distance*/
                 distance = distance/5;
                 printf("Distance: %f ",distance);
         }
         return EXIT_SUCCESS;
 }
```

## Compilation instructions

To compile the script, save it as `usonic.c` and compile using gcc, including the wiringPi library:

```bash
gcc -Wall -o usonic usonic.c -lwiringPi
```