Arduino: Sonic range finder with SRF05

By
larry
Published: 12 July 2009 4:55 PM UTCPosted in: Arduino, Arduino C/C++, Electronics, ProgrammingTags: , , , , , ,
Arduino: Sonic range finder with SRF05

A guide to using the SRF05 with Arduino in order to calculate distances from objects. In this case I’m also altering the output of an LED with PWM according to how close an object is to the sensor. So the nearer you are the brighter the LED.

So if we start with the SRF05, it’s an IC that works by sending an ultrasound pulse at around 40Khz. It then waits and listens for the pulse to echo back, calculating the time taken in microseconds (1 microsecond = 1.0 × 10-6 seconds). You can trigger a pulse as fast as 20 times a second and it can determine objects up to 3 metres away and as near as 3cm. It needs a 5V power supply to run.

Adding the SRF05 to the Arduino is very easy, only 4 pins to worry about. Power, Ground, Trigger and Echo. Since it needs 5V and Arduino provides 5V I’m obviously going to use this to power it. Below is a diagram of my SRF05, showing the pins. There are 2 sets of 5 pins, 1 set you can use, the other is for programming the PIC chip so don’t touch them!

SRF05 pin layout

Shopping list
220 Ohm resistor (Red, Red, Brown, Gold)
SRF05 Ultrasonic range finder
LED
Arduino Deumilanove w/ ATMEGA328
Breadboard / Prototyping board
Jumper/ Connector wires
Optional 9V power supply (here) or use the USB power for the Arduino

The circuit
Very, very simple circuit, I’ve used the breadboard to share the GND connection and to add the LED which I could probably have done with out the breadboard. You’ll see the most complex thing is the code later on.

Arduino-SRF05

The sketch
All the work is done here, I’ve added code that averages the distance readings to remove some of the jitter in the results as the SRF05 is calculating distances very rapidly and there can be a lot of fluctuation. Also I convert the time in microseconds to distance by dividing the time by 58.

Why 58? Well because if you take the time in microseconds for a pulse to be sent and  received e.g. for 1 meter it takes about 5764 microseconds – at least from my wall anyway. If I divide this time by the distance in cm in I will get 57.64 so I just round this up – you can calculate distance in any other unit with this method.

Here I’ve also decided that for every cm under 255 my LED will get 1 step brighter. I’ve been lazy here for the sake of the sensors 3 metre range I didn’t see the point in making this any more complicated. Otherwise I would calculate the brightness on the percentile of proximity out of total range.

// written at: luckylarry.co.uk
// variables to take x number of readings and then average them
// to remove the jitter/noise from the SRF05 sonar readings
 
const int numOfReadings = 10;			// number of readings to take/ items in the array
int readings[numOfReadings];    		// stores the distance readings in an array
int arrayIndex = 0;             		// arrayIndex of the current item in the array
int total = 0;                  		// stores the cumlative total
int averageDistance = 0;        		// stores the average value
 
// setup pins and variables for SRF05 sonar device
 
int echoPin = 2;             			// SRF05 echo pin (digital 2)
int initPin = 3;             			// SRF05 trigger pin (digital 3)
unsigned long pulseTime = 0;  			// stores the pulse in Micro Seconds
unsigned long distance = 0;  			// variable for storing the distance (cm)
 
// setup pins/values for LED
 
int redLEDPin = 9;   				// Red LED, connected to digital PWM pin 9
int redLEDValue = 0; 				// stores the value of brightness for the LED (0 = fully off, 255 = fully on)
 
//setup
 
void setup() {
 
  pinMode(redLEDPin, OUTPUT);   		// sets pin 9 as output
  pinMode(initPin, OUTPUT);			// set init pin 3 as output
  pinMode(echoPin, INPUT);			// set echo pin 2 as input
 
  // create array loop to iterate over every item in the array
 
  for (int thisReading = 0; thisReading < numOfReadings; thisReading++) {      
readings[thisReading] = 0;      
 }         
// initialize the serial port, lets you view the   
 // distances being pinged if connected to computer 
     Serial.begin(9600);		
 } 
 
// execute 
void loop() {   
digitalWrite(initPin, HIGH);	  		// send 10 microsecond pulse   
delayMicroseconds(10);	  		// wait 10 microseconds before turning off   
digitalWrite(initPin, LOW);			// stop sending the pulse   
pulseTime = pulseIn(echoPin, HIGH);		// Look for a return pulse, it should be high as the pulse goes low-high-low   
distance = pulseTime/58;	  		// Distance = pulse time / 58 to convert to cm.  
 total= total - readings[arrayIndex]; 		// subtract the last distance   
readings[arrayIndex] = distance; 		// add distance reading to array   
total= total + readings[arrayIndex]; 		// add the reading to the total   
arrayIndex = arrayIndex + 1;         		// go to the next item in the array                                 
// At the end of the array (10 items) then start again   
if (arrayIndex >= numOfReadings)  {
    arrayIndex = 0;
  }
 
  averageDistance = total / numOfReadings;  	// calculate the average distance
 
  // if the distance is less than 255cm then change the brightness of the LED
 
  if (averageDistance < 255) {
    redLEDValue = 255 - averageDistance;	// this means the smaller the distance the brighterthe LED.
  }
 
  analogWrite(redLEDPin, redLEDValue); 		// Write current value to LED pins
  Serial.println(averageDistance, DEC);		// print out the average distance to the debugger
  delay(100); 			  		// wait 100 milli seconds before looping again
 
}

After thoughts
Well this is going to make the sensor for a robot methinks. I’ll alter this to control a servo so turn left or right when blocked, or perhaps to alter the speed of the motors. Or maybe I’ll just give myself bat like senses, maybe even fight crime! Anyway below is the quick video of it in action:

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