I am going to show you how you can use an RGB LED and a Cds photocell as a colour sensor for a micro-controller. I will illustrate the method to retrieve a colour using Arduino, and I will show you a how you can verify the colour being scanned with a small Processing sketch.

We will be making this colour sensor on a breadboard, but it is easily transferred onto a prototyping board, and for those who fab their own boards, this would be an awesome kit that is super cheap to throw together. I am sure it would only take about two minutes to write a gerber file for this circuit and make a nice little finished sensor.

Step 1: Gather Some Parts

For this sensor you will need a breadboard (not required, but it is how I will walk you through it.) an RGB LED (alternatively you could use 3 LEDs)

A 220 ohm resistor

A CdS photocell (these can be salvaged out of all kinds of things like nightlights or garden lamps)

An Arduino, or a clone. I am using a RBB in this example

Tools you will need

A computer a cable to upload to your Arduino

Step 2: A Little Theory

Some of you might be wondering how a CdS photocell can detect colours. Well it is surprisingly simple and provides pretty accurate results.

We see colour as a frequency of light reflected from an object. So different colours reflect different wavelengths which our eyes then interpret as colours. (Maybe brain...I am no scientist)

A common CdS photocell has a very similar response to colour as the human eye.

Because colours absorb certain wavelengths and reflect certain wavelengths, we can use different wavelengths(colours) of light and take readings(from a sensor that has nearly human responses) and thereby make a pretty good guess at what colour the sensor is being exposed to.

Step 3: Build the Circuit

http://cdn.instructables.com/FUK/W7N9/GVVPCXSJ/FUKW7N9GVVPCXSJ.LARGE.jpg

I have included both images of the breadboard arrangment, and a small diagram to show you how to wire up the sensor to the Arduino.

The circuit is really simple. First we will look at the RGB LED half of the sensor. It is simply a common cathode RGB LED connected to pins 2,3, and 4 of the Arduino with a 220 ohm resistor going out to ground. This will allow us to turn each of the LEDs within the package on and off individually when we need to.

On the other side of the circuit we have a Cds photocell being fed 5 volts from the arduino. combined with the resistor going out to ground this effectively creates a voltage divider which allows us to read a changing analog value on analog pin 0.

This sensor works great on a breadboard, but it works even better if you put it into a more permanent enclosure to minimilize ambient light interference. The photo of the light tight (ish) enclosure was used in another one of my projects and is included here only to illustrate what I meant. (Feel free to check it out though,here.)

Step 4: Code the Arduino

Open up the Arduino environment, and create a new sketch by clicking File - New. For those that don`t want to follow along, I have included the code as a text file. Copy/Paste it into a new sketch in Arduino, and you are good to go.

Declarations

The beginning of a sketch is where we make our declarations, so type the following code into the environment window.// Define colour sensor LED pinsint ledArray[] = {2,3,4};

// boolean to know if the balance has been setboolean balanceSet = false;

//place holders for colour detectedint red = 0;int green = 0;int blue = 0;

//floats to hold colour arraysfloat colourArray[] = {0,0,0};float whiteArray[] = {0,0,0};float blackArray[] = {0,0,0};

//place holder for averageint avgRead; '//' is reserved for denoted comments, therefore you do not need the text on

lines that begin with '//' for the sketch to work. The rest of it are some names

that we are giving to some placeholders, and the declarations for what kind of

data they will contain. As you can see, we start by setting up an array to hold

the pin numbers. These correspond to the pins that the different colour LEDs

are conected to on the Arduino.Next we declare a Boolean value to check

whether or not balancing has been performed. A Boolean value is something

that returns true or false. We go on to create some place holders for the colour

values, and some arrays to use for holding the scan data and balancing the

detected colour. Although we are not finished yet, go ahead and save your

sketch. Give it a name that is meaningful to you, something like

coloursensor.pde maybe.

Setup

The next step in creating an Arduino sketch is to write the setup function.

Setup is run when the Arduino first boots up, so this is where we tell the

Arduino what how we want to use the pins and setup other features that we

may need such as serial communication.

Type the following below the code that you just entered.

void setup(){   //setup the outputs for the colour sensor  pinMode(2,OUTPUT);  pinMode(3,OUTPUT);  pinMode(4,OUTPUT);   //begin serial communication  Serial.begin(9600); } Not much to this section. Here we are just telling the Arduino that we intend on using pins 2,3,and 4 as outputs. This is neccesary if we want to light our LEDs...and we do, if only very briefly.

The next part is to tell the Arduino that we intend to make use of the serial port, and that it should be opened and ready to communicate at the baud rate specified (in brackets).

Loop

The loop function is typically where the magic happens in an Arduino. It is the code that is run over and over again once the Arduino is booted and setup has been passed through. Put the next bit of code under your setup function.

void loop(){

    checkBalance();    checkColour();

    printColour();    } Okay at first glance it looks like there really is not much there. But these are each calls to private functions. For some projects I find this type of approach works well. It makes it easier to read (I think) as each block of code is seperated with a meaningful name, and we can see what the sketch is going to do at a glance.

First it will check the balance, then it will check the colour, and then it will print the colour to the serial port so we can see the values that it read.

Let`s explore the first private function, checkBalance, and add it to our code. Type the following below your loop function. void checkBalance(){  //check if the balance has been set, if not, set it  if(balanceSet == false){    setBalance();  }} As you can see, if the balanceSet value (a boolean value) is set to false, then it makes a call to a secondary function called setBalance, where we set the white and black balancing readings. This will only happen when you boot up the arduino the one time. void setBalance(){  //set white balance   delay(5000);                              //delay for five seconds, this gives us time to get a white sample in front of our sensor  //scan the white sample.  //go through each light, get a reading, set the base reading for each colour red, green, and blue to the white array  for(int i = 0;i<=2;i++){     digitalWrite(ledArray[i],HIGH);     delay(100);     getReading(5);          //number is the number of scans to take for average, this whole function is redundant, one reading works just as well.     whiteArray[i] = avgRead;     digitalWrite(ledArray[i],LOW);     delay(100);  }

  //done scanning white, now it will pulse blue to tell you that it is time for the black (or grey) sample.   //set black balance    delay(5000);              //wait for five seconds so we can position our black sample   //go ahead and scan, sets the colour values for red, green, and blue when exposed to black  for(int i = 0;i<=2;i++){     digitalWrite(ledArray[i],HIGH);     delay(100);     getReading(5);     blackArray[i] = avgRead;     //blackArray[i] = analogRead(2);     digitalWrite(ledArray[i],LOW);     delay(100);  }   //set boolean value so we know that balance is set  balanceSet = true;  delay(5000);     //delay another 5 seconds to let us catch up  } If you noticed, we made yet another call to a function getReading.  We will add that right after we put in the checkColour function.

Once our balancing numbers have been set, we go on to finally read the colour. The function is basically the same as setBalance,, with the addition of the math that balances the reading. Let`s add it now. void checkColour(){    for(int i = 0;i<=2;i++){     digitalWrite(ledArray[i],HIGH);  //turn or the LED, red, green or blue depending which iteration     delay(100);                      //delay to allow CdS to stabalize, they are slow     getReading(5);                  //take a reading however many times     colourArray[i] = avgRead;        //set the current colour in the array to the average reading     float greyDiff = whiteArray[i] - blackArray[i];                    //the highest possible return minus the lowest returns the area for values in between     colourArray[i] = (colourArray[i] - blackArray[i])/(greyDiff)*255; //the reading returned minus the lowest value divided by the possible range multiplied by 255 will give us a value roughly between 0-255 representing the value for the current reflectivity(for the colour it is exposed to) of what is being scanned     digitalWrite(ledArray[i],LOW);   //turn off the current LED

     delay(100);  }  } We will also need to add our function getReading, otherwise our sketch won`t have real values. This function just allows us to take a few readings and average them out. This allows for a slightly smoother reading. void getReading(int times){  int reading;  int tally=0;  //take the reading however many times was requested and add them upfor(int i = 0;i < times;i++){   reading = analogRead(0);   tally = reading + tally;   delay(10);}//calculate the average and set itavgRead = (tally)/times;} Now we have a colour read into our colour holding array, all we need to do now is output it to the screen. Remember that we setup serial communication back in the setup function, all we have to do now is use the serial port to send out our data. The Arduino environment includes a serial monitor, which will allow you to see the data as it is passed from the Arduino. So let`s add the last function to the sketch. //prints the colour in the colour array, in the next step, we will send this to processing to see how good the sensor works.void printColour(){Serial.print("R = ");Serial.println(int(colourArray[0]));Serial.print("G = ");Serial.println(int(colourArray[1]));Serial.print("B = ");Serial.println(int(colourArray[2]));//delay(2000);}

 

This will print out some data that looks something like this R = 231G = 71B = 0 which would be the equivilent to a red-orange colour. I have left this sketch super simple. You can dress it up as you see fit. The idea is to show how to make and use the sensor, creative implementation is up to you.

Now you can save your file again and upload it to the Arduino. Your Arduino should start running the sketch, the little lights should be flickering.

The sketch is very basic. I have offered it as a building block and to show you how to use the sensor.