Tutorials Section

Photocell Tutorial with Arduino

Overview

Photocells are one of the coolest components for detecting light and darkness. These components are small, cost effective and so easy to use! Photocells are used in the general market everyday ranging from toys to indoor and outdoor light sensors.

In this tutorial, we will provide a short summary of the photocell and then we will put it into action with the Arduino Duemilanove.

Tutorial

Tools Required

• Arduino Development Board.
• 1 Photocell
• 1 Breadboard
• 1 LED (Light Emitting Diode)
• Hookup Wire
• 10K Ohm Resistor
• 1K Ohm Resistor
• A Power Source for the Arduino Development Board. (USB or Power Supply)
• Multimeter
• Thinking cap.

*Please note that these items are available separately or you can purchase the Photocell Kit in our store.

Learn

Photocells are designed to detect light and darkness through resistance in the cell. When light strikes the photocell, the internal resistance decreases.When there is no light striking on the photocell, then resistance increases. If we were to connect the photocell directly into a circuit, then current would be limited in darkness and current would move more freely during light. So in a nutshell, the photocells acts as a variable (changing) resistor. Here’s a visual explanation of how a photocell can be used in a circuit.

Note* Photocell is R2 aligned with a 10K Resistor R1.

Here are some pictures testing the theory of the resistance in the photocell.

As you see with the first picture (left), the photocell alone has a lower resistance. In the second picture (right), the covered photocell has a higher resistance.

Enjoy

Now that you have learned a little bit about photocells, let’s get the photocell working! First, we need to setup a circuit. We are going to have a 10K or a 1K Ohm resistor in series to the photocell. The analog output will be measured between the photocell and the resistor. See the picture below.

Now let’s program the Arduino and make it happen! Here’s the code:

int sensorPin = 0;    				// select the input pin for the photocell
int sensorValue = 0;  				// variable to store the value coming from the photocell

void setup() {
  Serial.begin(9600); 				//Set baud rate to 9600 on the Arduino
}

void loop() {
                                          // read the value from the sensor:
  sensorValue = analogRead(sensorPin);  //get the voltage value from input pin
  Serial.println(sensorValue); 		 //print the value to Serial monitor
  delay(2000);                        //delay for 2 seconds
}

The program code above reads the analog input of the photocell which is then converted into a digital value. The digital signal at it’s highest potential can be high 10 bits and the lowest potential can be zero bits. When the Arduino provides the value to the human viewer it will be viewed as the highest value of 1023 (10 bits) or 0 (0 bits). Thus, there is 1024 possibilities.

So if we applied 5 volts to the circuit and the resistance of the photocell is 5K Ohm with a 10K Ohm resistor in series, then using a voltage divider formula it would be: (10K/15K) * 5V = 3.33V. So the digital equivalance for 3.33V is (3.33V/ 5V) *1024 = 682.

Now let’s try using an LED (Light Emitting Diode) for feedback. We are going to hook up the LED to a digital port with a 1K Ohm Resistor in series with the LED. See the picture below.

Now let’s use a program that emits an LED when darkness appears. Please note to change the sensor threshold value to a desired level.

int sensorPin = 0;    			// select the input pin for the photocell
int sensorValue = 0;  			// variable to store the value coming from the photocell
int LEDpin = 8;					//LED Pin is on the Digital i/o pin number 8

void setup() {
  Serial.begin(9600);      //Set baud rate to 9600 on the Arduino
  pinMode(LEDpin, OUTPUT); //set the LED pin as an output on digital i/o pin 8
}

void loop() {

  sensorValue = analogRead(sensorPin);  //get the value from input pin
  Serial.println(sensorValue);  //print the value to Serial monitor
  delay(2000);

      if (sensorValue < 300) //if there is darkness then turn led on

      {
        digitalwrite(LEDpin,HIGH);
      }
      else
      {
       digitalwrite(LEDpin, LOW);  //else, keep the led off
      }

}

Imagine

Now that you have learned, programmed and hopefully enjoyed this tutorial, here are some ideas that you could use to spark the imagination:

Photocell and Serial LCD, read a value from the photocell and display on the LCD.

Photocell and a robot, get the sensor value of the photocell and move a robot into the light or away from the light.

Filed under: by Hanguru
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