.:Photo Resistors:.
(ARDX) .:Arduino Expermentation Kit:. (ARDX)

What We're Doing
Whilst getting input from a potentiometer can be useful for human controlled experiments, what do we use when we want an environmentally controlled experiment? We use exactly the same principles but instead of a potentiometer (twist based resistance) we use a photo resistor (light based resistance). The Arduino cannot directly sense resistance (it senses voltage) so we set up a voltage divider ( ). The exact voltage at the sensing pin is calculable, but for our purposes (just sensing relative light) we can experiment with the values and see what works for us. A low value will occur when the sensor is well lit while a high value will occur when it is in darkness.

The Circuit
The Parts
Breadboard Sheet
2 Pin Header
10k Ohm Resistor
560 Ohm Resistor
Green LED



breadboard layout sheet

assembly video


Code (no need to type everything in just)
Download the Code from ( )
(copy the text and paste it into an empty Arduino Sketch)

 * A simple programme that will change the intensity of
 * an LED based  * on the amount of light incident on 
 * the photo resistor.

//PhotoResistor Pin
int lightPin = 0; //the analog pin the photoresistor is 
                  //connected to
                  //the photoresistor is not calibrated to any units so
                  //this is simply a raw sensor value (relative light)
//LED Pin
int ledPin = 9;   //the pin the LED is connected to
                  //we are controlling brightness so 
                  //we use one of the PWM (pulse width
                  // modulation pins)
void setup()
  pinMode(ledPin, OUTPUT); //sets the led pin to output
 * loop() - this function will start after setup 
 * finishes and then repeat
void loop()
 int lightLevel = analogRead(lightPin); //Read the
                                        // lightlevel
 lightLevel = map(lightLevel, 0, 900, 0, 255); 
         //adjust the value 0 to 900 to
         //span 0 to 255

 lightLevel = constrain(lightLevel, 0, 255);//make sure the 
                                           //value is between 
                                           //0 and 255
 analogWrite(ledPin, lightLevel);  //write the value


Not Working? (3 things to try)

LED Remains Dark
This is a mistake we continue to make time and time again, if only they could make an LED that worked both ways. Pull it up and give it a twist.
It Isn't Responding to Changes in Light.
Given that the spacing of the wires on the photo-resistor is not standard, it is easy to misplace it. Double check its in the right place.
Still not quite working?
You may be in a room which is either too bright or dark. Try turning the lights on or off to see if this helps. Or if you have a flashlight near by give that a try.

Making it Better?
Reverse the response:
Perhaps you would like the opposite response. Don't worry we can easily reverse this response just change:
    analogWrite(ledPin, lightLevel);
          ----> analogWrite(ledPin, 255 - lightLevel);

Upload and watch the response change:
Night light:
Rather than controlling the brightness of the LED in response to light, lets instead turn it on or off based on a threshold value. Change the loop() code with.
void loop(){
  int threshold = 300;
  if(analogRead(lightPin) > threshold){
    digitalWrite(ledPin, HIGH);
    digitalWrite(ledPin, LOW);

Light controlled servo:
Lets use our newly found light sensing skills to control a servo (and at the same time engage in a little bit of Arduino code hacking). Wire up a servo connected to pin 9 (like in CIRC-04). Then open the Knob example program (the same one we used in CIRC-08) File > Examples > Library-Servo > Knob. Upload the code to your board and watch as it works unmodified.
Using the full range of your servo:
You'll notice that the servo will only operate over a limited portion of its range. This is because with the voltage dividing circuit we use the voltage on analog pin 0 will not range from 0 to 5 volts but instead between two lesser values (these values will change based on your setup). To fix this play with the val = map(val, 0, 1023, 0, 179); line. For hints on what to do visit .