Arduino Code

Simple Demonstration of Use

This sketch will take the analog voltage reading and use that to determine how bright the red LED is. The darker it is, the brighter the LED will be! Remember that the LED has to be connected to a PWM pin for this to work, I use pin 11 in this example.
These examples assume you know some basic Arduino programming. If you don't, maybe spend some time reviewing the basics at the Arduino tutorial?
/* Photocell simple testing sketch. 
 
Connect one end of the photocell to 5V, the other end to Analog 0.
Then connect one end of a 10K resistor from Analog 0 to ground 
Connect LED from pin 11 through a resistor to ground 
For more information see http://learn.adafruit.com/photocells */
 
int photocellPin = 0;     // the cell and 10K pulldown are connected to a0
int photocellReading;     // the analog reading from the sensor divider
int LEDpin = 11;          // connect Red LED to pin 11 (PWM pin)
int LEDbrightness;        // 
void setup(void) {
  // We'll send debugging information via the Serial monitor
  Serial.begin(9600);   
}
 
void loop(void) {
  photocellReading = analogRead(photocellPin);  
 
  Serial.print("Analog reading = ");
  Serial.println(photocellReading);     // the raw analog reading
 
  // LED gets brighter the darker it is at the sensor
  // that means we have to -invert- the reading from 0-1023 back to 1023-0
  photocellReading = 1023 - photocellReading;
  //now we have to map 0-1023 to 0-255 since thats the range analogWrite uses
  LEDbrightness = map(photocellReading, 0, 1023, 0, 255);
  analogWrite(LEDpin, LEDbrightness);
 
  delay(100);
}
You may want to try different pulldown resistors depending on the light level range you want to detect!

Simple Code for Analog Light Measurements

This code doesn't do any calculations, it just prints out what it interprets as the amount of light in a qualitative manner. For most projects, this is pretty much all thats needed!
/* Photocell simple testing sketch. 
 
Connect one end of the photocell to 5V, the other end to Analog 0.
Then connect one end of a 10K resistor from Analog 0 to ground
 
For more information see http://learn.adafruit.com/photocells */
 
int photocellPin = 0;     // the cell and 10K pulldown are connected to a0
int photocellReading;     // the analog reading from the analog resistor divider
 
void setup(void) {
  // We'll send debugging information via the Serial monitor
  Serial.begin(9600);   
}
 
void loop(void) {
  photocellReading = analogRead(photocellPin);  
 
  Serial.print("Analog reading = ");
  Serial.print(photocellReading);     // the raw analog reading
 
  // We'll have a few threshholds, qualitatively determined
  if (photocellReading < 10) {
    Serial.println(" - Dark");
  } else if (photocellReading < 200) {
    Serial.println(" - Dim");
  } else if (photocellReading < 500) {
    Serial.println(" - Light");
  } else if (photocellReading < 800) {
    Serial.println(" - Bright");
  } else {
    Serial.println(" - Very bright");
  }
  delay(1000);
}
To test it, I started in a sunlit (but shaded) room and covered the sensor with my hand, then covered it with a piece of blackout fabric.

BONUS!  Reading Photocells Without Analog Pins

Because photocells are basically resistors, its possible to use them even if you don't have any analog pins on your microcontroller (or if say you want to connect more than you have analog input pins). The way we do this is by taking advantage of a basic electronic property of resistors and capacitors. It turns out that if you take a capacitor that is initially storing no voltage, and then connect it to power (like 5V) through a resistor, it will charge up to the power voltage slowly. The bigger the resistor, the slower it is.

This capture from an oscilloscope shows whats happening on the digital pin (yellow). The blue line indicates when the sketch starts counting and when the couting is complete, about 1.2ms later.

This is because the capacitor acts like a bucket and the resistor is like a thin pipe. To fill a bucket up with a very thin pipe takes enough time that you can figure out how wide the pipe is by timing how long it takes to fill the bucket up halfway.

In this case, our 'bucket' is a 0.1uF ceramic capacitor. You can change the capacitor nearly any way you want but the timing values will also change. 0.1uF seems to be an OK place to start for these photocells. If you want to measure brighter ranges, use a 1uF capacitor. If you want to measure darker ranges, go down to 0.01uF.
/* Photocell simple testing sketch. 
Connect one end of photocell to power, the other end to pin 2.
Then connect one end of a 0.1uF capacitor from pin 2 to ground 
For more information see http://learn.adafruit.com/photocells */
 
int photocellPin = 2;     // the LDR and cap are connected to pin2
int photocellReading;     // the digital reading
int ledPin = 13;    // you can just use the 'built in' LED
 
void setup(void) {
  // We'll send debugging information via the Serial monitor
  Serial.begin(9600);   
  pinMode(ledPin, OUTPUT);  // have an LED for output 
}
 
void loop(void) {
  // read the resistor using the RCtime technique
  photocellReading = RCtime(photocellPin);
 
  if (photocellReading == 30000) {
    // if we got 30000 that means we 'timed out'
    Serial.println("Nothing connected!");
  } else {
    Serial.print("RCtime reading = ");
    Serial.println(photocellReading);     // the raw analog reading
 
    // The brighter it is, the faster it blinks!
    digitalWrite(ledPin, HIGH);
    delay(photocellReading);
    digitalWrite(ledPin, LOW);
    delay(photocellReading);
  }
  delay(100);
}
 
// Uses a digital pin to measure a resistor (like an FSR or photocell!)
// We do this by having the resistor feed current into a capacitor and
// counting how long it takes to get to Vcc/2 (for most arduinos, thats 2.5V)
int RCtime(int RCpin) {
  int reading = 0;  // start with 0
 
  // set the pin to an output and pull to LOW (ground)
  pinMode(RCpin, OUTPUT);
  digitalWrite(RCpin, LOW);
 
  // Now set the pin to an input and...
  pinMode(RCpin, INPUT);
  while (digitalRead(RCpin) == LOW) { // count how long it takes to rise up to HIGH
    reading++;      // increment to keep track of time 
 
    if (reading == 30000) {
      // if we got this far, the resistance is so high
      // its likely that nothing is connected! 
      break;           // leave the loop
    }
  }
  // OK either we maxed out at 30000 or hopefully got a reading, return the count
 
  return reading;
}
This guide was first published on Jul 29, 2012. It was last updated on Oct 22, 2018. This page (Arduino Code) was last updated on Dec 08, 2017.