Photoresistor Sensor (LDR)

Photoresistors, also known as light dependent resistors (LDR), are light sensitive devices most often used to indicate the presence or absence of light, or to measure the light intensity. In the dark, their resistance is very high, sometimes up to 1MΩ, but when the LDR sensor is exposed to light, the resistance drops dramatically, even down to a few ohms, depending on the light intensity. LDRs have a sensitivity that varies with the wavelength of the light applied and are nonlinear devices.

Documentation for this sensor is available here.

The typical reistor is 10kOhms but you can also find the LDRs (like the one shown in the figure below) that already contains the resistor, in which case you don't need to connect the resistor.

/********************
 * Program:  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 = A0; // the cell and 10K pulldown are connected to A0
int photocellReading; // the analog reading from the analog resistor divider

void setup() {
  // We'll send debugging information via the Serial monitor
  Serial.begin(38400);
}

void loop() {
  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(" - Black");
  } else if (photocellReading < 200) {
    Serial.println(" - Dark");
  } else if (photocellReading < 500) {
    Serial.println(" - Light");
  } else if (photocellReading < 800) {
    Serial.println(" - Luminous");
  } else {
    Serial.println(" - Bright");
  }
  delay(2000);
}