• They are devices designed to detect and respond to light levels or changes in light intensity.
  • Often referred to as photodetectors.


There are several types of light-sensitive sensors, each with its own principles of operation and applications:

Photodiodes:
Semiconductor devices that generate a current when exposed to light.




Phototransistors:
These are similar to photodiodes but with additional amplification, phototransistors are used for detecting light in a wider range.




Photoresistors:
These are sensors that change resistance based on the amount of light they receive.




Light Dependent Resistors or Photoresistors:


  • The resistance of an LDR is high in darkness because fewer photons are striking the semiconductor material.
  • As the intensity of incident light increases, more photons hit the material, causing more electrons to be excited across the semiconductor's valence and conduction bands.
  • This increases the conductivity of the material, resulting in a decrease in resistance.


Code:

int ledPin = 2;

void setup() 

{
    Serial.begin(9600);
    pinMode(ledPin,  OUTPUT);
}

void loop() {
    int value = analogRead(A0);

    Serial.println("Analog  Value: ");
    Serial.println(value);
    
    if (value > 450) {
        digitalWrite(ledPin, LOW);
    } else {
        digitalWrite(ledPin,  HIGH);
    }
    
    delay(250);
}