Using serial communication and a CdS cell as a primitive light meter.

LightMeter.ino

/*
 LightMeter
 Report the light intensity at an arbitrary scale to the serial port.
 */
 
const int inputPin = A0;  // use analog 0 (A0) input pin
int lightLevel = 0;       // used to store the input value
 
void setup() {
  Serial.begin(9600);  // initialize serial communication at 9600 bits/second
}
 
void loop() {
  lightLevel = analogRead(inputPin);  // get a reading      
  Serial.println(lightLevel);         // print out reading
  delay(500);                         // delay between reads
}