Simple code to light up a set of four LED's attached to an arduino depending on the intensity measured by the photodiode.

light_sensor_led.ino

const int P25 = 2;
const int P50 = 3;
const int P75 = 4;
const int P100 = 5;
const int SENSOR = A0;

void setup()
{
	pinMode(P25, OUTPUT);
    pinMode(P50, OUTPUT);
    pinMode(P75, OUTPUT);
    pinMode(P100, OUTPUT);
    pinMode(SENSOR, INPUT);
    digitalWrite(P25, LOW);
    digitalWrite(P50, LOW);
    digitalWrite(P75, LOW);
    digitalWrite(P100, LOW);
    Serial.begin(9600);
}

long level = 0;

void updateLeds(long level)
{
    if (level == 0) {
        digitalWrite(P25, LOW);
        digitalWrite(P50, LOW);
        digitalWrite(P75, LOW);
        digitalWrite(P100, LOW);    
    }    
    else if (level == 1) {
        digitalWrite(P25, HIGH);
        digitalWrite(P50, LOW);
        digitalWrite(P75, LOW);
        digitalWrite(P100, LOW);    
    }
    else if (level == 2) {
        digitalWrite(P25, HIGH);
        digitalWrite(P50, HIGH);
        digitalWrite(P75, LOW);
        digitalWrite(P100, LOW);        
    }
    else if (level == 3) {
        digitalWrite(P25, HIGH);
        digitalWrite(P50, HIGH);
        digitalWrite(P75, HIGH);
        digitalWrite(P100, LOW);      a  
    }
    else {
        digitalWrite(P25, HIGH);
        digitalWrite(P50, HIGH);
        digitalWrite(P75, HIGH);
        digitalWrite(P100, HIGH);
    }
 
}

void loop()
{
    Serial.println(analogRead(SENSOR));
    level = map(analogRead(SENSOR), 1005, 1022, 0, 4);
    updateLeds(level);
    Serial.print(level);
    Serial.print("\n");
}