LED As Lightsensor on the Arduino

I'm writing this instructable as an answer on some of Paolo's questions.

[I did some more experimenting on this subject in this: https://www.instructables.com/id/Better-LED-as-light-sensor/ Ible]

It is not hard to use an a LED as a sensor on the Arduino. Just put a LED with the anode (positive = long leg) in analog port 0 and the cathode (negative = short leg) in GND (ground).
You can also put a LED to react on the sensor with the anode in port 13 and the cathode in GND. (you can also watch the LED already on the Arduino board).
Different colors of LEDs will have better or worse effect, so experiment with that. 

Than upload the following sketch (program):

// this program is just made to experiment with a LED as a light-sensor.

int led = 13;
int basis = 0;
int sens = 0;
int x = 0;
int y = 0;
int totaal = 0;
int totaal1 = 0;

void setup() {
pinMode(led, OUTPUT);
}

void loop() {                                              // here we do 50 readings to set the sensors
   for(x =0; x < 50; x++) {
   sens = analogRead(0);
   totaal = totaal1 + sens; totaal1 = totaal;
   }
 sens = totaal/x;                          // divide the 50 readings by 50 again 
totaal = 0;
totaal1 = 0;
basis = sens-20;           // setting sensitivity - now it will react if the LED is 20 lower than the setting above
    for(y=0;y<1000;y++){            // after every 1000 tests the program will reset the led to cope with changing light
        for(x =0; x < 50; x++) {      // 50 readings to see if the sensor is in the dark
       sens = analogRead(0);
       totaal = totaal1 + sens;
       totaal1 = totaal;
       delay(10); }
    sens = totaal/x;
       if (sens < basis)                // testing is the led was in the dark
       digitalWrite(led, HIGH);  // turning the led in port 13 or on the board on if the sensor-led was 20 darker than now than in the setting
         else  
          digitalWrite(led, LOW); // turning it of if not
   totaal = 0;  
   totaal1 = 0;  
   delay(10); } }