But you can use this amazing concept to spark that geek inside you or just to impress your nerd friends.
I found that concept of using the LED as a light sensor.
The LED has nearly the same hardware structure as the light sensor.
This means that it has the ability to detect light exposure.
I found that LED can be used as a Light sensing element.
Of course, I cannot assure it can be used in practical application.
But I liked the concept when I knew it.
I've tested the concept both on Arduino and on Pinguino and it just worked well for me.
I followed that setup from instructables
//Copy the following sketch in your Arduino software and upload it to the Arduino board. (be sure to select the right board and com-port in the Arduino software)
int led = 13; // the pin where you will put the LED
int sensorpin = A3; // the analog pin where you put your sensorLED
int resetteller = 0; // the rest are counters and variables to calculate with
int sens = 0;
int teller = 0;
int basis = 1024;
int test = 1024;
int test2 = 1024;
int test3 = 1024;
// this are the values to play with to get better (or worse) results
int marge = 5; // the space between a positive and negative reading
int vertraging = 1; // the speed of the readings; a lower number is a higher speed
int samples = 70; // the amount of samples to compare to make one reading
int resetsamples = 30; // how many cycles to run the light on before you don't trust the value anymore
void setup() {
pinMode(led, OUTPUT);
Serial.begin(9600); // no real part of the program, just for debugging
for(teller =0; teller < samples; teller++) {// remember the lowest value out of many readings
sens = analogRead(sensorpin);
if (sens < basis){basis = sens;}
delay(vertraging); // the sensor needs a delay here to catch its breath
}
}
void loop() {
for(teller =0; teller < samples; teller++) {// remember the lowest value out of many readings
sens = analogRead(sensorpin);
delay(vertraging); // the sensor needs a delay here to catch its breath
if (sens < test){
test3 = sens; // remember the 3 lowest readings
test2 = test3;
test = test2;}
}
if (test < basis-marge && test2 < basis-marge && test3 < basis-marge){//all 3 low readings mus be < the basis reading
digitalWrite(led, HIGH);
resetteller++; // count how long the LED stays on
}
else{
digitalWrite(led, LOW);
basis = test; // if the lowest test reading is higher than the basis, basis will be reset
resetteller = 0;
}
if (resetteller > resetsamples){basis = test;}//if LED stays on to long, we don't trust it and reset basis
Serial.print(basis);Serial.print(" ");Serial.print(test);Serial.print(" ");Serial.println(sens);//just for debugging
test = 1024;
}
int led = 13; // the pin where you will put the LED
int sensorpin = A3; // the analog pin where you put your sensorLED
int resetteller = 0; // the rest are counters and variables to calculate with
int sens = 0;
int teller = 0;
int basis = 1024;
int test = 1024;
int test2 = 1024;
int test3 = 1024;
// this are the values to play with to get better (or worse) results
int marge = 5; // the space between a positive and negative reading
int vertraging = 1; // the speed of the readings; a lower number is a higher speed
int samples = 70; // the amount of samples to compare to make one reading
int resetsamples = 30; // how many cycles to run the light on before you don't trust the value anymore
void setup() {
pinMode(led, OUTPUT);
Serial.begin(9600); // no real part of the program, just for debugging
for(teller =0; teller < samples; teller++) {// remember the lowest value out of many readings
sens = analogRead(sensorpin);
if (sens < basis){basis = sens;}
delay(vertraging); // the sensor needs a delay here to catch its breath
}
}
void loop() {
for(teller =0; teller < samples; teller++) {// remember the lowest value out of many readings
sens = analogRead(sensorpin);
delay(vertraging); // the sensor needs a delay here to catch its breath
if (sens < test){
test3 = sens; // remember the 3 lowest readings
test2 = test3;
test = test2;}
}
if (test < basis-marge && test2 < basis-marge && test3 < basis-marge){//all 3 low readings mus be < the basis reading
digitalWrite(led, HIGH);
resetteller++; // count how long the LED stays on
}
else{
digitalWrite(led, LOW);
basis = test; // if the lowest test reading is higher than the basis, basis will be reset
resetteller = 0;
}
if (resetteller > resetsamples){basis = test;}//if LED stays on to long, we don't trust it and reset basis
Serial.print(basis);Serial.print(" ");Serial.print(test);Serial.print(" ");Serial.println(sens);//just for debugging
test = 1024;
}
project into 
Create page and followed the step-by-step guide to using 
