// ldr arduino
// Techatronic.com  
 void setup()  
 {  
Serial.begin(9600); // sensor buart rate  
pinMode(3,OUTPUT); // led pin  
}  
void loop()  
{  
int s1=analogRead(A0); // LDR Sensor output pin connected  
Serial.println(s1);  
delay(50);  
if(s1< 500 )  
{  
digitalWrite(3,HIGH); // led ON  
}  
else  
{  
digitalWrite(3,LOW); // Red led OFF  
}  
}