bool on = false ;

void setup() {
  // put your setup code here, to run once:
pinMode (10,OUTPUT);
attachInterrupt(0,ledChange,FALLING);

}

void loop() {
  // put your main code here, to run repeatedly:
delay (5000000);

}
void ledChange () {
  on =!on ;
if(on)
digitalWrite(10, LOW);
else
digitalWrite(10, HIGH);
}