int ledPin = 13;
int ledState = LOW;
unsigned long previousMillis = 0;
long interval = 1000;


void setup() {

pinMode(ledPin, OUTPUT);  // put your setup code here, to run once:

}

void loop() {
unsigned long currentMillis = millis();  // put your main code here, to run repeatedly:
if (currentMillis - previousMillis > interval){
  previousMillis = currentMillis ;
  if (ledState == LOW) 
  ledState= HIGH;
  else 
  ledState = LOW ;
  digitalWrite(ledPin, ledState);
  }
}