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

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

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