const int ledPin = 13;
unsigned long previousMillis = 0; 
const long interval = 500;
int ledState = LOW;
 
void setup() {
  pinMode(ledPin, OUTPUT);
}

void loop() {
  if (millis() - previousMillis >= interval) { 
    previousMillis = millis(); 
    ledState = !ledState;
    digitalWrite(ledPin, ledState); 
  }
}