int ledState = LOW;
int interval = 1000;
unsigned long predosli = 0;
unsigned long cas;

void setup() 
{
  pinMode(3, OUTPUT);
}

void loop() 
{
  cas = millis();
  led();
  
}

void led()
{
  if(cas - predosli >= interval)
  {
    predosli = cas;
    if(ledState == LOW)
    {
      ledState = HIGH;
    }
    else
    {
      ledState = LOW;
    }
    digitalWrite(3, ledState);
  }
}