unsigned long pre_millis=0;
const long interval=250;
int led=LOW;


void setup() {
 pinMode(2,OUTPUT);

}

void loop() {
  unsigned long cur_millis= millis ();
if (cur_millis-pre_millis>= interval)
{
  pre_millis=cur_millis ;
  if(led==LOW)
  led=HIGH;
  else 
  led=LOW;
  digitalWrite(2,led);   }
}