int ledPin = 13;
int switchPin = 5;
int ledState = LOW;
unsigned long previousMillis = 0;
long interval = 100;
boolean running = false;

void setup() {

pinMode(ledPin, OUTPUT);  // put your setup code here, to run once:
pinMode(switchPin, INPUT_PULLUP);
digitalWrite(switchPin, HIGH);
}

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