arduino_switch.c

int before_sw = HIGH;

void setup() {
  Serial.begin(9600);
  pinMode(9, INPUT_PULLUP);
  pinMode(13, OUTPUT);
}

void loop() {
  if(digitalRead(9) == LOW && before_sw == HIGH)
  {
    before_sw = LOW;
    if(digitalRead(13) == HIGH) digitalWrite(13, LOW);
    else digitalWrite(13, HIGH);
  }
  else if(digitalRead(9) == HIGH)
  {
    before_sw = HIGH;
  }
  
//  delay(100);

}