treba takhle :

int start = 12;
int rele = 13;
int stav = LOW;

void setup() {
  pinMode(rele, OUTPUT);
  pinMode(start, INPUT);
  digitalWrite(rele, stav);
}

void loop() {
    if digitalRead(start) { //je stisknuto
          stav= !stav;    // prepnu stav
          digitalWrite(rele,stav);
          delay(100);                   //spi 0.1s pro debounce
          while digitalRead(start) { }; // cekej na pusteni
          delay(100);                   //spi 0.1s pro debounce
    }
}