int pin = 12;
int utrip = 0;

void setup() {
  Serial.begin(115200);
  Serial.println("Test");
  pinMode(pin, OUTPUT);

}

void loop() {
    if(Serial.available()){
      char ch = Serial.read();
      if(ch >= '0' && ch <= '9'){
        utrip = ch * 10;
      }
      Serial.println(ch);
    }
  utripanje();
}

void utripanje(){
  digitalWrite(pin, HIGH);
  delay(utrip);
  digitalWrite(pin, LOW);
  delay(utrip);
}