#define LED 3
int interval,interval2 = 1;
bool stav;
unsigned long previousMilis_LED = 0;
String text;

void setup() {
  pinMode(LED, OUTPUT);
  Serial.begin(9600);
}

void loop() {
unsigned long currentMilis = millis();

if(Serial.available() > 0 ){
  text = Serial.readString();
  text.trim();
  interval = text.toFloat();

  if(interval > 0 && interval <= 10){
    Serial.println("interval je: " + String(text) + " sekund.");
    interval2 = interval;
  }
}
if(currentMilis - previousMilis_LED >= interval2*1000){
  stav= !stav;
  digitalWrite(LED, stav);
  previousMilis_LED = currentMilis;
}
}