void setup() {
  pinMode(A0, INPUT);
  pinMode(13, OUTPUT);
  Serial.begin(9600); // 9600 bits/s
}
void loop() {
  float BETA = 3950;
  int analogValue = analogRead(A0);
  float celsius = 1 / (log(1 / (1023. / analogValue - 1 )) /  BETA + 1.0 / 298.15) - 273.15;
  Serial.println(celsius);

  if (celsius >= 25) {
    digitalWrite(13, HIGH);
  } else {
    digitalWrite(13, LOW);
  }

}