Temperature alarm: thermistor on A0, RGB on 10, 11 and 12, on an Arduino Uno

// A temperature alarm: a thermistor on A0 and an RGB LED that changes color.
//
//   10k from 5V -> A0; the thermistor from A0 -> GND
//   RGB red -> 220 ohm -> pin 10   green -> 220 ohm -> pin 11
//   RGB blue -> 220 ohm -> pin 12   the long common leg -> GND
//
// A thermistor's resistance falls as it warms, so the bottom of the divider
// shrinks and A0 reads lower. The beta equation turns the reading back into
// degrees. Below 18 C the light is blue, above 28 C it is red, and between
// the two it is green.
//
// Drag the temperature on the thermistor.

const int SENSOR = A0;
const int RED = 10;
const int GREEN = 11;
const int BLUE = 12;
const float SERIES_OHMS = 10000.0;

// The thermistor's temperature from the divider's reading, by the beta
// equation: 1/T = 1/T0 + ln(R/R0)/B, with T in kelvin.
float celsius(int reading) {
  float r = SERIES_OHMS * reading / (1023.0 - reading);
  float kelvin = 1.0 / (1.0 / (25.0 + 273.15) + log(r / 10000.0) / 3950.0);
  return kelvin - 273.15;
}

void color(int r, int g, int b) {
  digitalWrite(RED, r);
  digitalWrite(GREEN, g);
  digitalWrite(BLUE, b);
}

void setup() {
  pinMode(RED, OUTPUT);
  pinMode(GREEN, OUTPUT);
  pinMode(BLUE, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: temperature alarm, thermistor on A0");
}

void loop() {
  float c = celsius(analogRead(SENSOR));
  Serial.print(c);
  if (c < 18) {
    color(LOW, LOW, HIGH);
    Serial.println(" C: cold");
  } else if (c > 28) {
    color(HIGH, LOW, LOW);
    Serial.println(" C: too hot!");
  } else {
    color(LOW, HIGH, LOW);
    Serial.println(" C: comfortable");
  }
  delay(500);
}