Level light on an Arduino Uno

// Level light: a potentiometer sets a level, and a two-color LED shows it,
// red when it is low, through orange and yellow, to green when it is full.
// The same light a battery charger or a fuel gauge uses.
//
//   two-color LED   R -> pin 9 through 220 ohm   G -> pin 10 through 220 ohm
//                   C (the middle leg) -> GND
//   potentiometer   ends to 5 V and GND, wiper -> A0
//
// A two-color LED is two LEDs, a red one and a yellow-green one, under one
// lens with a shared leg. Each needs its own resistor, as two LEDs would, and
// the KY-011 module has none on it. Light both and the lens glows somewhere
// between the two: the eye adds red and green light into orange and yellow.
//
// So the sketch walks the mix: below halfway the red die is full and the
// green one comes up with the knob; above halfway the green die is full and
// the red one goes down. Turn the knob and watch the lens.

const int RED_PIN = 9;     // PWM
const int GREEN_PIN = 10;  // PWM

int shown = -1;

void setup() {
  Serial.begin(9600);
  pinMode(RED_PIN, OUTPUT);
  pinMode(GREEN_PIN, OUTPUT);
  Serial.println("Level light: turn the knob");
}

void loop() {
  int level = analogRead(A0);  // 0 to 1023
  int red = level < 512 ? 255 : (1023 - level) / 2;
  int green = level < 512 ? level / 2 : 255;
  analogWrite(RED_PIN, red);
  analogWrite(GREEN_PIN, green);

  // A line when the level moves by a whole percent.
  int percent = (long)level * 100 / 1023;
  if (percent != shown) {
    shown = percent;
    Serial.print("level ");
    Serial.print(percent);
    Serial.print("%: red ");
    Serial.print(red);
    Serial.print(", green ");
    Serial.println(green);
  }
  delay(20);
}