Night light with a light sensor on an Arduino Uno

// Night lamp: a real light sensor fades a lamp up when the room goes dark.
//
//   A0     the middle of the divider: 5 V, the LDR, A0, 10k, GND
//   pin 9  the lamp, through 220 ohm to GND
//   pin 8  the indicator, through 220 ohm to GND
//
// The light-dependent resistor and the fixed 10k make a divider, and A0 reads
// the middle of it. Light on the cell drops its resistance, so the middle goes
// up: a bright room reads high and a covered cell reads low. That is the whole
// sensor, and it is why an LDR always comes with a resistor.
//
// # The threshold, and why there are two of them
//
// One threshold gives a lamp that flutters on and off while the light sits on
// it. Two (switch on below DARK, switch off above LIGHT, and do nothing in
// between) is hysteresis, and it is what every thermostat, every light
// sensor and every comparator circuit with a bit of feedback does. The gap
// here is about a fifth of the range, which is wide enough that a hand moving
// past the cell does not set it off.
//
// # The fade
//
// Pin 9 is one of the PWM outputs, so analogWrite gives any brightness from 0
// to 255. It walks there rather than jumping: a night light that snaps on is a
// torch. Eight counts every twenty milliseconds is about six tenths of a second
// end to end, and the indicator on pin 8 follows the sensor instantly, so the
// two disagree for exactly as long as the ramp takes.

const int SENSOR = A0;
const int LAMP = 9;
const int INDICATOR = 8;

// Out of 1023. Below DARK it is night; above LIGHT it is day.
const int DARK = 380;
const int LIGHT = 580;

const int STEP = 8;
const int FULL = 255;

int level = 0;
bool night = false;
int said = -1;

void setup() {
  pinMode(LAMP, OUTPUT);
  pinMode(INDICATOR, OUTPUT);
  analogWrite(LAMP, 0);
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: night lamp");
  Serial.println("drag the slider on the LDR to change the light in the room");
}

void loop() {
  int light = analogRead(SENSOR);

  // Two thresholds with a gap between them, so the lamp does not flutter.
  if (!night && light < DARK) {
    night = true;
  } else if (night && light > LIGHT) {
    night = false;
  }
  digitalWrite(INDICATOR, night ? HIGH : LOW);

  int want = night ? FULL : 0;
  if (level < want) {
    level += STEP;
    if (level > want) {
      level = want;
    }
  } else if (level > want) {
    level -= STEP;
    if (level < want) {
      level = want;
    }
  }
  analogWrite(LAMP, level);

  if (level == want && level != said) {
    said = level;
    Serial.print(night ? "dark (" : "light (");
    Serial.print(light);
    Serial.print("): lamp at ");
    Serial.println(level);
  }

  delay(20);
}