LM358 dark switch with an Arduino on an Arduino Uno

// LM358 dark switch: the op-amp decides, the Arduino listens.
//
//   10 k from 5 V and an LDR to GND make a divider:  middle -> LM358 1IN+ and A0
//   a 10 k pot across 5 V and GND:                    wiper  -> LM358 1IN- and A1
//   LM358 1OUT -> pin 2, and through 470 ohm to an LED
//
// Amplifier 1 of the LM358 has no feedback, so it is a comparator: when the
// divider (it rises as the room gets darker) is above the pot's wiper, the
// output goes high and lights the LED; below it, the output drops to ground.
// The Arduino does no comparing at all. It reads the comparator's answer on
// pin 2, and reads both voltages on A0 and A1 so you can see why.
//
// An LM358's output stops 1.5 V under its supply, and the LED's current
// drops a little more inside the chip, so "high" here is 3.15 V rather than
// 5 V. An ATmega328P on 5 V is only sure of a HIGH above 3.0 V, so pin 2 reads
// it, with little to spare: that is why the LED has 470 ohm and not 220,
// which would pull the output down to 2.9 V.

const int COMPARATOR_PIN = 2;
const int SENSOR_PIN = A0;
const int THRESHOLD_PIN = A1;

bool dark = false;
bool first = true;
unsigned long printedAt = 0;

long millivolts(int pin) {
  return (long)analogRead(pin) * 5000L / 1023L;
}

void setup() {
  Serial.begin(9600);
  pinMode(COMPARATOR_PIN, INPUT);
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.println("LM358 dark switch");
}

void loop() {
  bool nowDark = digitalRead(COMPARATOR_PIN) == HIGH;
  if (first || nowDark != dark) {
    first = false;
    dark = nowDark;
    digitalWrite(LED_BUILTIN, dark ? HIGH : LOW);
    Serial.println(dark ? "dark: comparator HIGH, lamp on" : "light: comparator LOW, lamp off");
  }
  if (millis() - printedAt >= 1000) {
    printedAt = millis();
    Serial.print("sensor ");
    Serial.print(millivolts(SENSOR_PIN));
    Serial.print(" mV, threshold ");
    Serial.print(millivolts(THRESHOLD_PIN));
    Serial.println(" mV");
  }
}