Photocell light meter on an Arduino Uno

// Light meter: a photocell on A0 and a bar of eight LEDs on a 74HC595.
//
//   photocell      5V -> A0
//   10k resistor   A0 -> GND
//   74HC595        DS -> pin 4, SHCP (clock) -> pin 5, STCP (latch) -> pin 6
//                  Q0 to Q7 -> the eight LEDs, each through 220 ohm to GND
//
// A photocell's resistance falls as light falls on it. With the 10k under it,
// more light pulls A0 up toward 5 V, so the reading rises with the light.
// The sketch turns the reading into a number of LEDs, from none in the dark
// to all eight in bright light, and shifts that bar out to the 74HC595.

const int DATA = 4;
const int CLOCK = 5;
const int LATCH = 6;

int shownBars = -1;

void showBar(int bars) {
  // bars ones from the bottom: 0 -> 00000000, 3 -> 00000111, 8 -> 11111111
  byte pattern = (bars >= 8) ? 0xFF : (byte)((1 << bars) - 1);
  digitalWrite(LATCH, LOW);
  shiftOut(DATA, CLOCK, MSBFIRST, pattern);
  digitalWrite(LATCH, HIGH);
}

void setup() {
  pinMode(DATA, OUTPUT);
  pinMode(CLOCK, OUTPUT);
  pinMode(LATCH, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: photocell light meter");
}

void loop() {
  int reading = analogRead(A0);
  int bars = reading / 114;  // 0..1023 into 0..8
  if (bars > 8) bars = 8;
  if (bars != shownBars) {
    shownBars = bars;
    showBar(bars);
    Serial.print("reading ");
    Serial.print(reading);
    Serial.print(", LEDs lit ");
    Serial.println(bars);
  }
  delay(50);
}