Photocell bar graph on a 74HC595, on an Arduino Uno

// Photocell light meter: the brighter it is, the more LEDs light.
//
//   74HC595 DS (pin 14)    -> Uno pin 12   (data)
//   74HC595 SHCP (pin 11)  -> Uno pin 9    (clock)
//   74HC595 STCP (pin 12)  -> Uno pin 11   (latch)
//   74HC595 VCC and MR -> 5V      GND and OE -> GND
//   Q0 to Q7 -> eight LEDs, each through 220 ohm to GND
//   5V -> photocell -> A0 -> 1k resistor -> GND
//
// A photocell's resistance falls as light lands on it. With it on top of a
// 1k resistor, A0 reads more in bright light. The sketch maps the reading,
// 0 to 1023, onto a count of LEDs, 0 to 8, and lights that many.
//
// Click the photocell and drag its light level to see the bar move.

const int LIGHT_PIN = A0;
const int DATA_PIN = 12;
const int CLOCK_PIN = 9;
const int LATCH_PIN = 11;

int shown = -1;

void show(byte value) {
  digitalWrite(LATCH_PIN, LOW);
  shiftOut(DATA_PIN, CLOCK_PIN, MSBFIRST, value);  // bit 0 lands on Q0
  digitalWrite(LATCH_PIN, HIGH);
}

void setup() {
  pinMode(DATA_PIN, OUTPUT);
  pinMode(CLOCK_PIN, OUTPUT);
  pinMode(LATCH_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: photocell on A0, eight LEDs on a 74HC595");
}

void loop() {
  int reading = analogRead(LIGHT_PIN);
  int count = map(reading, 0, 1023, 0, 9);
  if (count > 8) count = 8;
  byte bar = 0;
  for (int i = 0; i < count; i++) bitSet(bar, i);
  show(bar);
  if (count != shown) {
    shown = count;
    Serial.print("light ");
    Serial.print(reading);
    Serial.print(": ");
    Serial.print(count);
    Serial.println(" LEDs");
  }
  delay(100);
}