Photoresistor bar graph on a 74HC595 (pins 9, 12 and 11), on an Arduino Uno

// Photoresistor light meter: the brighter it is, the more LEDs light.
//
//   74HC595 DS (pin 14)    -> Uno pin 9    (data)
//   74HC595 STCP (pin 12)  -> Uno pin 11   (latch)
//   74HC595 SHCP (pin 11)  -> Uno pin 12   (clock)
//   74HC595 VCC and MR -> 5V      GND and OE -> GND
//   Q0 to Q7 -> eight LEDs, each through 220 ohm to GND
//   5V -> photoresistor -> A0 -> 1k resistor -> GND
//
// A photoresistor's resistance falls as light lands on it. Sitting on top of
// a 1k resistor, it makes A0 read more in bright light. When the cell is down
// to 1k, A0 reads half of 1023, and that is where all eight LEDs light: the
// reading is cut into nine bands of 57, from none lit to all eight.
//
// Click the photoresistor and drag its light level to see the bar move.

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

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(LATCH_PIN, OUTPUT);
  pinMode(CLOCK_PIN, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: photoresistor on A0, 74HC595 on pins 9, 11 and 12");
}

void loop() {
  int reading = analogRead(LIGHT_PIN);
  int lit = reading / 57;
  if (lit > 8) lit = 8;
  byte leds = 0;
  for (int i = 0; i < lit; i++) bitSet(leds, i);
  show(leds);
  if (lit != shown) {
    shown = lit;
    Serial.print("reading ");
    Serial.print(reading);
    Serial.print(", LEDs lit ");
    Serial.println(lit);
  }
  delay(50);
}