Falling rain on eight LEDs (74HC595, pins 11, 12 and 13), on an Arduino Uno

// Falling rain: a drop of light with a fading tail runs along eight LEDs.
//
//   74HC595 DS -> pin 11   ST_CP -> pin 12   SH_CP -> pin 13
//   VCC and MR -> 5V   OE and GND -> GND   Q0..Q7 -> 220 ohm -> LEDs -> GND
//
// The 74HC595 has no PWM, so the sketch makes its own: each pass of 64 steps
// switches an LED on for as many steps as its brightness in TAIL, and the
// eye averages it. Sliding the window over TAIL moves the drop along.

const int LATCH = 12;  // ST_CP
const int CLOCK = 13;  // SH_CP
const int DATA = 11;   // DS

void write595(byte value, int order) {
  digitalWrite(LATCH, LOW);
  shiftOut(DATA, CLOCK, order, value);
  digitalWrite(LATCH, HIGH);
}

const byte TAIL[24] = {0, 0, 0, 0, 0, 0, 0, 0, 64, 48, 32, 16, 8, 4, 2, 1, 0, 0, 0, 0, 0, 0, 0, 0};

void setup() {
  pinMode(LATCH, OUTPUT);
  pinMode(CLOCK, OUTPUT);
  pinMode(DATA, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: falling rain on a 74HC595 (11, 12 and 13)");
}

void loop() {
  static byte offset = 0;
  static unsigned int frames = 0;
  if (frames++ % 8 == 0) offset = offset < 15 ? offset + 1 : 0;
  for (int step = 0; step < 64; step++) {
    byte bits = 0;
    for (int j = 0; j < 8; j++) {
      if (step < TAIL[j + offset]) bits |= 1 << j;
    }
    write595(bits, MSBFIRST);
  }
}