Show any number

// Show any number on chained 4" seven-segment modules with a 74HC595.
//
// Wiring, at the module's IN edge (reading the back, IN is on the left):
//   12 V supply +   -> 12V            two-pin header
//   12 V supply -   -> GND            two-pin header
//   Uno 5V          -> 3V3/5V         six-pin header
//   Uno GND         -> GND            six-pin header
//   Uno D8          -> DATAIN (SER)
//   Uno D11         -> LATCH (RCLK)
//   Uno D12         -> CLOCK (SRCLK)
//   Uno D9          -> PWM            or leave PWM empty and solder the
//                                     PWM pad shut (then PWM_PIN is -1)
//
// The board ships with the PWM pad OPEN. With nothing on PWM the digit
// has no supply and stays dark, however correct the number is.
//
// The 12 V supply's ground and the Uno's ground must be the same wire.
//
// Arduino IDE: Tools > Board > Arduino AVR Boards > Arduino Uno, then
// Tools > Port and pick the port the board appears on. No libraries.

#define NUM_DIGITS 2

#define SER  8         // DATAIN (SER)
#define LAT 11         // LATCH  (RCLK)
#define CLK 12         // CLOCK  (SRCLK)
#define PWM_PIN 9      // PWM; use -1 if you shorted the PWM pad instead

const uint8_t d[] = {
  0x3F, 0x06, 0x5B, 0x4F, 0x66, 0x6D, 0x7D, 0x07, 0x7F, 0x6F
};

const uint8_t BLANK = 0x00;   // no segments
const uint8_t MINUS = 0x40;   // segment G on its own
const uint8_t POINT = 0x80;   // the decimal point

void updateDisplay(const uint8_t *buf) {
  digitalWrite(LAT, LOW);
  for (int i = 0; i < NUM_DIGITS; i++)
    shiftOut(SER, CLK, MSBFIRST, buf[i]);
  digitalWrite(LAT, HIGH);
}

// Highest number this chain can hold: 9, 99, 999 …
unsigned long chainLimit() {
  unsigned long limit = 1;
  for (int i = 0; i < NUM_DIGITS; i++) limit *= 10;
  return limit - 1;
}

// true if it was shown, false if the number needs more modules than you
// have. Without this check the top digits vanish and nothing says so.
bool showNumber(unsigned long value, bool leadingZeros = true) {
  if (value > chainLimit()) return false;

  uint8_t b[NUM_DIGITS];
  unsigned long t = value;
  for (int i = NUM_DIGITS - 1; i >= 0; i--) {
    b[i] = d[t % 10];       // rightmost digit of what is left
    t /= 10;                // drop it
  }

  if (!leadingZeros) {      // blank the noughts in front of the number
    for (int i = 0; i < NUM_DIGITS - 1 && b[i] == d[0]; i++) b[i] = BLANK;
  }

  updateDisplay(b);
  return true;
}

void blank() {
  uint8_t b[NUM_DIGITS];
  for (int i = 0; i < NUM_DIGITS; i++) b[i] = BLANK;
  updateDisplay(b);
}

void setup() {
  pinMode(SER, OUTPUT);
  pinMode(LAT, OUTPUT);
  pinMode(CLK, OUTPUT);
  if (PWM_PIN >= 0) {
    pinMode(PWM_PIN, OUTPUT);
    digitalWrite(PWM_PIN, HIGH);   // high = the digit is supplied
  }
  blank();

  if (!showNumber(42, false)) blank();   // 42 on a two-module chain
}

void loop() {
  // nothing: the display holds what it was last given
}