The Rotary Dial: A Number Sent as a Count of Breaks

/*
  dial_decoder.ino -- decode a rotary dial's pulses into digits.

  A rotary dial's PULSE contacts open once per step as the dial returns. Wire them (or, to
  try it without a dial, a push button pressed quickly N times) between pin 2 and GND.
  The input uses the internal pull-up, so:  contacts closed = LOW, contacts open = HIGH.

  A digit is complete when no new break arrives for GAP_MS. Ten breaks means 0.
*/

const int PIN_PULSE = 2;
const unsigned long DEBOUNCE_MS = 10;    // contacts bounce; ignore changes shorter than this
const unsigned long GAP_MS      = 300;   // no break for this long = end of the digit
const unsigned long NUMBER_MS   = 3000;  // no digit for this long = end of the number

int lastStable = LOW;
int lastRead = LOW;
unsigned long lastChange = 0;
unsigned long lastBreak = 0;
int count = 0;
bool numberOpen = false;

void setup() {
  pinMode(PIN_PULSE, INPUT_PULLUP);
  Serial.begin(9600);
  Serial.println(F("Dial a number."));
}

void loop() {
  int r = digitalRead(PIN_PULSE);
  unsigned long now = millis();
  if (r != lastRead) { lastRead = r; lastChange = now; }

  // a change that has held for DEBOUNCE_MS is real
  if (now - lastChange >= DEBOUNCE_MS && r != lastStable) {
    lastStable = r;
    if (r == HIGH) {             // contacts just opened: one break
      count++;
      lastBreak = now;
    }
  }

  if (count > 0 && lastStable == LOW && now - lastBreak >= GAP_MS) {
    int digit = (count == 10) ? 0 : count;
    if (count > 10) {
      Serial.print(F("?"));      // more than ten breaks: a fault, not a digit
    } else {
      Serial.print(digit);
    }
    count = 0;
    numberOpen = true;
  }

  if (numberOpen && count == 0 && now - lastBreak >= NUMBER_MS) {
    Serial.println();
    numberOpen = false;
  }
}