countdown.ino

// A B C D E F G DP
// 6 5 2 3 4 7 8 9
int pins[] = {6, 5, 2, 3, 4, 7, 8, 9};
char segments[] = {'A', 'B', 'C', 'D', 'E', 'F', 'G', 'D','P'};

const int NUM_PINS = 8;

void setup() {
  for (int i = 0; i < NUM_PINS; i++) {
    pinMode(pins[i], OUTPUT);
  }
}

const char* renderDigit(int digit) {
  switch (digit) {
    case 0: return "ABCDEF";
    case 1: return "BC";
    case 2: return "ABGED";
    case 3: return "ABGCD";
    case 4: return "FGBC";
    case 5: return "AFGCD";
    case 6: return "AFEDCG";
    case 7: return "ABC";
    case 8: return "ABCDEFG";
    case 9: return "GFABCD";
    default: return "";
  }
}

// Given a segment letter, return its pin number
int segmentToPin(char seg) {
  // A=6, B=5, C=2, D=3, E=4, F=7, G=8, DP=9
  switch (seg) {
    case 'A': return 6;
    case 'B': return 5;
    case 'C': return 2;
    case 'D': return 3;
    case 'E': return 4;
    case 'F': return 7;
    case 'G': return 8;
    default:  return -1;
  }
}

void clearDisplay() {
  for (int i = 0; i < NUM_PINS; i++) {
    digitalWrite(pins[i], LOW);
  }
}

void loop() {
  // Countdown from 9 to 0
  for (int digit = 9; digit >= 0; digit--) {
    clearDisplay();

    const char* schema = renderDigit(digit);

    // Turn on only the segments for this digit
    for (int j = 0; schema[j] != '\0'; j++) {
      int pin = segmentToPin(schema[j]);
      if (pin >= 0) {
        digitalWrite(pin, HIGH);
      }
    }

    delay(1000);
  }
}