Таймер с обратным отсчётом на 4-разрядном семисегментном индикаторе

#include <math.h>

const int digitPins[] = {A1, A2, A3, A4};    // катоды разрядов
const int segmentPins[] = {2, 3, 4, 5, 6, 7, 8, A0}; // a,b,c,d,e,f,g,dp
const int buzzerPin = 9;

const int btnPlus = 10;
const int btnMinus = 11;
const int btnStart = 12;
const int btnReset = 13;

unsigned long defaultTime = 30;      // стартовое время в секундах
unsigned long lastSetTime = defaultTime;
unsigned long remainingTime = 0;
unsigned long previousMillis = 0;
bool running = false;
bool buzzerActive = false;

const byte numbers[10] = {
  B11111100, // 0
  B01100000, // 1
  B11011010, // 2
  B11110010, // 3
  B01100110, // 4
  B10110110, // 5
  B10111110, // 6
  B11100000, // 7
  B11111110, // 8
  B11110110  // 9
};

void setup() {
  for (int i = 0; i < 4; i++) pinMode(digitPins[i], OUTPUT);
  for (int i = 0; i < 8; i++) pinMode(segmentPins[i], OUTPUT);
  pinMode(buzzerPin, OUTPUT);
  pinMode(btnPlus, INPUT_PULLUP);
  pinMode(btnMinus, INPUT_PULLUP);
  pinMode(btnStart, INPUT_PULLUP);
  pinMode(btnReset, INPUT_PULLUP);

  remainingTime = lastSetTime;
  displayTime(remainingTime);
}

void loop() {
  unsigned long currentMillis = millis();

  // обработка кнопок
  if (!running) {
    if (digitalRead(btnPlus) == LOW) { adjustTime(1); delay(10); }
    if (digitalRead(btnMinus) == LOW) { adjustTime(-1); delay(10); }
  }
  if (digitalRead(btnStart) == LOW) { running = !running; delay(200); }
  if (digitalRead(btnReset) == LOW) {
    remainingTime = lastSetTime;
    running = false;
    buzzerActive = false;
    noTone(buzzerPin);
    delay(200);
  }

  // обратный отсчёт
  if (running && currentMillis - previousMillis >= 1000) {
    previousMillis = currentMillis;
    if (remainingTime > 0) remainingTime--;
    else {
      running = false;
      buzzerActive = true;
      tone(buzzerPin, 1000);
    }
    displayTime(remainingTime);
  }

  // звуковой сигнал по окончании
  if (buzzerActive && currentMillis % 1000 < 500) tone(buzzerPin, 1000);
  else if (buzzerActive) noTone(buzzerPin);

  displayTime(remainingTime);
}

void adjustTime(int dir) {
  static unsigned long lastAdjust = 0;
  unsigned long now = millis();
  int speed = (now - lastAdjust < 500) ? 10 : 1;
  remainingTime += dir * speed;
  if (remainingTime > 3599) remainingTime = 0;
  lastSetTime = remainingTime;
  displayTime(remainingTime);
  lastAdjust = now;
}

void displayTime(unsigned long seconds) {
  int mins = seconds / 60;
  int secs = seconds % 60;
  int digits[4] = {mins / 10, mins % 10, secs / 10, secs % 10};

  for (int d = 0; d < 4; d++) {
    digitalWrite(digitPins[d], HIGH);
    for (int s = 0; s < 8; s++) {
      digitalWrite(segmentPins[s], bitRead(numbers[digits[d]], 7 - s));
    }
    delay(2);
    digitalWrite(digitPins[d], LOW);
  }
}