Digital Alarm Clock

Published: January 16, 2026

This project is a digital alarm clock built with an Arduino UNO, a 16x2 LCD display, and a Real-Time Clock (RTC) module. It can keep accurate time even when the Arduino is turned off, lets you set the time and alarm using push buttons, and sounds a buzzer when the alarm goes off. You can switch between 12-hour and 24-hour display modes and stop the alarm with a button press.

The full breakdown of how to assemble this project in detail can be found in my beginners Arduino guide here →

📊 Preview

Digital Alarm Clock Preview

⚙️ How It Works:

📦 Components Used:

ComponentPurpose
Arduino UNOMain microcontroller
LCD1602 (16-pin)Displays the time + alarm
DS1307 RTC ModuleKeeps accurate time
Push Buttons (x3)Set/stop/toggle alarm
Passive or Active BuzzerSounds the alarm
Potentiometer (10kΩ)Controls LCD contrast
Resistors (10kΩ + 220Ω)Pull-downs + backlight
Breadboard & wiresAll connections

🔌 Wiring Instructions & Diagram

Wiring Diagram

📜 Arduino Code

Language: C++

// Arduino LCD Alarm Clock (1602) — Final Sketch with 12/24h toggle, AM/PM display,
// 12h-aware time/alarm setting, passive-buzzer alarm pattern, and improved alarm row.
//
// Hardware
// - LCD1602 (parallel): RS->7, E->8, D4..D7->9..12, RW->GND, V0 via 10k pot, LED+ (15) via 220Ω to 5V, LED- (16) to GND
// - Buttons: SET_TIME->D2, SET_ALARM->D3, STOP->D4 (wired with external 10k pulldown to GND, button to +5V)
// - Buzzer (passive): D5 -> + pin, other pin to GND
// - RTC: DS3231/DS1307 via I2C (SDA/SCL)
//
// Usage
// - NORMAL screen: double-press STOP to toggle 12HR/24HR (top-right shows mode).
// - SET TIME: press SET_TIME to enter. In hour screen: 
//     * Press SET_TIME to increment.
//     * In 12h mode: short-press STOP toggles AM/PM.
//     * Press SET_ALARM (or long-press SET_TIME/SET_ALARM) to move to minutes.
//   Minutes screen: press SET_TIME to increment; press SET_ALARM (or long-press) to save.
// - SET ALARM: press SET_ALARM to enter; same controls as TIME. Saving enables the alarm.
// - When alarm rings: press STOP to silence.

#include <Wire.h>
#include <RTClib.h>
#include <LiquidCrystal.h>

// ---------- LCD pins (RS, EN, D4, D5, D6, D7)
LiquidCrystal lcd(7, 8, 9, 10, 11, 12);

// ---------- RTC
RTC_DS3231 rtc; // Works with DS1307 too via RTClib

// ---------- Pins
const int BTN_SET_TIME  = 2;
const int BTN_SET_ALARM = 3;
const int BTN_STOP      = 4;
const int BUZZER_PIN    = 5; // passive buzzer

// ---------- Alarm state
int  alarmHour   = 7;   // 0..23 (stored internally in 24h)
int  alarmMinute = 0;
bool alarmEnabled = false;
bool alarmRinging = false;

// ---------- Display mode
bool is12HourMode = false; // toggled by STOP double-press in NORMAL

// ---------- Button handling (debounce + edge + long-press)
struct BtnState {
  int pin;
  bool lastLevel;
  bool pressedEdge;
  bool longPressEdge;
  unsigned long pressedAt;
  BtnState(int p): pin(p), lastLevel(false), pressedEdge(false), longPressEdge(false), pressedAt(0) {}
};

BtnState bTime(BTN_SET_TIME);
BtnState bAlarm(BTN_SET_ALARM);
BtnState bStop(BTN_STOP);

const unsigned long DEBOUNCE_MS = 30;
const unsigned long LONG_MS     = 800;

bool readBtn(BtnState &b) {
  bool level = digitalRead(b.pin); // active HIGH (external pulldown)
  bool rising  =  level && !b.lastLevel;
  bool falling = !level &&  b.lastLevel;

  b.pressedEdge   = false;
  b.longPressEdge = false;

  if (rising)  b.pressedAt = millis();
  if (falling) {
    unsigned long held = millis() - b.pressedAt;
    if (held >= LONG_MS)          b.longPressEdge = true;
    else if (held >= DEBOUNCE_MS) b.pressedEdge   = true;
  }
  b.lastLevel = level;
  return level;
}

// ---------- Modes
enum Mode { NORMAL, SET_TIME_H, SET_TIME_M, SET_ALARM_H, SET_ALARM_M, ALARM_RING };
Mode mode = NORMAL;

// ---------- Edit buffers
int  newHour = 0, newMinute = 0;            // TIME editing
bool editPM = false;                        // for 12h time edit
int  newAlarmHour = 0, newAlarmMinute = 0;  // ALARM editing
bool editAlarmPM = false;                   // for 12h alarm edit

// ---------- STOP double-press detection
unsigned long lastStopPressAt = 0;
const unsigned long DOUBLE_MS = 450; // window for double-press

// ---------- Passive buzzer alarm pattern (non-blocking)
unsigned long lastToneMs = 0;
bool toneFlip = false; // alternate between two tones

void startAlarmTone() {
  lastToneMs = millis();
  toneFlip = false;
  tone(BUZZER_PIN, 1200); // start high
}

void updateAlarmTone() {
  // Alternate tones every 250 ms for an alarm-y sound
  unsigned long now = millis();
  if (now - lastToneMs >= 250) {
    toneFlip = !toneFlip;
    tone(BUZZER_PIN, toneFlip ? 800 : 1200);
    lastToneMs = now;
  }
}

void stopAlarmTone() {
  noTone(BUZZER_PIN);
}

// ---------- UI helpers
void clearRow(int row) {
  lcd.setCursor(0, row);
  for (int i = 0; i < 16; i++) lcd.print(' ');
}

void printTimeRow(const DateTime& now) {
  // Row 0: time and mode tag on the far right (cols 12..15)
  lcd.setCursor(0, 0);
  // Clear row to avoid artifacts when switching modes
  for (int i = 0; i < 16; i++) lcd.print(' ');

  int h = now.hour();
  int m = now.minute();

  if (is12HourMode) {
    int h12 = h % 12; if (h12 == 0) h12 = 12;
    const char* ampm = (h < 12) ? "AM" : "PM";
    // HH:MM AM
    lcd.setCursor(0, 0);
    char buf[8];
    snprintf(buf, sizeof(buf), "%02d:%02d", h12, m);
    lcd.print(buf);
    lcd.setCursor(6, 0);
    lcd.print(ampm);
  } else {
    // HH:MM (24h)
    lcd.setCursor(0, 0);
    char buf[8];
    snprintf(buf, sizeof(buf), "%02d:%02d", h, m);
    lcd.print(buf);
  }
  // Mode tag at far right
  lcd.setCursor(12, 0);
  lcd.print(is12HourMode ? "12HR" : "24HR");
}

void printAlarmRow() {
  // Row 1 shows the alarm time in either 24h or 12h with AM/PM,
  // and places the ON/OFF status at the far right (cols 12..15).
  clearRow(1);
  lcd.setCursor(0, 1);

  if (is12HourMode) {
    int h24 = alarmHour;
    int h12 = h24 % 12; if (h12 == 0) h12 = 12;
    const char* ampm = (h24 < 12) ? "AM" : "PM";

    // Print: "AL %02d:%02d" at cols 0..7
    char tbuf[9];
    snprintf(tbuf, sizeof(tbuf), "AL %02d:%02d", h12, alarmMinute);
    lcd.print(tbuf);

    // AM/PM at cols 9..10
    lcd.setCursor(9, 1);
    lcd.print(ampm);

    // Status at cols 12..15
    lcd.setCursor(12, 1);
    if (alarmEnabled) lcd.print(" ON "); else lcd.print("OFF ");
  } else {
    // 24h: no AM/PM text
    char tbuf[9];
    snprintf(tbuf, sizeof(tbuf), "AL %02d:%02d", alarmHour, alarmMinute);
    lcd.print(tbuf);

    // Status at cols 12..15
    lcd.setCursor(12, 1);
    if (alarmEnabled) lcd.print(" ON "); else lcd.print("OFF ");
  }
}

void showSetPrompt(const char* title, int val, bool showAMPM, bool pmFlag) {
  // Row 0: title
  clearRow(0);
  lcd.setCursor(0, 0);
  lcd.print(title);
  // Row 1: value (+ optional AM/PM)
  clearRow(1);
  lcd.setCursor(0, 1);
  char buf[5];
  snprintf(buf, sizeof(buf), "%02d", val);
  lcd.print(buf);
  if (showAMPM) {
    lcd.print(' ');
    lcd.print(pmFlag ? "PM" : "AM");
  }
  lcd.setCursor(10, 1);
  lcd.print("(next)");
}

void stopAlarm() {
  stopAlarmTone();
  alarmRinging = false;
  mode = NORMAL;
}

void ringAlarm() {
  alarmRinging = true;
  mode = ALARM_RING;
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("  *** ALARM ***");
  lcd.setCursor(0, 1);
  lcd.print("Press STOP Btn");
  startAlarmTone();
}

// ---------- Setup
void setup() {
  pinMode(BTN_SET_TIME,  INPUT);
  pinMode(BTN_SET_ALARM, INPUT);
  pinMode(BTN_STOP,      INPUT);
  pinMode(BUZZER_PIN,    OUTPUT);

  lcd.begin(16, 2);
  lcd.print("Digital Clock...");
  delay(1200);
  lcd.clear();

  Wire.begin();
  if (!rtc.begin()) {
    lcd.print("RTC not found!");
    while (1);
  }
  if (rtc.lostPower()) {
    rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));
  }
}

// ---------- Loop
void loop() {
  // Poll buttons
  readBtn(bTime);
  readBtn(bAlarm);
  readBtn(bStop);

  // Handle alarm ringing (non-blocking tone pattern)
  if (mode == ALARM_RING) {
    updateAlarmTone();
    if (bStop.pressedEdge || bStop.longPressEdge) {
      stopAlarm();
      lcd.clear();
    }
    return; // stay here until stopped
  }

  // NORMAL clock display & alarm check
  if (mode == NORMAL) {
    DateTime now = rtc.now();

    // Display
    printTimeRow(now);
    printAlarmRow();

    // Alarm trigger (once at second==0)
    if (alarmEnabled && !alarmRinging &&
        now.hour() == alarmHour && now.minute() == alarmMinute && now.second() == 0) {
      ringAlarm();
      return;
    }

    // Enter set modes
    if (bTime.pressedEdge || bTime.longPressEdge) {
      // Init edit buffers from current time
      DateTime n = rtc.now();
      if (is12HourMode) {
        editPM = (n.hour() >= 12);
        int h12 = n.hour() % 12; if (h12 == 0) h12 = 12;
        newHour = h12;
      } else {
        newHour = n.hour();
      }
      newMinute = n.minute();
      mode = SET_TIME_H;
      lcd.clear();
    } else if (bAlarm.pressedEdge || bAlarm.longPressEdge) {
      // Init edit buffers from current alarm
      if (is12HourMode) {
        editAlarmPM = (alarmHour >= 12);
        int h12 = alarmHour % 12; if (h12 == 0) h12 = 12;
        newAlarmHour = h12;
      } else {
        newAlarmHour = alarmHour;
      }
      newAlarmMinute = alarmMinute;
      mode = SET_ALARM_H;
      lcd.clear();
    }

    // STOP double-press to toggle 12/24h mode
    if (bStop.pressedEdge) {
      unsigned long nowMs = millis();
      if (nowMs - lastStopPressAt <= DOUBLE_MS) {
        is12HourMode = !is12HourMode;
        lastStopPressAt = 0; // reset window
      } else {
        lastStopPressAt = nowMs;
      }
    }

    delay(120);
    return;
  }

  // -------- TIME SET MODE
  if (mode == SET_TIME_H) {
    // Show Hour prompt
    showSetPrompt("Set Time: Hour", newHour, is12HourMode, editPM);

    // Increment hour on TIME press
    if (bTime.pressedEdge) {
      if (is12HourMode) {
        newHour++; if (newHour > 12) newHour = 1;
      } else {
        newHour = (newHour + 1) % 24;
      }
      showSetPrompt("Set Time: Hour", newHour, is12HourMode, editPM);
    }

    // In 12h mode, STOP short toggles AM/PM
    if (is12HourMode && bStop.pressedEdge) {
      editPM = !editPM;
      showSetPrompt("Set Time: Hour", newHour, true, editPM);
    }

    // Next field
    if (bAlarm.pressedEdge || bTime.longPressEdge || bAlarm.longPressEdge) {
      mode = SET_TIME_M;
      lcd.clear();
    }

    // Cancel
    if (bStop.longPressEdge) {
      mode = NORMAL;
      lcd.clear();
    }
    return;
  }

  if (mode == SET_TIME_M) {
    showSetPrompt("Set Time: Min", newMinute, false, false);

    if (bTime.pressedEdge) {
      newMinute = (newMinute + 1) % 60;
      showSetPrompt("Set Time: Min", newMinute, false, false);
    }

    // Save
    if (bAlarm.pressedEdge || bTime.longPressEdge || bAlarm.longPressEdge) {
      // Build 24h value from edit fields
      int saveH = newHour;
      if (is12HourMode) {
        saveH = newHour % 12;
        if (editPM && saveH != 12) saveH += 12; // 1..11 PM => 13..23
        if (!editPM && newHour == 12) saveH = 0; // 12 AM => 0
      }
      DateTime cur = rtc.now();
      rtc.adjust(DateTime(cur.year(), cur.month(), cur.day(), saveH, newMinute, 0));
      mode = NORMAL;
      lcd.clear();
    }

    // Cancel
    if (bStop.longPressEdge) {
      mode = NORMAL;
      lcd.clear();
    }
    return;
  }

  // -------- ALARM SET MODE
  if (mode == SET_ALARM_H) {
    showSetPrompt("Set Alarm: Hour", newAlarmHour, is12HourMode, editAlarmPM);

    if (bTime.pressedEdge) {
      if (is12HourMode) {
        newAlarmHour++; if (newAlarmHour > 12) newAlarmHour = 1;
      } else {
        newAlarmHour = (newAlarmHour + 1) % 24;
      }
      showSetPrompt("Set Alarm: Hour", newAlarmHour, is12HourMode, editAlarmPM);
    }

    if (is12HourMode && bStop.pressedEdge) {
      editAlarmPM = !editAlarmPM;
      showSetPrompt("Set Alarm: Hour", newAlarmHour, true, editAlarmPM);
    }

    // Next field
    if (bAlarm.pressedEdge || bTime.longPressEdge || bAlarm.longPressEdge) {
      mode = SET_ALARM_M;
      lcd.clear();
    }

    // Cancel
    if (bStop.longPressEdge) {
      mode = NORMAL;
      lcd.clear();
    }
    return;
  }

  if (mode == SET_ALARM_M) {
    showSetPrompt("Set Alarm: Min", newAlarmMinute, false, false);

    if (bTime.pressedEdge) {
      newAlarmMinute = (newAlarmMinute + 1) % 60;
      showSetPrompt("Set Alarm: Min", newAlarmMinute, false, false);
    }

    // Save
    if (bAlarm.pressedEdge || bTime.longPressEdge || bAlarm.longPressEdge) {
      int saveH = newAlarmHour;
      if (is12HourMode) {
        saveH = newAlarmHour % 12;
        if (editAlarmPM && saveH != 12) saveH += 12;
        if (!editAlarmPM && newAlarmHour == 12) saveH = 0;
      }
      alarmHour = saveH;
      alarmMinute = newAlarmMinute;
      alarmEnabled = true;
      mode = NORMAL;
      lcd.clear();
    }

    // Cancel
    if (bStop.longPressEdge) {
      mode = NORMAL;
      lcd.clear();
    }
    return;
  }
}

📆 Installing the Required Libraries

🚀 Uploading the Code

  1. Open Arduino IDE
  2. Paste the full sketch into a new file
  3. Select "Arduino UNO" as your board
  4. Choose the correct COM port
  5. Click Upload

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