Digital Clock in Arduino Serial Monitor

Introduction

In this project, we will design a digital clock using Arduino that works in the Serial Monitor.
Unlike simple timer programs, this clock will start only after the user enters the present time manually in AM/PM format.

This project is very useful for beginners because it helps to understand:

Serial communication
Time calculation using millis()
String input handling
AM/PM logic
Digital clock formatting

The user must enter the time in this format:


HH:MM:SS AM
or
HH:MM:SS PM

Example:


10:30:00 PM

After entering the time, the clock starts running like a real digital clock.

Components Required

Arduino UNO (or any Arduino board)

USB cable
Arduino IDE
Serial Monitor

(No other external hardware is required.)

Working Principle

When the program starts, Arduino waits for the user to enter the current time.
The clock does not run until valid time input is received.
After setting the time, Arduino counts seconds using the millis() function.
Every second, it updates:
- Seconds
- Minutes
- Hours
- AM/PM
The updated time is displayed in the Serial Monitor continuously.

Time Input Format

The user must enter time in the following format:


HH:MM:SS AM
HH:MM:SS PM

Examples:


08:45:00 AM
11:20:30 PM

Arduino Program Code


int hours = 0;
int minutes = 0;
int seconds = 0;
bool isPM = false;

bool timeSet = false;   // clock will start only after time is entered

unsigned long previousMillis = 0;

void setup() {
  Serial.begin(9600);
  Serial.println("Enter time in format: HH:MM:SS AM or PM");
  Serial.println("Example: 10:25:30 PM");
}

void loop() {

  // Wait for user to enter time
  if (!timeSet && Serial.available()) {
    String input = Serial.readStringUntil('\n');
    input.trim();

    int h, m, s;
    char ampm[3];

    if (sscanf(input.c_str(), "%d:%d:%d %s", &h, &m, &s, ampm) == 4) {
      hours = h;
      minutes = m;
      seconds = s;

      if (ampm[0] == 'P' || ampm[0] == 'p')
        isPM = true;
      else
        isPM = false;

      timeSet = true;   // start clock
      previousMillis = millis();

      Serial.println("Time Set! Clock Started...");
    } else {
      Serial.println("Wrong format! Use: HH:MM:SS AM/PM");
    }
  }

  // Run clock ONLY after time is set
  if (timeSet) {
    unsigned long currentMillis = millis();

    if (currentMillis - previousMillis >= 1000) {
      previousMillis = currentMillis;

      seconds++;
      if (seconds == 60) {
        seconds = 0;
        minutes++;
      }
      if (minutes == 60) {
        minutes = 0;
        hours++;
      }
      if (hours == 12) {
        isPM = !isPM;   // toggle AM/PM
      }
      if (hours == 13) {
        hours = 1;
      }

      // Print time
      if (hours < 10) Serial.print("0");
      Serial.print(hours);
      Serial.print(":");

      if (minutes < 10) Serial.print("0");
      Serial.print(minutes);
      Serial.print(":");

      if (seconds < 10) Serial.print("0");
      Serial.print(seconds);

      Serial.print(isPM ? " PM" : " AM");
      Serial.println();
    }
  }
}

How to Use

Upload the program to Arduino
Open Serial Monitor
Set:
- Baud Rate = 9600
- Line Ending = Newline
Enter time like:
```
09:30:00 AM
```
Press Enter
The digital clock will start running

Sample Output


09:30:01 AM
09:30:02 AM
09:30:03 AM
09:30:04 AM

Detailed Program Explanation

This program uses variables to store hours, minutes, seconds, and AM/PM status. A flag variable (timeSet) ensures the clock does not start until the user enters time. The millis() function is used to create a 1-second interval without using the delay() function.

The program reads the input time from the Serial Monitor, separates hours, minutes, seconds, and AM/PM using sscanf(), and stores them in variables. Once time is set, the clock increments seconds every second. When seconds reach 60, minutes increase. When minutes reach 60, hours increase. When hours reach 12, AM/PM toggles. If hours reach 13, they are reset to 1 to maintain 12-hour format.

Finally, the time is printed in HH:MM:SS AM/PM format with leading zeros.

Advantages

No RTC module required
Works in Tinkercad
Easy to understand
Suitable for beginners
Supports AM/PM format

Applications

Learning Arduino timing
Digital clock project
Serial communication practice
Base code for alarm clock
Base code for LCD clock

Conclusion

This project demonstrates how to build a digital clock using Arduino Serial Monitor that starts only after entering the present time in AM/PM format. It is an excellent beginner project to understand time logic and serial input handling. The same program can be extended to include LCD display, alarm feature, or RTC module for real-time accuracy.

Basic Electronics for Beginners

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