In recent blog posts (part 1, part 2) I talked about building an alarm clock with Arduino. I finally had some leftover time to build a similar thing with Arduino.

The parts I used are:

  • Arduino Mega
  • Tiny RTC
  • Seven Segment display with four numbers

The initial idea was to use a bluetooth shield to modify the time and to set the alarm. In the end I didn’t used this, so at this moment it only tells the user the time. Before I used the Tiny RTC I had two buttons to modify time and to switcht between modes, so you had a mode for displaying time and a mode for setting the time. But because of the RTC I didn’t need the setting time, because it’s done through uploading a sketch to the Arduino. These features could be incorporated into one sketch for the Arduino with so it will use different functions.

I 3d printed a casing for it so it would hold the parts and the breadboard. I haven’t soldered any parts together, this would be the next step, but as an early prototype this would be sufficient enough.

It runs on a 9v battery, but the code requires a lot of power, because of the constant calls made between the RTC module and the Arduino. So the code needs to be made more efficient.

For the code I used an external library that makes driving the display a lot easier. I modified it a bit to accept Strings as a time and not only numbers.

#include "SevSeg.h"
#include "Wire.h"
#define DS1307_ADDRESS 0x68

SevSeg sevseg; //Instantiate a seven segment controller object
String minuteString = "";

void setup() {
  byte numDigits = 4;   
  byte digitPins[] = {2, 3, 4, 5};
  byte segmentPins[] = {6, 7, 8, 9, 10, 11, 12, 13};

  sevseg.begin(COMMON_ANODE, numDigits, digitPins, segmentPins);


void loop() {
    // Reset the register pointer

  byte zero = 0x00;

  Wire.requestFrom(DS1307_ADDRESS, 7);

  int second = bcdToDec(;
  int minute = bcdToDec(;
  int hour = bcdToDec( & 0b111111); //24 hour time
  int weekDay = bcdToDec(; //0-6 -> sunday - Saturday
  int monthDay = bcdToDec(;
  int month = bcdToDec(;
  int year = bcdToDec(;
  if(minute < 10)
    minuteString = "0" + String(minute);  
    minuteString = String(minute);  
  String hourString = String(hour);
  String time = String(hourString + minuteString);
  sevseg.setNumber(time.toInt(), 2);
  sevseg.refreshDisplay(); // Must run repeatedly

byte bcdToDec(byte val)  {
// Convert binary coded decimal to normal decimal numbers
  return ( (val/16*10) + (val%16) );

Here are some pictures of the end result:

Alarm clock part 3
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