Introduction: Coronavirus Quarantine Clock

About: I am an IT professional working in Big Data. I've also been messing around with IoT devices such as Arduino, Raspberry PI and electronics for decades. I also have a passion for both giving and receiving knowle…

It is October, 2020 - right in the middle of the Covid-19 pandemic.

My wife and I have found ourselves in hotel quarantine for 14 days as a result of undertaking some international travel. 14 days stuck in a hotel room 24 hours a day is a very, very, very long time - even with company! Fortunately, I decided to bring along some Arduino stuff to help pass the time.

For some strange reason, the hotel into which we have been placed does not have any clocks in the room. This quickly became a problem for me as one of the things that I discovered to be quite helpful while in hotel quarantine is to have the ability to observe that time is indeed progressing and that there is light at the end of the quarantine tunnel, and most importantly as it turns out, just how far away the end of the tunnel is!

Personally I find it much more convenient (and less frustrating) to simply glance at a clock as opposed to finding a handheld device and activating it to see what time it is. So, since I had some electronics components with me and plenty of time on my hands, I decided to build this quarantine clock.

The clock uses a real time clock module to track the date and time. It has two displays:

  1. A traditional clock display that shows the time - with a blinking ":" just like a real clock!
  2. A LED Bar panel which counts down the days remaining until release!

The first image above shows that it is 15:42 with just 3 days to go. The second image 9:47 with 8 days to go.

It is a fairly simple build - despite the number of connections. However, you will need an Arduino Mega for this specific version. At the end of this post, I make some suggestions for things you can do to help pass your time. One suggestion is converting the project to make it work on an Uno or Leonardo (I didn't have the necessary additional bits to do this during my quarantine, hence my use of the Mega).

As a bonus, I've used an interrupt driven programming model to ensure a rock solid steady display on the clock display. It is also possible to convert this to a more "traditional" while loop model that uses millis() to drive the clock display. The bonus part is that you can switch between the two models by (un)commenting one line of code. I also include a discussion and an experiment you can try to actually see the benefits of the interrupt model -vs- the "traditional" while loop + millis() construct to do things on a timely basis (namely keep the clock display updated).

If you find my projects helpful, please consider supporting me by buying me a coffee.


To build the clock, you will require:

  • 1 x Arduino Mega (the project requires lots of Digital I/O pins, so it can't work with the "smaller" devices)
  • 1 x medium sized breadboard
  • 1 x Duinotech Real Time Clock module such as this RTC module. The RTC should be compatible with the RTClib Arduino library.
  • 1 x 4 digit common cathode clock display such as this one Vishay TDCG1060M - clock display module
  • Alternatively, a bunch of seven segment common cathode LED digits.
  • 1 x 10 segment (or more) LED bar panel such as this one 10 segment LED bar panel
  • Alternatively, 10 (or more) regular LED's.
  • 18 x 470 ohm resistors (or higher but not higher then 2K ohm)
  • hookup wire (4 x M-F and lots of M-M)

Sorry I can't provide a single supplier parts list, but as mentioned these are components that I just happened to have with me and I can only find similar examples just now - but they do correspond to the parts that I used.

You may wish to make substitutions as required, the project should be quite amenable to substitutions, just update the declarations near the top of the program to reflect your environment.

Step 1: Hooking Up the Components

The diagrams show the hookup of the various components to the Arduino mega.

There are lots of connections, but it is pretty straight forward. Since I use the dual in line socket connectors on the Arduino Mega (pins 22-43), it can be a bit hard to see from the breadboard diagram where the wires go. So, I've also included a schematic which may show the wiring a bit more clearly. Use either or both as a guide when hooking things up. It may help to print them and cross off each wire as you install it.

The Real Time Clock (RTC)

The Real Time Clock (RTC) module uses I2C for communications. This can only work on certain Arduino pins. On the Mega, these are pins 20 and 21. Make sure you connect SDA on the RTC to pin 20 on the Arduino and SCL to pin 21. My RTC module could not be mounted on the breadboard, so I had to use M-F hookup wires to connect it. Don't forget to connect +5V and GND!

Substituting the Arduino

If for some reason you wish to switch to another Arduino model, by all means do so, but make sure you hookup the SDA and SCL pins to the correct DIO pins on that particular model. You may need to hack the program a bit as well. Try to keep the LED segments connected to a single MCU port. In this project, I am using the MCU port A which corresponds to DIO pins 22-29 on the Arduino Mega. The program uses direct I/O to the entire port register on the MCU to output the LED image. You can see this as PORTA = ... in the code.

Substituting the clock display module

If you can't get the (or don't have) the LED module I used, feel free to use another, but make sure you get its datasheet. My observation so far is that there isn't a standard for the pins on these modules. So, if you do use a different clock display module, you will probably need to connect it to the Arduino differently (see below for tips about this).

The following instructions show the design that I used for an LED panel that must be driven directly by the Arduino and how it can easily accommodate a LED module with a different pin-out configuration. If you use individual 7 segment LED digits, then these instructions will also apply.

Another possibility is a module that uses some sort of serial communications (e.g. I2C) to control it. That is, the display is driven by the module itself (e.g. it has an integrated MAX7219 chip). In this case, the connections (8 x current limiting resistors A-F and L1,L2) I have shown will need to be completely replaced with whatever mechanism your panel needs.

The rest of the instructions in this step are for 7 segment LED panels such as the one that I am using (or individual 7 segment digits) that must be driven by the Arduino.

You will note that there are 8 resistors to the right of the clock display (breadboard view). These are connected to the Arduino in such a way that the rightmost resistor corresponds to the A segment on the display. The next one along is the B segment and so on. The leftmost resistor is the colon (named L1+L2 on this particular display).

I have labelled the breadboard diagram with these LED segment identifiers.

So, if you do swap out the clock display with a different unit, simply ensure that the connection from the rightmost resistor (i.e. the bottom side of the resistor) goes to the A segment pin on the display module. The second rightmost resistor goes to the B segment and so on. There should be no need to adjust the connections between the Arduino and the top side of these resistors.

Step 2: Loading the Program

The program code can be obtained from my github. There is also a fritzing diagram of the circuit at the same github location.

You will need to install the RTCLib as shown in the library manager image.

Simply open the ino file in the Arduino IDE, connect the Arduino Mega (with the required hardware) to your computer and upload the program. You may want to set your quarantine end date by adjusting the values in the variable quarantineEndDate (about line 54).

There are plenty of comments in the program, so hopefully, modifying it will be relatively straightforward.

For those of you who prefer to copy and paste, knock yourself out with the following.

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

* Quarantine Clock.
* ----------------
* By gm310509
* 2020-09-26
* A simple program built for hotel quarantine.
* Form some reason, my hotel did not have a clock. I find it very convenient to have a clock
* so that I can just glance at the current time without having to pick up and activate a handheld device such as
* a phone or tablet (and I couldn't be bothered putting on my watch every day, and I had nothing better to do while in
* quarantine).
* So, I made one from some parts that I had with me.
* I happened to have a 10 segment bar LED, so I decided to add this to show the number of days remaining in my quarantine.
* Hence this quarantine clock program was created.
* The program needs a lot of I/O ports (I didn't have any multiplexors or shift registers), so it can only run
* on an Arduino with a high I/O port count - such as the Arduino Mega.
* All up, it requires 20 I/O pins.
* Additionally with the current design, it assumes that at least one MCU port (port A in this
* version - DIO pins 22-29 on the Arduino Mega) is fully accessible via the Arduino's Digital I/O connectors.
#define VERSION ""

#define CHECK_TIME_INTERVAL 1000 /* Interval between RTC time checks = 1 second */
#define LED_STROBE_INTERVAL 1 /* Interval between clock LED strobe steps = 1 millisecond (or 1000 times per second */
#define CLOCK_COLON_DISPLAY_TIME 500 /* How long the clock colon is turned on = 500 millisecond (or 1/2 a second) */

/* Use interrupts - program configuration constant
* If defined (the value is unimportant) then the program
* will be configured to use interrupts to control the
* refresh of the clock display.
* If not defined, the program will be configured to use
* polling to refresh the clock display. This will have side
* effects when long running processes are active.
* Best option: define the USE_INTERRUPTS symbol.

* This is the quarantine end date.
* Modify this to reflect your quarantine end date.
* Mine was 9th of October 2020.
DateTime quarantineEndDate (2020, 10, 9, 0, 0, 0); // The date that quarantine will end.

unsigned long checkTimeTO = 0; // Variable used to track the *next* time we check the RTC.
unsigned long systemUpTime = 0; // Counts the number of seconds the system has been running.

#if ! defined( USE_INTERRUPTS)
unsigned long strobeClockTO = 0; // Variable used to track the next time we need to strobe the LED display.
// The strobeClockTO is only relevant for the non-Interrupt driven version of the program.

// Variables for the LED strobe

/* The ledFont array defines the "font" for the LED's.
* Each byte corresponds to the "image" of a "character" that is displayed on a 7 segment LED digit.
* On the Arduino Mega, the font value is simply output to Port A (digital pins 22-29 inclusive).
* font
* font value
* value binary
* Character hex X G F E D C B A
* 0 3f 0 0 1 1 1 1 1 1
* 1 06 0 0 0 0 1 0 1 0
* 2 5b 0 1 0 1 1 0 1 1
* 3 4f 0 1 0 0 1 1 1 1
* 4 66 0 1 1 0 1 0 1 0
* 5 6d 0 1 1 0 1 1 0 1
* 6 75 0 1 1 1 0 1 0 1
* 7 07 0 0 0 0 0 1 1 1
* 8 7f 0 1 1 1 1 1 1 1
* 9 67 0 1 1 0 0 1 1 1
* - 40 0 1 0 0 0 0 0 0
* space 00 0 0 0 0 0 0 0 0
* The value 1 turns an LED segment on, a 0 turns it off. The X bit is unused for the font.
* The segments map to the Arduino mega's digital outputs as follows:
* Clock Mega 2560 Arduino
* Display Port A Digital
* Segment bit I/O Port
* A 0 22
* B 1 23
* C 2 24
* D 3 25
* E 4 26
* F 5 27
* G 6 28
* DP n/c n/c
* L1,L2 7 29
int ledFont[] = {
0x3f, 0x06, 0x5b, 0x4f, // 0, 1, 2, 3
0x66, 0x6d, 0x7d, 0x07, // 4, 5, 6, 7
0x7F, 0x67, 0x40, 0x00 // 8, 9, -, space

volatile int clockDisplay[4] = {0, 0, 0, 0}; // The display image to be displayed on each of the clock LEDs.
int currLed = 0; // The LED currently being shown:
// 0 = tens of hours, 1 = hours, 2 = tens of minutes, 3 = minutes.
// This is used by the clock display routine as an index into the clockDisplay array.

volatile int colonDisplayTime = 0; // How long the clock's colon has been on.
// When the second clicks over, this is set to CLOCK_COLON_DISPLAY_TIME
// and the clock display routine counts it down to zero.
// if the value is non zero, the colon is tuned on. When zero
// the colon is turned off.

const int NUM_LEDS = sizeof (clockDisplay) / sizeof (clockDisplay[0]); // the number of clock display digits as defined by the size of clockDisplay.
const int clockLedPin[NUM_LEDS] = { 3, 4, 5, 6 }; // The DIO pins to which the common cathode of the 7 segment LED's is attached.
// [0] is the tens of hours.
// [1] is the units of hours.
// [2] is the tens of minutes.
// [3] is the units of minutes.

// Definition of the number of days remaining in quarantine bar graph LED panel.
const int barGraphLed[] = { // The digital I/O pins used to control the 10 segment BAR LED.
34, 35, 36, 37, 38, // [0] is the least significant (rightmost in my design) LED used to represent 1 day left.
39, 40, 41, 42, 43 // [9] is the most significant (leftmost in my design) LED used to represent 10 days left.
const int BAR_GRAPH_NUM_LEDS = sizeof(barGraphLed) / sizeof (barGraphLed[0]); // Number of bar LED's as defined by barGraphLed.
// If you wanted to add more LED's (e.g. to show the full 14 days,
// it should be a simple matter of extend barGraphLed with the extra DIO pin numbers.
// NB: I haven't tested that, but the program is intended to adapt to the number
// of elements in barGraphLed to define the number of quarantine day LEDs.

RTC_DS1307 rtc; // The realtime clock interface.

* Strobe the clock LED.
* Strobing involves turning off the current clock digit (as defined by currLed).
* This is achieved by setting the associated pin (from clockLedPin) to HIGH.
* Then apply the next digit's image from the fonts (clockDisplay) and turn on that LED so
* its image can be displayed.
* This function also manages the blinking of the clock panels colon LED.
void _strobeClockLed() {
digitalWrite(clockLedPin[currLed], HIGH); // Turn the current digit off.

currLed = (currLed + 1) % NUM_LEDS; // identify the next digit (wrap around to 0 when NUM_LEDs is reached)
int ledImage = clockDisplay[currLed]; // Get the image of the next digit from the fonts array.
if (colonDisplayTime != 0) { // Work out whether the colon should be on or off.
ledImage |= 0x80; // It should be on, so set bit 7 of the display image to 1
colonDisplayTime--; // count this display time.
PORTA = ledImage; // Output the entire image to PORTA (DIO pins 22-29 on the Arduino Mega)
// Serial.print("Clock LED "); Serial.print(currLed); Serial.print(": Writing: "); Serial.print(clockDisplay[currLed], HEX);
// Serial.print(", pin: "); Serial.println(clockLedPin[currLed]);
digitalWrite(clockLedPin[currLed], LOW); // Finally turn on the digit that should display the image.

// Timer 2 compare match Interrupt Service Routine
// -----------------------------------------------
// This routine controls the strobing of the LED display.
// What does strobing mean? basically we turn just one digit of the LED's on at any one
// time (the other three LED's are off).
// Why would we do this?
// Because the LED digits all share the control signals (wires) that determine which
// LED segment(s) must be turned on to display the information we need.
// We only have 8 control signals, but we have 32 (4 digits x 8 LED's per digit). So, we strobe,
// or turn on just one of the LED's one at a time, and simultaneously output the correct "image"
// data to the 8 LED segment control lines.
// This routine is called very rapidly - see the description in setup () for how rapidly. This
// gives the illusion of a clear steady display.
#if defined(USE_INTERRUPTS)

* Strobe the Clock LED.
* This function is called continuously to determine when it is time to strobe the clock LED display.
* If it is time to strobe the clock display, then the function determines the next time that it must be
* strobed and calls the strobe function.
* This routine can have a side effect when long running operations are executed. Long running operations
* prevent this routine from being called in a timely fashion. This can result in the clock display flickering
* or freezing on a single digit.
* ** If we are using interrupts to manage the clock display, we don't need this function.
#if ! defined( USE_INTERRUPTS)
void strobeClockLed(unsigned long _now) {
if (_now >= strobeClockTO) { // Is it time to strobe the clock display?
strobeClockTO = _now + LED_STROBE_INTERVAL; // Yep, calculate the next time to do so and
_strobeClockLed(); // strobe the display.

* Calculate the number of quarantine days remaining (including today)
* return the number of days remaining.
int getQuarantineDaysRemaining(DateTime _now) {
// create a datetime for midnight on the date specified.
DateTime _today = DateTime(_now.year(), _now.month(),, 0, 0, 0);
TimeSpan quarantineTimeRemaining = quarantineEndDate - _today; // Work out the time remaining in days.

int daysRemaining = quarantineTimeRemaining.days();
// Serial.print(F("QDays remaining: ")); Serial.print(quarantineTimeRemaining.days());
// Serial.print(F(", daysRemaining = ")); Serial.println(daysRemaining);

if (daysRemaining < 0) { // if we have passed the quarantine end date, then
daysRemaining = 0; // simply return 0 days remaining (as opposed to a negative number).
return daysRemaining;

* Given a date+time, calculate the number of days remaining in quarantine.
* Update the bar graph to show that number.
void setDaysRemaining(DateTime _now) {
int daysRemaining = getQuarantineDaysRemaining(_now);
for (int i = 0; i < BAR_GRAPH_NUM_LEDS; i++) {
digitalWrite(barGraphLed[i], i < daysRemaining); // if i is within the number of remaining days, then turn the bar LED on otherwise turn it off
Serial.print("Setting days remaining to "); Serial.println(daysRemaining);

// Serial.print("Quarantine days remaining: "); Serial.println(quarantineTimeRemaining.days() + 1);
// DateTime rnd = DateTime(2020, 10, 8, 10, 0, 0);
// TimeSpan rndTs = quarantineEndDate - rnd;
// Serial.print("Random Date diff: "); Serial.println(rndTs.days());


* Output a formatted date/time.
void printDate(DateTime dttm) {
Serial.print("DateTime: ");
Serial.print(dttm.year()); Serial.print("/"); Serial.print(dttm.month()); Serial.print("/"); Serial.print(;
Serial.print(" ");
Serial.print(dttm.hour()); Serial.print(":");Serial.print(dttm.minute()); Serial.print(":");Serial.println(dttm.second());

// Variables used to track the previous second or day.
// These are used to detect if the second or day has changed and thus trigger an appropriate action (Defined below).
int prevSecond = 99;
int prevDay = 99;

* Checks the Real Time Clock (RTC) time and date.
* If the time/date has changed, then update the relevant parameters including:
* - the time to display on the clock LED.
* - the quarantine days remaining
* - whether the clock ':' should be on or off (to give the once per second blinking effect).
void checkTime(unsigned long _now) {
if (_now >= checkTimeTO) { // Is it time to check the RTC?
checkTimeTO = _now + CHECK_TIME_INTERVAL; // Yep, work out when to next check it.

DateTime dttm =; // Read the current date and time.
int day =; // Extract the components of interest.
int hour = dttm.hour();
int minute = dttm.minute();
int second = dttm.second();
// Have we started a new day? If so, work out the remaining quarantine days.
if (day != prevDay /*&& second >= 1*/) { // NB: We need to wait to at least one second into the new day for the calculation to be correct.
prevDay = day;

// Work out what characters (digits) to put into the clock display.
if (hour < 10) { // is the hour a signle digit?
clockDisplay[0] = ledFont[11]; // Yes, output a blank (as opposed to a leading zero.
} else {
clockDisplay[0] = ledFont[hour / 10]; // No, work out what digit image to display for the 10's of the hour.
clockDisplay[1] = ledFont[hour % 10]; // The units digit for the hours.
clockDisplay[2] = ledFont[minute / 10]; // The 10's digit for the minutes (this will display a leading zero if needed).
clockDisplay[3] = ledFont[minute % 10]; // The units digit for the minutes.

if (second != prevSecond) { // Has the second changed?
colonDisplayTime = CLOCK_COLON_DISPLAY_TIME; // Yep, reset the colon blink time
prevSecond = second;
systemUpTime++; // Count 1 second for the system up time.

* The following set of functions and variables are used
* to interact with the clock (e.g. set the quarantine date, interrogate the system status and others).
* The list of valid commands.
char *commands [] = {
"date", // Command: 0
"time", // 1
"qdate", // 2
"status" // 3

* A description of the commands. This is only used by the showCommands method.
char *commandDesc [] = {
"Set Date (yyyy-mm-dd)",
"Set Time (hh:mm:ss)",
"Set Quarantine End Date (yyyy-mm-dd).",
"Print system status"

* Output a list of the commands along with there description and some generic information.
void showCommands() {
Serial.println(F("Available commands:"));
for (int i = 0; i < sizeof(commands) / sizeof(commands[0]); i++) {
Serial.print(" ");
Serial.print(": ");
Serial.println(F("the format of any parameters is shown in parenthesis"));
Serial.println(F("For example, use the following to set the time to 1:15 PM and 22 seconds:"));
Serial.println(F(" time 13:15:22"));
Serial.println(F("In most cases, you can omit the parameter and the corresponding value will be displayed."));
Serial.println(F("For example, to show the current date, simply enter the date command with no parameter."));
Serial.println(F(" ** NOTE ** there is no validation on the values entered. Make sure you enter a valid date or time."));

* Given some text, attempt to extract up to 3 integers from that text.
* The integers can be delimited by any non numeric character (including '-' and '.'). Thus, negative
* numbers will not be identified. By definition, fractional numbers will be treated as two seperate numbers
* as this function only parses integers.
* This function is used to extract the digits from three digit structures such as a date or a time.
* It can also be used for shorter structures (e.g. a single or dual digit structure) if need be.
* Return: The actual number of integers found in the text is returned.
int parse3integers(char * pData, int parameterValues[]) {
int pIndex = 0; // the index where the integer will be stored.

while (pIndex < 3 && *pData) {
// find the start of the next/first number.
while ((*pData < '0' || *pData > '9') && *pData) {
pData++; // Skip characters until we get a digit or end of text.
if (! *pData) { // If we are at the end of the text, just return what we have found so far.
return pIndex; // return the actual number of digits found.
// Great, we have found a digit, so convert it to a number.
int acc = 0; // Initialise an accumulator to accumalate the digit values.
while (*pData >= '0' && *pData <= '9') {
acc = acc * 10 + *pData - '0'; // for each digit, multiply the accumulator by 10 and add in the current digit's value.
pData++; // point to the next character.
//Serial.print("Storing "); Serial.print(acc); Serial.print(" at index "); Serial.println(pIndex);
parameterValues[pIndex++] = acc; // Store this integer and loop back for more.
return pIndex; // Finally return the number of digits captured.

// A buffer to accumulate characters received from Serial.
char buffer[80];
int bufferPtr = 0; // A "pointer" into buffer defining where the next character shall be placed.

* A command has been supplied from Serial.
* Identify the command entered and if valid, execute it.
void processControlInput() {
Serial.print("Command: '"); // To begin, simply echo what we have received.
// check the command is valid.

int cmdNo = -1; // Next try to identify the command from the list of available commands found in *commands[]
for(int i = 0; i < sizeof (commands) / sizeof(commands[0]); i++) {
char *res = strcasestr(&buffer[0], commands[i]); // Does the buffer contain the command keyword?
// Serial.print("Trying: "); Serial.print(commands[i]); Serial.print(", "); Serial.println(res == buffer ? "match" : "no match" );
if (res == buffer) { // if res is the same address as the buffer's address, then we have a command match.
cmdNo = i; // identify the matched command
break; // and terminate the loop.

if (cmdNo == -1) { // Check that we have a valid command, if not, output the help and return.
Serial.print(F("Invalid command entered: '")); Serial.print(buffer); Serial.println(F("'"));

// Find where the command's parameter value starts
// first, find the end of the command.
char *pData = &buffer[0]; // Skip over the command text.
while (*pData != ' ' && *pData != '\t' && *pData) {

// next find the end of the whitespace.
while (*pData == ' ' || *pData == '\t') {

//Serial.print("Data Portion of message: "); Serial.println(pData);

// Commands can accept up to 3 numeric values (e.g. year, month and day in a date).
// So, prepare to receive those three values.
int parameterValues[3];
int parameterCount = parse3integers(pData, & parameterValues[0]);

// Finally, try to execute the command.
if (cmdNo == 0 || cmdNo == 1) { // date or time command
if (parameterCount == 3) { // did we get three numerics as parameters?
DateTime dttm =; // Read the current Time.
int year = dttm.year();
int month = dttm.month();
int day =;

int hour = dttm.hour();
int minute = dttm.minute();
int second = dttm.second();
if (cmdNo == 0) { // date command - override the current date with the supplied values.
year = parameterValues[0];
month = parameterValues[1];
day = parameterValues[2];
} else { // time command - override the current time with the supplied values.
hour = parameterValues[0];
minute = parameterValues[1];
second = parameterValues[2];
// This line sets the RTC with an explicit date & time, for example to set
// January 21, 2014 at 3am you would call:
//rtc.adjust(DateTime(2014, 1, 21, 3, 0, 0));
// Set the date and time in accordance with the supplied values.
rtc.adjust(DateTime(year, month, day, hour, minute, second));
Serial.print(F("Setting date/time: "));
Serial.print(year); Serial.print("/"); Serial.print(month); Serial.print("/"); Serial.print(day);
Serial.print(" ");
Serial.print(hour); Serial.print(":"); Serial.print(minute); Serial.print(":"); Serial.println(second);
} else { // We did not receive three integers for the date / time commands.
if (parameterCount > 0) { // If we got any parameters, then there is a validation error. Show the help.
Serial.println(F("** Failed to parse Date or Time"));
} else { // We got 0 parameters, so this is a request to show the current date/time.
Serial.print(F("Current date/time: ")); printDate(;
} else if (cmdNo == 2) { // qdate command
if (parameterCount == 3) { // Did we get three parameters? If so, set the new quarantine end date.
quarantineEndDate = DateTime(parameterValues[0], parameterValues[1], parameterValues[2], 0, 0, 0);
Serial.println(F("Quarantine end date set to: ")); printDate(quarantineEndDate);
setDaysRemaining(; // Update the days remaining value and display.
} else { // We did not get three parameters.
if (parameterCount > 0) { // Did we get any parameters? If so, there is an invalid input, so display error and help info.
Serial.println(F("** Failed to parse Date or Time"));
} else { // We got zero parameters, so this is a request to show the quarantine end date information.
Serial.print(F("Quarantine end date: "));
Serial.print(F("Days remaining: "));
} else if (cmdNo == 3) { // Status command - takes no parameters, so just output the system status.
DateTime _now =;
Serial.println(F("Current system status"));
Serial.print(F(" Firmware Version: ")); Serial.println(F(VERSION));
Serial.print(F(" Current Date/Time: ")); printDate(_now);
Serial.print(F(" Quarantine end Date: ")); printDate(quarantineEndDate);
Serial.print(F(" Quarantine days remaining: ")); Serial.println(getQuarantineDaysRemaining(_now));
Serial.print(F(" System uptime: ")); Serial.print(systemUpTime); Serial.println(F("s"));
#if defined(USE_INTERRUPTS)
Serial.println(F(" LED Clock refresh: Interrupt driven"));
Serial.println(F(" LED Clock refresh: Best effort polling"));
} else { // We should not get here, but just in case, output an error.
Serial.println(F("Unexpected command entered"));

* Check to see if there is any input on Serial.
* If so, accumulate the input up to a Carriage Return (CR).
* Once a CR has been received the input is processed.
void checkControllerInput() {
if (Serial.available() > 0) {
char ch =;
if (ch == '\n') { // We have a CR which marks the end of the input.
buffer[bufferPtr++] = '\0'; // Null terminate the string.
processControlInput(); // Process the input
bufferPtr = 0; // Reset the buffer Pointer for the next input.
} else if (ch == '\r') {
// ignore carriage returns.
} else if (ch == '\b') { // Just in case we are using a terminal,
bufferPtr--; // process a backspace by removing a character from the input.
if (bufferPtr < 0) {
bufferPtr = 0;
} else {
// Check for buffer overflow (and avoid it).
if (bufferPtr < sizeof(buffer) - 1) {
buffer[bufferPtr++] = ch; // Not a CR and not a LF, so just accumulate the character.

* setup routine.
* Initialise everything including:
* - the Serial port for status messages and command input
* - set the various ports for output.
* - display a cutsie "test pattern" on the bar LED.
* - setup Timer2 to generate our interrupts (only for the Interrupt driven version of the program).
* - initialise the RTC (real time clock)
* - output some helpful information.
void setup() {
Serial.begin(115200); // Initialise the Serial port.
//Serial.begin(1200); // Use this Serial.begin if you want to try the experiment described below
// in the interrupts section of setup().
int to = 1000;
while (! Serial && to > 0) { // Wait for the Serial port to be ready, but no longer than 1 second.

Serial.print(F("Dimming Quarantine Clock - version: "));

DDRA = 0xff; // Set Port A to be output. Port A is used to output the image to display on one of the clock digits.
PORTA = 0x00; // turn the clock LED off.

// Set the digital I/O pins for the clock display's common cathodes.
for (int i = 0; i < NUM_LEDS; i++) {
pinMode(clockLedPin[i], OUTPUT);
digitalWrite(clockLedPin[i], HIGH);

// Set the digital I/O pins for the remaining days bar LED panel.
for (int i = 0; i < BAR_GRAPH_NUM_LEDS; i++) {
pinMode (barGraphLed[i], OUTPUT);
digitalWrite(barGraphLed[i], LOW);

// Output a cute "test pattern" on the bar LED.
for (int i = 0; i < BAR_GRAPH_NUM_LEDS; i++) {
digitalWrite(barGraphLed[i], HIGH); // Turn each LED on one by one and pause after each one.
delay(500); // Leave the LED's on for a short time.
for (int i = BAR_GRAPH_NUM_LEDS - 1; i >= 0 ; i--) {
digitalWrite(barGraphLed[i], LOW); // Turn each LED off one by one and pase after each one.

#if defined (USE_INTERRUPTS)
// Setup an interrupt Service Routine to manage the display
// of the digits on the clock (which must be stobed).
// This is handled by our Interrupt Service Routine (ISR).
// To begin, disable interrupts temporarily. Why bother with this?
// If we didn't disable interrupts, the registers that control interrupts will
// be active. This means that we could risk an interrupt being fired while we are
// part way through setting up the registers. Should that happen, the results will
// be unpredictable, random and in the worst case disastrous. Granted this is low
// probability, but if you are unlucky, you will regret not having done this!
// So, we turn of interrupts, configure what we need and finally, renable interrupts.

// We will setup up Timer2 (which is also used by the tone function) to trigger an
// interrupt 1,000 times per second.
// Out interrupt Service Routine (ISR) will stobe the Clock digits to give us a nice
// crisp display.
// Why would we bother with this complexity?
// The easiest way to explain why is via an experiment:
// 1) Set the Serial monitor to a low speed (e.g. 1200 baud).
// Remember to change this in the Serial.begin as well.
// 2) use the non-interrupt version of this program.
// Comment out the following line near the top of the program
// #define USE_INTERRUPTS 1
// This may be enough, but if not:
// 3) Cause a large volume of data to be displayed on the serial monitor (the easiest way
// is to use the "status" command or enter an invalid command such as an "X" or to
// display the help information).
// What you should see is that the clock display will pause from time to time with just one LED lit,
// or it may or have similar undesirable characteristics.
// Finally:
// 4) repeat the test using the "interrupt enabled" version of the program (uncomment the #define
// from step 2.
// 5) Reset the Serial.begin and serial monitor to a more sensible baud rate (e.g. 115200).
// So, what is happening is that the strobing of the LED's needs to be performed at a consistant rate to maintain
// the crispness of the display. If the "refresh rate" slows down too much (or stops altogether), then the clock
/// display may start to flicker or even freeze periodically.
// This will happen as a result of other activity (e.g. Serial.println, delay and long running code).
// To avoid this, the interrupt mechanism will reliably call our LED strobing ISR 1,000 times per second
// no matter what other long running tasks may be running.
// After all of the above, you might be thinking:
// - don't set the Serial monitor to be so slow
// - don't output so many messages.
// or
// - some other work around.
// Yes, you could do those things. But, messages are useful. And, workarounds can tend to grow out of proportion
// in the "difficulty department" as you try to deal with other side affects.
// Maybe it is just me, but I feel that I can sometimes see the clock flicker slightly even at higher baud rates.
// Indeed at 115200 baud, when you run the status command or display the help, the display will flicker.
// Finally, and this is probably the best reason, I'm stuck in quarantine, so I haven't got much else
// to do! That is also probably why I am using the single line comment for all of this rather than a
// /* multi-line comment block */ !!!!!! Although that is starting to get tedious, so I might switch
// to multi-line comment blocks for the rest of this documentation! :-)
// And if you don't appreciate the quarantine reason, this use case (i.e. ensuring things happen when they are supposed
// to happen despite other things that are going on) is what interrupts are for.
// ------------------------
// Details about the timer registers we will be using can be found in the AVR data sheet.
// In the case of the Mega (AVR Mega2560 data sheet dated 02/2014):
// - Chapter 20 describes all aspects of the features of timer 2 (Chapter 18 on Uno)
// - Chapter 20.10 has a detailed description of the timer 2 registers (chapter 18.11 on Uno)
// - Chapter 33 has a summary of all registers (chapter 36 on Uno)
// Set the TCCR2 registers to 0. This effectively cancels any other configuration
// that might be left over in the timer 2 registers.
// TCCR2 is the Timer Counter / Control register for timer2.
// TCCR2 is in 2 parts (i.e. A and B).
TCCR2A = 0; // set timer 2 control registers 0 to turn off
TCCR2B = 0; // all functions.
TCNT2 = 0; // initialize the timer 2 counter value to 0

// set compare match register for 1khz increments
// = (16*10^6) / (1000*64) - 1 (must be <256)
// = 16 MHz clock / (1000 hz * 64x prescaler) - 1
// = 16,000,000 / 64,000 - 1
// = 250 - 1
// = 249
// OCR2A is a single byte (and thus must be < 256)
// so a combination of "prescaler" and frequency is used
// to determine how high to count before generating an interrupt
OCR2A = 249; // = Clock speed / (desired frequency * prescaler value).

// Set CS22 bit for 32x prescaler
// - Refer to section 17.10 of the datasheet
// Basically this divides the 16MHz clock by 32 (= 0.5MHz)
// for the purposes of driving Timer2.
// (CS22 = 1, CS21 = 0, CS20 = 0)
TCCR2B |= (1 << CS22);
// The above two settings cause the counter (TCCNT2) to be incremented by
// one every time the scaled clock (16MHz / 64x) ticks.
// this will occur roughly once every 16,000,000 / 64,000 = 1/250th of a second
// or put another way, 250 times per second.
// so once we get to 249 (remember, we started counting from 0) 1 second will have
// passed.

// Turn on CTC (Clear Timer on Compare match) mode
// Basically, when TCNT2 reaches OCR2A (i.e. 249) TCNT2 is reset to zero.
// This is not terribly useful for our purposes, but stay tuned for more.
// Refer to chapter 20.10.1 for register setting specfics
// (WGM2 = 0, WGM1 = 1, WGM0 = 0) and chapter 20.4 for a
// on the various modes of operation.
TCCR2A |= (1 << WGM21);

// Enable timer compare (Timer 2 Output Compare Match A) interrupt.
// This is "the good bit", it is what we have been working towards.
// Basically when the counter (TCNT2) reaches our limit (OCR2A) an interrupt
// will be generated.
// The interrupt is the Timer 2 Output Compare Match A Interrupt.
// This interrupt is interrupt vector 14 on the Arduino Mega AVR2560
// (on the UNO, it is vector 8).
// In our code, the Interrupt Service Routine (ISR) is nominated by the rather
// curious looking function definition as follows:
// // ISR code goes here
// }
TIMSK2 |= (1 << OCIE2A);

// Now that everything is setup, reenable the interrupts.
// From this point on, the ISR will be called every 1KHz (i.e. 1000 times per second).
// Irrespective of anything else that is going on (such as slow Serial.println calls).

Wire.begin(); // Initialise the interface to the RTC.

if (! rtc.isrunning()) {
Serial.println(F("**** RTC is not running. Please set the time and date."));
Serial.println(); Serial.println(F("Use these commands to set the clock"));

* The main loop.
* Give the perdiodic activities a chance to run. These include:
* - Checking if there is any input on Serial.
* - Check whether the time has ticked over and format the new time for display.
* - strobe the clock LED display (only applies for the non-interrupt version of the program).
void loop() {
// put your main code here, to run repeatedly:
unsigned long _now = millis();

// If we are using interrupts to manage the clock display, we don't need this.
#if ! defined( USE_INTERRUPTS)

Step 3: Setting the Date/time

Once the program is loaded and running, you can interact with it via the serial monitor.

Ensure you have the correct communications settings (115200 baud, newline). You can then enter commands into the little text box at the top of the serial monitor and click the "send" button to send your text to the Arduino for consideration. Refer to the image for the location of the various controls.

If you enter an invalid command, the list of valid commands will be shown in the monitor. Try entering "help" (which is not a recognised command) to produce the list along with some other generic, hopefully, helpful information. The "help" information is also shown when the Arduino is reset.

To see what the RTC thinks is the current date, simply enter "date" and click send. The current date+time will be displayed in the serial monitor's output panel.

To set the date simply enter the command "date" followed by the date. For example if today is the 19th of September, 2020, enter "date 2020-09-19" and click send. The date will now be set. Obviously, the correct date is important for working out the number of days remaining in quarantine.

Similarly, the time can be set using the time command, For example, to set the time to 10:52 AM and 5 seconds (the seconds are required), enter "time 10:51:05" and click send. The clock should update immediately to show the new time. You can also retrieve the current time by entering the time command by itself and clicking send.

You can also set and interrogate the quarantine end date via the "qdate" command.

Finally, you can get a system status report by entering the "status" command.

There is no validation on the inputs and any delimiter is accepted. So for example, each of the following commands will produce the exact same result:

date 2020/10/9

date 2020-10-9

date 2020 10 9

date 2020.10.9

date 2020abcde10defghi9

By no validation, I mean that assuming that the simple rule that a date (or time) has 3 numeric parts is met, then the values are sent to the RTC for processing. So, it is up to you to ensure that you enter valid information. But if you really want to find out what happens if you enter invalid data, by all means try something like "date 2020-42-88'. That input will be accepted and passed onto the RTC exactly as you entered it! So, buyer/builder beware!

Step 4: If You Have Spare Time...

... you almost certainly will if you find your self in quarantine!

Here is a TODO list of things you could do to keep yourself occupied while stuck in quarantine.

  1. Add a date display - this could be by using an additional LED Panel, or simply switching the display from clock to date and back again periodically.
    Perhaps leave the colon permanently on when showing the date to distinguish the date from the time.
  2. Add a seconds display - this would most likely require an additional 2 digits (and maybe another colon?)
  3. Put some data validation on the date/time entry (i.e. reject commands with invalid values like "date 2020-42-88" or date "2020-0-0").
  4. Set an alarm - in our quarantine we had to take our temperatures twice daily. Once at 9AM and once at 3PM. Having an alarm sound at the appropriate time would be a great feature (admittedly, I did use my phone for this :-)).
    This will require some sort of sound generating device (e.g. a piezo buzzer) and a button (you will want to cancel the alarm) and potentially some additional commands to set/interrogate the alarm(s).
  5. Lap counter - while stuck in quarantine there is little opportunity to exercise. One of the things we did was to walk around the room 25 times 3 times a day. Keeping track of how many laps we had done proved to be tricky as we (my wife and I) were talking while walking. So a lap counter could come in handy. Some ideas to make this simple include:
    1. Use an RFID reader to count laps - swipe your room card (or some other tap style of card) to register a lap each time you go past.
    2. Use an IR sender / receiver - when you break the beam count one lap (or one lap when you break the beam twice if you must retrace your steps to complete one lap - like we had to as our room forced us to do a weird reverse P shaped lap).
    3. Something else - I'd be interested to hear your method to count the laps.
      You will probably need some buttons to activate and terminate lap counting. Perhaps use the buzzer to sound an alarm when you have reached your target and/or show the lap count on the LED. You could perhaps define a letter 'L' in the font table and display it on the clock display when operating in lap mode...
  6. Use external circuitry to reduce the required Digital I/O pin count and thus allow this to run on a "smaller" device such as an Arduino Uno or Leonardo. Some examples of external components might include:
  • A BCD generator (e.g. 74HC4511) with either a Multiplexer/latch (e.g. 4512) or a shift register (e.g. 74HC164) to control the selection of LED digits and the bar graph LED,
  • An all in 1 display driver (e.g. MAX7219) to manage the entire clock display.
  • A display panel with an integrated MAX7219 such as this MAX7219 display panel
  • A 16 bit latch/multiplexer (e.g. MAX306) to manage the LED bar graph.
    If you want to light up all of the remaining days (e.g. 3 days left = 3 LEDs on), you might also want a handful of diodes - I'll leave it as an exercise for you to figure out how to use the diodes...
  • No doubt there are plenty of other options.

No doubt there are other cool ideas that could be added on to further enhance this project. I'd be interested to hear about any enhancements you have made in the comments section...

Enjoy your quarantine / self isolation and remember it will come to an end eventually - maybe sooner than you expect if you keep yourself occupied...