You will also learn about the Nokia 5110 display and the EA DOG display series. And finally, you will learn how to send an Arduino Pro Mini into deep sleep mode and wake it up again. All in one post!<\/p>
Expectation management<\/strong>: This article is not a ready-made blueprint for an alarm clock with circuit board layout, 3D print template, etc. I have also refrained from defining the acoustic alarm. For me, it was about addressing the biggest stumbling blocks from my point of view. I’ll leave the rest to your creativity.<\/p>\n\n I will not go into detail about the DS3231 module in this article. Take a look here<\/a> if you want to find out more. Nevertheless, to start with, we will begin by \u2018warming up\u2019 with the bare clock without any setting functions.<\/p> I am initially using a classic Arduino Nano to control the project. An I2C-controlled LCD is used as the display. You can replace the board and display relatively easily with your favorite devices – as long as we don’t go into battery operation.<\/p> I use Adafruit’s RTCLib<\/a> library to control the DS3231 module. You can install it via the library manager of the Arduino IDE. The same applies to the LiquidCrystal_I2C<\/a> library from John Rickman, which I use to control the LCD.<\/p> All components run on 5 volts and are controlled via I2C, which makes wiring simple:<\/p>\n\n I use the DS3231 module without a battery. Applying battery and<\/em> external power supply is not a good idea (see my DS3231 article<\/a>).<\/p>\n\n Here is the corresponding sketch:<\/p>\n\n <\/p>\r\n<\/div> The current time is set with Here is the result on the display:<\/p>\n In version 2, we turn our clock into an alarm clock. The clock time and the alarm time are set using two buttons. Here is the circuit:<\/p>\n\n <\/p>\n\n The buttons have dual functions:<\/p> Once again, in detail: If you press the “Item Key”, the display will ask you to do so for one second. When the following message appears, release it:<\/p>\n\n At first, if necessary, you can set the year. Pressing the “Value Key” increases the year by 1. Pressing it continuously increases the value in a continuous loop. After 2050, the value returns to 2025. Pressing the “Item Key” accepts the value. Then it’s the turn of the months, then the day and so on down to the minutes:<\/p>\n\n As the seconds are “zeroed” when the minutes are confirmed, you can set the time to the exact second.<\/p>\n\n The DS3231 is very flexible when it comes to setting alarms (see my article<\/a> on this). I have decided to only implement a daily alarm at a specific time. If you press the “Value Key” for one second, you enter the alarm setting mode:<\/p>\n\n As in time setting mode, you increment the value with the “Value Key” and accept it with the “Item Key”. You do this down to the minutes:<\/p>\n\n The alarm time and the activation status then appear on the display:<\/p>\n\n If you now press the “Value Key”, the display changes from “Activate: no” to “Activate: yes”. Press the “Item Key” to complete the setting. The time appears on the display again, and the activated alarm is indicated by an “A” at the bottom right:<\/p>\n\n When the alarm goes off, the following display flashes:<\/p>\n\n Of course, just a flashing display would not wake you up. This is where you need to expand the concept accordingly, for example to wake you up with your favorite song. You could use an MP3 player module such as the DFPlayer Mini<\/a> or the YX5300<\/a>.<\/p>\n\n I have programmed the alarm to stop by itself after 60 seconds. If you are not at home and your alarm clock makes a lot of noise, your neighbors will be grateful for this function. You can extend the maximum alarm period as you wish ( \r\nIf the maximum alarm time has not yet been exceeded, you can switch to snooze mode by briefly pressing one of the two buttons. I have preset the snooze time to one minute. It starts at the time you press the button. After the snooze time has elapsed, a new alarm goes off.<\/p> Snooze mode is indicated by an “S” on the display:<\/p>\n\n To switch off the alarm, go to the alarm settings mode and switch to the “Activate: no” setting.<\/p>\n\n Here is the sketch for version 2:<\/p>\n\n <\/p>\r\n<\/div> I don’t want to go through the sketch line by line and hope that it is reasonably understandable with the comments in the sketch and the following notes.<\/p> In version 2, I was annoyed by the process of switching off the alarm. I don’t like the idea of having to work my way through a setting procedure in the morning. That’s why, in version 3, I added a slide switch to Arduino pin 4 that turns the alarm on or off:<\/p>\n\n Nothing else changes in the operation. In the sketch, the corresponding item at the end of the alarm time setting has been removed. However, it is now necessary to check in <\/p>\r\n<\/div>\n\n This alarm clock concept has a current consumption of ~44 mA. The display draws around 24 mA, the Nano around 16.5 mA and the DS3231 a good 3.5 mA. This is far too much for a battery power supply.<\/p> \r\nIf you supply the DS3231 module with power via a button cell and cut the line to VCC, the module’s consumption is no significant. You could save approx. 20 mA by switching off the display’s backlight. To do this, you could insert the following lines into If you press one of the setting buttons, the backlight comes on for 20 seconds. This might suit you if the bright display is annoying at night. But admittedly, this is not ideal. At least during the day you would like to be able to read the display without having to press any buttons. And the remaining ~20 mA is still a killer for battery operation, unless you want to change batteries every few days.<\/p>\n\n So what we need is a low-power microcontroller board and a low-power display.<\/p>\n\n As the board, I chose the 3.3 V \/ 8 MHz version<\/strong> of the Arduino Pro Mini. Like the classic Arduino Nano, the Pro Mini is based on the ATmega328P. In normal operation, the ATmega328P is not a world champion in power saving, but in deep sleep mode its consumption can be reduced to less than < 1 \u00b5A (see also my article on sleep modes here<\/a>). <\/p> To wake up every second, we use a pin change interrupt<\/a>, which we trigger via a second alarm of the DS3231. The interrupts triggered via the push-buttons also wake up the Pro Mini. <\/p> Like almost all boards, the Pro Mini has an LED that lights up permanently when it is supplied with power. You should remove this. This can be done with a soldering iron or simply with some force, e.g. by carefully scratching it off with a screwdriver. <\/p>\n\n During my research for energy-efficient displays, I found the following alternatives:<\/p> In the following, I will discuss the Nokia 5110, the DOGS164 display from the EA DOGS series and the ST7567 based display. You find an example with an e-paper display (MH-ET LIVE) in the appendix, but only as a simple clock.<\/p> \r\nE-paper displays are the world champions when it comes to saving energy, but they do have certain peculiarities. Among other things, they require a lot of RAM and most do not have a backlight, which is not good for an alarm clock in the dark. Or you can build your own display lighting.<\/p>\n\n The DS3231 module consumes very little power in battery mode (i.e. with a button cell in the module). However, I had problems waking up the Arduino Pro Mini with the alarm interrupt of the DS3231 in battery mode. I therefore switched back to power supply via VCC. As the power LED then lights up and consumes power, I simply removed it. This means that the DS3231 module consumes around 80 \u00b5A which I considered as acceptable. <\/p>\n\n First of all, the bad news about the Nokia 5110 display: The quality of most displays leaves a lot to be desired. I have now ordered a whole series of displays from various stores. In Germany, I ordered three displays twice from a well-known store and both times two out of three did not work straight away. However, I was able to get three of the non-functioning displays to work. More on this in the appendix 1<\/a>.<\/p> Then I made a few test purchases on AliExpress. Here I got three different versions. One version was the same as the one I had already bought in Germany – but I couldn’t get it to work at all. The other versions had other problems. More about this in appendix_2<\/a>. <\/p>\n\n The good sides of this display (if it works!) are:<\/p> The following setup was used:<\/p>\n\n You can use the top or bottom connectors of the Nokia 5110 display pins, it doesn’t matter. I have connected them as follows: <\/p> The backlight lights up (in this version!) when you connect the LIGHT pin to GND. Or you can connect the LIGHT pin to an I\/O pin and switch the light on by setting the I\/O pin to OUTPUT\/LOW. Via PWM or The Nokia 5110 display is incompatible with 5 volts. However, as we are using a 3.3 volt Arduino Pro Mini, we do not need a level shifter or voltage divider. A USB-to-TTL adapter, which can be set to 3.3 volts, is ideal for programming the Arduino Pro Mini. In standard operation, you can alternatively supply the circuit with higher voltages via the 3.3-volt voltage regulator of the Arduino Pro Mini. To do this, use the RAW pin. But once again: this only works in this form with the 3.3 volt version of the Pro Mini!<\/p>\n\n I used the Nokia 5110 LCD library<\/a> from Dimitris Platis(platisd<\/a>) to control the Nokia 5110 display. I won’t go into the details here. A special feature of the library is that the cursor position is specified vertically in lines (0 to 5) and horizontally in pixels (0 to 83).<\/p> Then the hardware-specific sleep functions for the ATmega328P are added. If you want to know more details about this, take a look at this article<\/a>. The pin change interrupt functions<\/a> are also hardware-specific.<\/p> As alarm1 offers the option of triggering an alarm every second (DS3231_A1_PerSecond), we use it to wake up the Pro Mini regularly from sleep mode. We moved the alarm clock’s wake-up function to alarm 2.<\/p> I think that’s basically it. The code is simply too long to go through line by line.<\/p>\n\n <\/p>\r\n<\/div>\n\n This is how the date and time appear on the display.<\/p>\n\n With this setup (including the removed LEDs), I measured a current consumption of 0.224 mA in the sleep phase. I have determined a power consumption of 2.8 mA for the waking phase. The waking phase takes 166 ms and is therefore quite relevant. The total current consumption is approx. 0.65 mA. You can move more in the direction of 0.224 mA by dispensing with the display of seconds. We’ll get to that shortly.<\/p>\n\n If you want to do without the seconds display, only a few changes are required in the sketch:<\/p>\n\n For alarm clock version 3, I have shown you how to switch on the backlight for a limited period. This does not work if you are using sleep mode, as the timer0 responsible for Alternatively, you can use the RTC instead of The DOGS164 belongs to the EA DOG display family. They are based on an interesting modular concept. You put together the display, the backlight module and, if necessary, a touch panel according to your wishes. You can choose between various text or graphic displays. An overview is available here<\/a>.<\/p> The letter after “DOG” gives you an indication of the (font) size, i.e. S, M, L or XL. For the text displays, the next two numbers indicate the number of characters per line. The third number tells you the number of lines. <\/p> You can then select how the characters are to be shown on the display. For example, there is a DOGS164B<\/strong>, DOGS164W<\/strong> and a DOGS164N<\/strong>: <\/p> The backlight panels are also available in different versions, e.g. in amber or green\/red\/white (switchable). So take a close look at what you order! The B version, for example, is only really legible with a backlight.<\/p> I opted for the DOGS164W text display with amber-colored backlight. I’ll show you the result in advance: <\/p>\n\n A certain disadvantage of the EA DOG displays is that once they are fixed to the backlight panel, they can no longer be inserted into breadboards. The pins are then too short. I cut off jumper cables on one side and soldered them to the pins for my trials: <\/p> According to the data sheet, the backlight connections should be soldered “with a little tin from the top”. Well, it worked anyway. <\/p>\n\n The various EA DOG displays differ in terms of the number and assignment of their pins. With some displays, you have the choice between SPI (3-wire), SPI (4-wire) and I2C. The DOGS164 can be addressed via I2C or SPI (3-wire). As the DS3231 module is already connected via I2C, I did the same with the DOG display. This saved three Arduino pins compared to SPI.<\/p>\n\n Here is the circuit without connected backlight:<\/p>\n\n A few more tips (especially for the DOGS164 \/ I2C):<\/p>\r\n
Preparation: the “basic clock” (version 1)<\/h2>\n\n
![\"Basic](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/03\/DS3231_basic_clock-1024x380.png\")
<\/a>#include <Wire.h>\r\n#include <RTClib.h> \/\/ Adafruit library for the DS3231\r\n#include <LiquidCrystal_I2C.h> \/\/ library for the LCD display\r\n\r\nRTC_DS3231 rtc; \/\/ create RTC_DS3231 object\r\nLiquidCrystal_I2C lcd(0x27,20,2); \/\/ create display object (address, columns, rows)\r\n\r\nvoid setup () {\r\n Serial.begin(115200);\r\n\r\n lcd.init(); \r\n lcd.backlight();\r\n lcd.clear();\r\n\r\n if (! rtc.begin()) {\r\n Serial.println(F(\"Couldn't find RTC\"));\r\n while(1){}\r\n }\r\n rtc.adjust(DateTime(F(__DATE__), F(__TIME__))); \/\/ set rtc to date\/time of compilation\r\n rtc.disable32K(); \/\/ stop signal at the 32K pin\r\n rtc.writeSqwPinMode(DS3231_OFF); \/\/ stop signal at the SQW pin\r\n}\r\n\r\nvoid loop() {\r\n if(millis()%1000 == 0){ \/\/ update date\/time every second\r\n printDateTime();\r\n delay(1);\r\n }\r\n}\r\n\r\nvoid printDateTime(){\r\n DateTime now = rtc.now();\r\n\r\n char datBuf[] = \"DDD, DD.MM.YYYY\"; \/\/ define the date format\r\n char timeBuf[] = \"hh:mm:ss\"; \/\/ define the time format\r\n Serial.println(now.toString(datBuf)); \/\/ print date\r\n Serial.println(now.toString(timeBuf)); \/\/ print time\r\n lcd.setCursor(0,0);\r\n lcd.print(datBuf); \/\/ display date \r\n lcd.setCursor(0,1);\r\n lcd.print(timeBuf); \/\/ display time\r\n}\r\n<\/pre>\r\n
\n\nrtc.adjust(DateTime(F(__DATE__), F(__TIME__)));<\/code>. This is the system time of your computer when the sketch is compiled. Since the upload takes some time, the clock is somewhat delayed.<\/p>![\"Alarm](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/03\/ds3231_alarm_clock_normal-1-1024x182.jpg\")
<\/a>Alarm clock – Version 2 with two buttons<\/h2>\n\n
The operating concept<\/h3>\n\n
![\"Circuit](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/03\/DS3231_alarm_clock_2_buttons-1024x368.png\")
<\/a>\r\n
\r\n
\r\n
Time setting<\/h4>\n\n
![\"Time](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/03\/ds3231_alarm_clock_se_date_time-1024x181.jpg\")
<\/a>![\"Time](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/03\/ds3231_alarm_clock_set_mins-1024x182.jpg\")
<\/a>Alarm setting<\/h4>\n\n
![\"Alarm](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/03\/ds3231_alarm_clock_set_alarm-1024x180.jpg\")
<\/a>![\"Alarm](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/03\/ds3231_alarm_clock_set_alarm_mins-1024x178.jpg\")
<\/a>![\"Alarm](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/03\/ds3231_alarm_clock_activate_alarm-1024x179.jpg\")
<\/a>![\"Date\/time](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/03\/ds3231_alarm_clock_alarm_activated-1024x181.jpg\")
<\/a>Alarm event<\/h3>\n\n
![\"Alarm](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/03\/ds3231_alarm_clock_alarm_2-1024x180.jpg\")
<\/a>#define MAX_ALARM_DURATION<\/code>).<\/p>![\"Display](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/03\/ds3231_alarm_clock_snooze-1024x183.jpg\")
<\/a>The alarm clock sketch (version 2)<\/h3>\n\n
#include <Wire.h>\r\n#include <RTClib.h> \/\/ library for the DS3231\r\n#include <LiquidCrystal_I2C.h> \/\/ library for the LCD display\r\n#define SNOOZE_TIME 60 \/\/ set snooze time in seconds \r\n#define SET_INCREMENT_DELAY 260 \/\/ incrementation time for settings\r\n#define MAX_ALARM_DURATION 60000 \/\/ maximum duration of an alarm in milliseconds\r\nvolatile bool setItemKeyPressed = false; \/\/ flag for item key\r\nvolatile bool setValueKeyPressed = false; \/\/ flag for value key\r\nvolatile unsigned long lastLow; \/\/ needed to handle bouncing\r\nbool alarmOn = false; \/\/ alarm activated?\r\nconst int setItemPin = 2; \/\/ choose item to set \/ time setting mode\r\nconst int setValuePin = 3; \/\/ set (increment) value \/ alarm setting mode\r\n\r\ntypedef enum TIME_UNIT { \/\/ needed for struct \"timeElement\"\r\n YEAR, MONTH, DAY, HOUR, MINUTE, SECOND\r\n} timeUnit;\r\n\r\ntypedef enum SET_MODE { \/\/ setting modes\r\n DATE_TIME, ALARM\r\n} setMode;\r\n\r\nstruct timeElement{ \/\/ for date\/time\/alarm setting\r\n timeUnit tUnit; \r\n uint16_t tMin; \/\/ minimum value\r\n uint16_t tMax; \/\/ maximum value\r\n char tname[7]; \/\/ name to display\r\n};\r\n\r\ntimeElement tElementYear = {YEAR, 2025, 2050, \"Year\"};\r\ntimeElement tElementMonth = {MONTH, 1, 12, \"Month\"};\r\ntimeElement tElementDay = {DAY, 1, 31, \"Day\"};\r\ntimeElement tElementHour = {HOUR, 0, 23, \"Hour\"};\r\ntimeElement tElementMinute = {MINUTE, 0, 59, \"Minute\"};\r\n\r\nRTC_DS3231 rtc; \/\/ \/\/ create RTC_DS3231 object\r\nDateTime alarmTime = DateTime(2014, 1, 1, 7, 0, 0);\r\nLiquidCrystal_I2C lcd(0x27,20,2); \/\/ create display object (address, columns, rows)\r\n\r\nvoid setup () {\r\n Serial.begin(115200);\r\n\r\n lcd.init(); \r\n lcd.backlight();\r\n lcd.clear();\r\n\r\n if (! rtc.begin()) {\r\n Serial.println(F(\"Couldn't find RTC\"));\r\n while(1){}\r\n }\r\n rtc.adjust(DateTime(F(__DATE__), F(__TIME__))); \/\/ \/\/ set rtc to date\/time of compilation\r\n rtc.disable32K(); \/\/ stop signal at the 32K pin\r\n rtc.clearAlarm(1); \/\/ clear alarm\r\n rtc.clearAlarm(2);\r\n rtc.disableAlarm(1); \/\/ disable alarm function\r\n rtc.disableAlarm(2);\r\n rtc.writeSqwPinMode(DS3231_OFF); \/\/ stop signal at the SQW pin\r\n \r\n \/* pressing the item or value button will cause a low signal at the corresponding pin\r\n and trigger an interrupt *\/\r\n pinMode(setItemPin, INPUT_PULLUP); \r\n pinMode(setValuePin, INPUT_PULLUP);\r\n attachInterrupt(digitalPinToInterrupt(setItemPin), setItemKeyISR, FALLING);\r\n attachInterrupt(digitalPinToInterrupt(setValuePin), setValueKeyISR, FALLING);\r\n setItemKeyPressed = false;\r\n setValueKeyPressed = false;\r\n}\r\n\r\nvoid loop() {\r\n if(millis()%1000 == 0){ \/\/ update date\/time every second\r\n printDateTime();\r\n delay(1); \/\/ avoids multiple updates per second\r\n }\r\n \r\n if(setItemKeyPressed){ \/\/ item key = date\/time setting key\r\n goIntoSetMode(DATE_TIME);\r\n }\r\n\r\n if(setValueKeyPressed){ \/\/ value key = alarm key\r\n goIntoSetMode(ALARM); \r\n }\r\n \r\n if(rtc.alarmFired(1) && alarmOn){\r\n alarmAction(); \/\/ act on alarm\r\n }\r\n}\r\n\r\nvoid goIntoSetMode(setMode settingMode){\r\n lcd.clear();\r\n lcd.setCursor(0,0);\r\n lcd.print(F(\"Press 1 sec\"));\r\n delay(1000); \/\/ press 1 second to go into setting mode\r\n if (digitalRead(setItemPin) == LOW || digitalRead(setValuePin) == LOW){\r\n lcd.setCursor(0,0);\r\n lcd.print(F(\"Setting Mode\"));\r\n lcd.setCursor(0,1);\r\n if(settingMode == DATE_TIME){\r\n lcd.print(F(\"Date\/Time\"));\r\n }\r\n else{\r\n lcd.print(F(\"Alarm\"));\r\n }\r\n \/* wait until button is released *\/\r\n while(digitalRead(setItemPin) == LOW){}\r\n while(digitalRead(setValuePin) == LOW){}\r\n delay(50); \/\/ debouncing\r\n if(settingMode == DATE_TIME){\r\n setDateTime();\r\n }\r\n else{\r\n setAlarmTime();\r\n }\r\n }\r\n setItemKeyPressed = false;\r\n setValueKeyPressed = false;\r\n}\r\n\r\nvoid setDateTime(){ \/\/ date\/time setting procedure\r\n DateTime now = rtc.now(); \/\/ get current time\r\n uint16_t tItem = 0;\r\n tItem = setDateTimeElement(&tElementYear, now.year());\r\n adjustRTC(YEAR, tItem);\r\n now = rtc.now();\r\n tItem = setDateTimeElement(&tElementMonth, now.month());\r\n adjustRTC(MONTH, tItem);\r\n now = rtc.now();\r\n tItem = setDateTimeElement(&tElementDay, now.day());\r\n adjustRTC(DAY, tItem);\r\n now = rtc.now();\r\n tItem = setDateTimeElement(&tElementHour, now.hour());\r\n adjustRTC(HOUR, tItem);\r\n now = rtc.now();\r\n tItem = setDateTimeElement(&tElementMinute, now.minute());\r\n adjustRTC(MINUTE, tItem);\r\n adjustRTC(SECOND, 0);\r\n lcd.clear();\r\n if(alarmOn){\r\n lcd.setCursor(15,1);\r\n lcd.print(\"A\"); \/\/ display \"alarm is on\" flag\r\n }\r\n printDateTime();\r\n}\r\n\r\nvoid setAlarmTime(){ \/\/ procedure to set an alarm\r\n uint16_t value = 0;\r\n value = setDateTimeElement(&tElementHour, alarmTime.hour());\r\n alarmTime = DateTime(alarmTime.year(), alarmTime.month(), alarmTime.day(), value, alarmTime.minute(), 0);\r\n value = setDateTimeElement(&tElementMinute, alarmTime.minute());\r\n alarmTime = DateTime(alarmTime.year(), alarmTime.month(), alarmTime.day(), alarmTime.hour(), value, 0);\r\n lcd.clear();\r\n char alarmTimeBuf[] = \"hh:mm\";\r\n alarmTime.toString(alarmTimeBuf);\r\n lcd.setCursor(0,0);\r\n lcd.print(\"Alarm: \");\r\n lcd.print(alarmTimeBuf);\r\n lcd.setCursor(0,1);\r\n alarmOn = false;\r\n lcd.print(F(\"Activate: no \"));\r\n setItemKeyPressed = false;\r\n setValueKeyPressed = false;\r\n while(!setItemKeyPressed && !setValueKeyPressed){}\r\n if(setValueKeyPressed){\r\n setValueKeyPressed = false;\r\n while(!setItemKeyPressed){\r\n if(digitalRead(setValuePin) == LOW){\r\n if(alarmOn){\r\n lcd.setCursor(10,1);\r\n lcd.print(\"no \");\r\n alarmOn = false;\r\n rtc.disableAlarm(1);\r\n delay(500); \r\n }\r\n else{\r\n lcd.setCursor(10,1);\r\n lcd.print(F(\"yes \"));\r\n alarmOn = true;\r\n rtc.setAlarm1(alarmTime, DS3231_A1_Hour);\r\n delay(500);\r\n }\r\n } \r\n }\r\n }\r\n lcd.clear();\r\n if(alarmOn){\r\n lcd.setCursor(15,1);\r\n lcd.print(\"A\");\r\n }\r\n printDateTime();\r\n}\r\n\r\nvoid alarmAction(){\r\n lcd.clear();\r\n unsigned long alarmStart = millis();\r\n \/* alarm was fired, not key pressed, and alarm did not exceed max. duration *\/\r\n while(!setItemKeyPressed && !setValueKeyPressed && (millis() - alarmStart < MAX_ALARM_DURATION)){\r\n lcd.setCursor(5,0);\r\n lcd.print(F(\"Alarm!\"));\r\n delay(500); \r\n lcd.clear();\r\n delay(500);\r\n if(millis() < alarmStart) { \/\/ if millis() overflow occured\r\n alarmStart = 0;\r\n }\r\n }\r\n \/* wait till buttons are released: *\/\r\n while(digitalRead(setItemPin) == LOW || digitalRead(setValuePin) == LOW){} \r\n rtc.clearAlarm(1);\r\n delay(50); \/\/ debouncing\r\n DateTime now = rtc.now();\r\n if(millis() - alarmStart < MAX_ALARM_DURATION){\r\n alarmTime = now + TimeSpan(SNOOZE_TIME); \/\/ set new alarm (snooze)\r\n rtc.setAlarm1(alarmTime, DS3231_A1_Hour);\r\n lcd.setCursor(15,1);\r\n lcd.print(\"S\"); \/\/ display snooze flag\r\n }\r\n else{\r\n alarmOn = false;\r\n }\r\n setItemKeyPressed = false;\r\n setValueKeyPressed = false;\r\n}\r\n\r\n\/* This functions sets a time element. Pressing the value key increments the value until\r\n the maximum value is reached. Pressing the item key will return. *\/ \r\nuint16_t setDateTimeElement(timeElement *tE, uint16_t currentSet){\r\n char buf[13];\r\n clearLineLCD(0);\r\n lcd.setCursor(0,0);\r\n sprintf(buf, \"Set %s: \", tE->tname);\r\n lcd.print(buf);\r\n lcd.setCursor(12,0);\r\n sprintf(buf, \"%02d\", currentSet);\r\n lcd.print(buf);\r\n setItemKeyPressed = false;\r\n while(!setItemKeyPressed){ \r\n if(setValueKeyPressed){\r\n while(digitalRead(setValuePin) == LOW){\r\n currentSet++;\r\n if(currentSet > tE->tMax){\r\n currentSet = tE->tMin;\r\n }\r\n lcd.setCursor(12,0);\r\n sprintf(buf, \"%02d\", currentSet);\r\n lcd.print(buf);\r\n delay(SET_INCREMENT_DELAY); \r\n }\r\n }\r\n }\r\n setItemKeyPressed = false;\r\n return currentSet;\r\n}\r\n\r\nvoid printDateTime(){ \/\/ display date\/time \r\n DateTime now = rtc.now();\r\n\r\n char datBuf[] = \"DDD, DD.MM.YYYY\"; \/\/ define the date format\r\n char timeBuf[] = \"hh:mm:ss\"; \/\/ \/\/ define the time format\r\n Serial.println(now.toString(datBuf)); \/\/ print date\r\n Serial.println(now.toString(timeBuf)); \/\/ print time\r\n lcd.setCursor(0,0);\r\n lcd.print(datBuf); \/\/ display date \r\n lcd.setCursor(0,1);\r\n lcd.print(timeBuf); \/\/ display time\r\n}\r\n\r\nvoid adjustRTC(timeUnit tU, uint16_t value){ \/\/ adjust year, month, day etc.\r\n DateTime now = rtc.now();\r\n switch(tU){\r\n case(YEAR):\r\n rtc.adjust(DateTime(value, now.month(), now.day(), now.hour(), now.minute(), now.second()));\r\n break;\r\n case(MONTH):\r\n rtc.adjust(DateTime(now.year(), value, now.day(), now.hour(), now.minute(), now.second()));\r\n break;\r\n case(DAY):\r\n rtc.adjust(DateTime(now.year(), now.month(), value, now.hour(), now.minute(), now.second()));\r\n break;\r\n case(HOUR):\r\n rtc.adjust(DateTime(now.year(), now.month(), now.day(), value, now.minute(), now.second()));\r\n break;\r\n case(MINUTE):\r\n rtc.adjust(DateTime(now.year(), now.month(), now.day(), now.hour(), value, now.second()));\r\n break;\r\n case(SECOND):\r\n rtc.adjust(DateTime(now.year(), now.month(), now.day(), now.hour(), now.minute(), value));\r\n break;\r\n }\r\n delay(10);\r\n}\r\n\r\nvoid clearLineLCD(int line){ \/\/ clears on line on the display\r\n char buf[17];\r\n sprintf(buf, \"%16s\", \" \");\r\n lcd.setCursor(0, line);\r\n lcd.print(buf);\r\n}\r\n\r\nvoid setItemKeyISR(){ \/\/ ISR for pressed item key\r\n if(millis()-lastLow >= 300){ \/\/ ignore bouncing when button is released\r\n setItemKeyPressed = true;\r\n }\r\n lastLow = millis();\r\n}\r\n\r\nvoid setValueKeyISR(){ \/\/ ISR for pressed value key\r\n if(millis()-lastLow >= 50){ \/\/ ignore bouncing when button is released\r\n setValueKeyPressed = true;\r\n }\r\n lastLow = millis();\r\n}<\/pre>\r\n
\n\nExplanations of the code<\/h4>\n\n
if(millis()-lastLow >= 300)<\/code> ensures that bouncing is not evaluated as a button press when the button is released. <\/li>\r\nsetItemKeyPressed<\/code> \/ setValueKeyPressed<\/code>), which causes the setting function goIntoSetMode()<\/code> to be called (provided the sketch is in loop()<\/code> ).<\/li>\r\ngoIntoSetMode()<\/code> it then goes to setDateTime()<\/code> or setAlarmTime()<\/code>. As the setting of years, months, days, hours and minutes always follows the same pattern, I have written the function setDateTimeElement()<\/code> for this purpose. This function is passed a structure of the type timeElement<\/code>, which specifies what the value is, its minimum and maximum and the designation on the display. Furthermore, the current value of the time element is passed and the value to be set is returned. <\/li>\r\nadjustRTC()<\/code>.<\/li><\/ul>\n\nAlarm clock – Version 3 with two buttons and a slide switch<\/h2>\n\n
![\"Alarm](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/03\/DS3231_alarm_clock_with_switch-1024x355.png\")
<\/a>loop()<\/code> whether the alarm has been switched on or off. To do this, the status of pin 4 is compared with the alarmOn flag.\r\n<\/p>
\n\n#include <Wire.h>\r\n#include <RTClib.h> \/\/ library for the DS3231\r\n#include <LiquidCrystal_I2C.h> \/\/ library for the LCD display\r\n#define SNOOZE_TIME 60 \/\/ set snooze time\r\n#define SET_INCREMENT_DELAY 260 \/\/ incrementation time for settings\r\n#define MAX_ALARM_DURATION 20000 \/\/ maximum duration of an alarm\r\nvolatile bool setItemKeyPressed = false; \/\/ flag for item key\r\nvolatile bool setValueKeyPressed = false; \/\/ flag for value key\r\nvolatile unsigned long lastLow; \/\/ needed to handle bouncing\r\nbool alarmOn = false; \/\/ alarm activated\r\nconst int setItemPin = 2; \/\/ choose item to set \/ time setting mode\r\nconst int setValuePin = 3; \/\/ set (increment) value \/ alarm setting mode\r\nconst int alarmPin = 4; \/\/ Pin for the alarm switch\r\n\r\ntypedef enum TIME_UNIT { \/\/ needed for struct \"timeElement\"\r\n YEAR, MONTH, DAY, HOUR, MINUTE, SECOND\r\n} timeUnit;\r\n\r\ntypedef enum SET_MODE { \/\/ setting modes\r\n DATE_TIME, ALARM\r\n} setMode;\r\n\r\nstruct timeElement{ \/\/ for date\/time\/alarm setting\r\n timeUnit tUnit;\r\n uint16_t tMin; \/\/ minimum value\r\n uint16_t tMax; \/\/ maximum value\r\n char tname[7]; \/\/ name to display\r\n};\r\n\r\ntimeElement tElementYear = {YEAR, 2025, 2050, \"Year\"};\r\ntimeElement tElementMonth = {MONTH, 1, 12, \"Month\"};\r\ntimeElement tElementDay = {DAY, 1, 31, \"Day\"};\r\ntimeElement tElementHour = {HOUR, 0, 23, \"Hour\"};\r\ntimeElement tElementMinute = {MINUTE, 0, 59, \"Minute\"};\r\n\r\nRTC_DS3231 rtc; \/\/ create RTC_DS3231 object\r\nDateTime alarmTime = DateTime(2014, 1, 1, 7, 0, 0);\r\nLiquidCrystal_I2C lcd(0x27,20,2); \/\/ create display object (address, columns, rows)\r\n\r\nvoid setup () {\r\n Serial.begin(115200);\r\n\r\n lcd.init(); \r\n lcd.backlight();\r\n lcd.clear();\r\n\r\n if (! rtc.begin()) {\r\n Serial.println(F(\"Couldn't find RTC\"));\r\n while(1){}\r\n }\r\n rtc.adjust(DateTime(F(__DATE__), F(__TIME__))); \/\/ \/\/ set rtc to date\/time of compilation\r\n rtc.disable32K(); \/\/ stop signal at the 32K pin\r\n rtc.clearAlarm(1);\r\n rtc.clearAlarm(2);\r\n rtc.disableAlarm(1);\r\n rtc.disableAlarm(2);\r\n rtc.writeSqwPinMode(DS3231_OFF); \/\/ stop signal at the SQW pin\r\n \r\n \/* pressing the item or value button will cause a low signal at the corresponding pin\r\n and trigger an interrupt *\/\r\n pinMode(setItemPin, INPUT_PULLUP);\r\n pinMode(setValuePin, INPUT_PULLUP);\r\n pinMode(alarmPin, INPUT_PULLUP);\r\n attachInterrupt(digitalPinToInterrupt(setItemPin), setItemKeyISR, FALLING);\r\n attachInterrupt(digitalPinToInterrupt(setValuePin), setValueKeyISR, FALLING);\r\n setItemKeyPressed = false;\r\n setValueKeyPressed = false;\r\n}\r\n\r\nvoid loop() {\r\n if(millis()%1000 == 0){ \/\/ update date\/time every second\r\n printDateTime();\r\n delay(1);\r\n }\r\n \r\n if(setItemKeyPressed){ \/\/ item key = date\/time setting key\r\n goIntoSetMode(DATE_TIME);\r\n }\r\n\r\n if(setValueKeyPressed){ \/\/ value key = alarm key\r\n goIntoSetMode(ALARM); \r\n }\r\n \r\n if(rtc.alarmFired(1) && alarmOn){\r\n alarmAction(); \/\/ act on alarm\r\n }\r\n\r\n checkAlarmSetting(); \/\/ check whether alarm was (de-)activated\r\n \r\n}\r\n\r\nvoid checkAlarmSetting(){\r\n if(!digitalRead(alarmPin) && !alarmOn){\r\n alarmOn = true;\r\n lcd.setCursor(15,1);\r\n lcd.print(\"A\"); \/\/ display alarm flag\r\n rtc.setAlarm1(alarmTime, DS3231_A1_Hour);\r\n }\r\n else if (digitalRead(alarmPin) && alarmOn){\r\n alarmOn = false;\r\n lcd.setCursor(15,1);\r\n lcd.print(\" \"); \/\/ delete alam flag\r\n rtc.disableAlarm(1);\r\n }\r\n}\r\n\r\nvoid goIntoSetMode(setMode settingMode){\r\n alarmOn = false;\r\n lcd.clear();\r\n lcd.setCursor(0,0);\r\n lcd.print(F(\"Press 1 sec\"));\r\n delay(1000); \/\/ press 1 second to go into setting mode\r\n if (digitalRead(setItemPin) == LOW || digitalRead(setValuePin) == LOW){\r\n lcd.setCursor(0,0);\r\n lcd.print(F(\"Setting Mode\"));\r\n lcd.setCursor(0,1);\r\n if(settingMode == DATE_TIME){\r\n lcd.print(F(\"Date\/Time\"));\r\n }\r\n else{\r\n lcd.print(F(\"Alarm\"));\r\n }\r\n \/* wait until button is released *\/\r\n while(digitalRead(setItemPin) == LOW){}\r\n while(digitalRead(setValuePin) == LOW){}\r\n delay(50); \/\/ debouncing\r\n if(settingMode == DATE_TIME){\r\n setDateTime();\r\n }\r\n else{\r\n setAlarmTime();\r\n }\r\n }\r\n setItemKeyPressed = false;\r\n setValueKeyPressed = false;\r\n}\r\n\r\nvoid setDateTime(){ \/\/ date\/time setting procedure\r\n DateTime now = rtc.now(); \/\/ get current time\r\n uint16_t tItem = 0; \r\n tItem = setDateTimeElement(&tElementYear, now.year());\r\n adjustRTC(YEAR, tItem);\r\n now = rtc.now();\r\n tItem = setDateTimeElement(&tElementMonth, now.month());\r\n adjustRTC(MONTH, tItem);\r\n now = rtc.now();\r\n tItem = setDateTimeElement(&tElementDay, now.day());\r\n adjustRTC(DAY, tItem);\r\n now = rtc.now();\r\n tItem = setDateTimeElement(&tElementHour, now.hour());\r\n adjustRTC(HOUR, tItem);\r\n now = rtc.now();\r\n tItem = setDateTimeElement(&tElementMinute, now.minute());\r\n adjustRTC(MINUTE, tItem);\r\n adjustRTC(SECOND, 0);\r\n lcd.clear();\r\n if(alarmOn){\r\n lcd.setCursor(15,1);\r\n lcd.print(\"A\"); \/\/ display \"alarm is on\" flag\r\n }\r\n printDateTime();\r\n}\r\n\r\nvoid setAlarmTime(){ \/\/ procedure to set an alarm\r\n uint16_t value = 0;\r\n value = setDateTimeElement(&tElementHour, alarmTime.hour());\r\n alarmTime = DateTime(alarmTime.year(), alarmTime.month(), alarmTime.day(), value, alarmTime.minute(), 0);\r\n value = setDateTimeElement(&tElementMinute, alarmTime.minute());\r\n alarmTime = DateTime(alarmTime.year(), alarmTime.month(), alarmTime.day(), alarmTime.hour(), value, 0);\r\n lcd.clear();\r\n char alarmTimeBuf[] = \"hh:mm\";\r\n alarmTime.toString(alarmTimeBuf);\r\n lcd.setCursor(0,0);\r\n lcd.print(\"Alarm: \");\r\n lcd.print(alarmTimeBuf);\r\n delay(1000); \/\/ Show Alarm Time for one second\r\n lcd.clear();\r\n printDateTime();\r\n}\r\n\r\nvoid alarmAction(){\r\n lcd.clear();\r\n unsigned long alarmStart = millis();\r\n \/* alarm was fired, not key pressed, and alarm did not exceed max. duration *\/\r\n while(!setItemKeyPressed && !setValueKeyPressed && (millis() - alarmStart < MAX_ALARM_DURATION)){\r\n lcd.setCursor(5,0);\r\n lcd.print(F(\"Alarm!\"));\r\n delay(500); \r\n lcd.clear();\r\n delay(500);\r\n if(millis() < alarmStart) { \/\/ if millis() overflow occured\r\n alarmStart = 0;\r\n }\r\n }\r\n \/* wait till buttons are released: *\/\r\n while(digitalRead(setItemPin) == LOW || digitalRead(setValuePin) == LOW){}\r\n rtc.clearAlarm(1);\r\n delay(50); \/\/ debouncing\r\n DateTime now = rtc.now();\r\n if(millis() - alarmStart < MAX_ALARM_DURATION){\r\n alarmTime = now + TimeSpan(SNOOZE_TIME); \/\/ set new alarm (snooze)\r\n rtc.setAlarm1(alarmTime, DS3231_A1_Hour);\r\n lcd.setCursor(15,1);\r\n lcd.print(\"S\"); \/\/ display snooze flag\r\n }\r\n else{\r\n alarmOn = false;\r\n }\r\n setItemKeyPressed = false;\r\n setValueKeyPressed = false;\r\n}\r\n\r\n\/* This functions sets a time element. Pressing the value key increments the value until\r\n the maximum value is reached. Pressing the item key will return. *\/ \r\nuint16_t setDateTimeElement(timeElement *tE, uint16_t currentSet){\r\n char buf[13];\r\n clearLineLCD(0);\r\n lcd.setCursor(0,0);\r\n sprintf(buf, \"Set %s: \", tE->tname);\r\n lcd.print(buf);\r\n lcd.setCursor(12,0);\r\n sprintf(buf, \"%02d\", currentSet);\r\n lcd.print(buf);\r\n setItemKeyPressed = false;\r\n while(!setItemKeyPressed){ \r\n if(setValueKeyPressed){\r\n while(digitalRead(setValuePin) == LOW){\r\n currentSet++;\r\n if(currentSet > tE->tMax){\r\n currentSet = tE->tMin;\r\n }\r\n lcd.setCursor(12,0);\r\n sprintf(buf, \"%02d\", currentSet);\r\n lcd.print(buf);\r\n delay(SET_INCREMENT_DELAY); \r\n }\r\n }\r\n }\r\n setItemKeyPressed = false;\r\n return currentSet;\r\n}\r\n\r\nvoid printDateTime(){ \/\/ display date\/time\r\n DateTime now = rtc.now();\r\n\r\n char datBuf[] = \"DDD, DD.MM.YYYY\"; \/\/ define the date format\r\n char timeBuf[] = \"hh:mm:ss\"; \/\/ define the time format\r\n Serial.println(now.toString(datBuf)); \/\/ print date \r\n Serial.println(now.toString(timeBuf)); \/\/ print time \r\n lcd.setCursor(0,0);\r\n lcd.print(datBuf); \/\/ display date\r\n lcd.setCursor(0,1);\r\n lcd.print(timeBuf); \/\/ display time\r\n}\r\n\r\nvoid adjustRTC(timeUnit tU, uint16_t value){ \/\/ adjust year, month, day etc.\r\n DateTime now = rtc.now();\r\n switch(tU){\r\n case(YEAR):\r\n rtc.adjust(DateTime(value, now.month(), now.day(), now.hour(), now.minute(), now.second()));\r\n break;\r\n case(MONTH):\r\n rtc.adjust(DateTime(now.year(), value, now.day(), now.hour(), now.minute(), now.second()));\r\n break;\r\n case(DAY):\r\n rtc.adjust(DateTime(now.year(), now.month(), value, now.hour(), now.minute(), now.second()));\r\n break;\r\n case(HOUR):\r\n rtc.adjust(DateTime(now.year(), now.month(), now.day(), value, now.minute(), now.second()));\r\n break;\r\n case(MINUTE):\r\n rtc.adjust(DateTime(now.year(), now.month(), now.day(), now.hour(), value, now.second()));\r\n break;\r\n case(SECOND):\r\n rtc.adjust(DateTime(now.year(), now.month(), now.day(), now.hour(), now.minute(), value));\r\n break;\r\n }\r\n delay(10);\r\n}\r\n\r\nvoid clearLineLCD(int line){ \/\/ clears on line on the display\r\n char buf[17];\r\n sprintf(buf, \"%16s\", \" \");\r\n lcd.setCursor(0, line);\r\n lcd.print(buf);\r\n}\r\n\r\nvoid setItemKeyISR(){ \/\/ ISR for pressed item key\r\n if(millis()-lastLow >= 300){ \/\/ ignore bouncing when button is released\r\n setItemKeyPressed = true;\r\n }\r\n lastLow = millis();\r\n}\r\n\r\nvoid setValueKeyISR(){ \/\/ ISR for pressed value key\r\n if(millis()-lastLow >= 50){ \/\/ ignore bouncing when button is released\r\n setValueKeyPressed = true;\r\n }\r\n lastLow = millis();\r\n}<\/pre>\r\nPower consumption of versions 1 – 3<\/h2>\n\n
loop()<\/code>: <\/p>\n\nif(millis()-lastLow < 20000){\r\n lcd.backlight();\r\n}\r\nelse lcd.noBacklight();<\/pre>\n\nConcept for battery operation<\/h2>\n\n
Microcontroller board<\/h3>\n\n
Display <\/h3>\n\n
\r\n
DS3231<\/h3>\n\n
Alarm clock – Version 4 with Nokia 5110 display<\/h2>\n
![\"Nokia](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/04\/nokia_5110.jpg\")
<\/a>\r\n
\n\n![\"\"](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/04\/ds3231_nokia5110_alarm_clock-1-1024x491.png\")
<\/a>\r\n
![\"Nokia](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/04\/nokia_5110_displ_backjpg.jpg\")
<\/a>A few remarks<\/h4>\n\n
analogWrite()<\/code> you could dim it.<\/p>The alarm clock sketch – Version 4<\/h3>\n\n
#include <Wire.h>\r\n#include <RTClib.h> \/\/ library for the DS3231\r\n#include <Nokia_LCD.h> \/\/ Nokia 5110 display lib\r\n#include \"Bold_LCD_Fonts.h\" \/\/ bold fonts for time display\r\n#include <avr\/sleep.h> \/\/ hardware-specific sleep library for AVR MCUs\r\n#define SNOOZE_TIME 60 \/\/ set snooze time\r\n#define SET_INCREMENT_DELAY 260 \/\/ incrementation time for settings\r\n#define MAX_ALARM_DURATION 20000 \/\/ maximum duration of an alarm\r\nvolatile bool setItemKeyPressed = false; \/\/ flag for item key\r\nvolatile bool setValueKeyPressed = false; \/\/ flag for value key\r\nvolatile unsigned long lastLow; \/\/ needed to handle bouncing\r\nbool alarmOn = false; \/\/ alarm activated\r\nconst int setItemPin = 2; \/\/ choose item to set \/ time setting mode\r\nconst int setValuePin = 3; \/\/ set (increment) value \/ alarm setting mode\r\nconst int alarmPin = 4; \/\/ Pin for the alarm switch\r\nconst int wakeUpPin = 5;\r\nconst int backlightPin = 11;\r\n\r\ntypedef enum TIME_UNIT { \/\/ needed for struct \"timeElement\"\r\n YEAR, MONTH, DAY, HOUR, MINUTE, SECOND\r\n} timeUnit;\r\n\r\ntypedef enum SET_MODE { \/\/ setting modes\r\n DATE_TIME, ALARM\r\n} setMode;\r\n\r\nchar daysOfTheWeek[7][12] = {\"Sunday \", \"Monday \", \"Tuesday \", \"Wednesday\", \"Thursday \", \"Friday \", \"Saturday \"};\r\n\r\nstruct timeElement{ \/\/ for date\/time\/alarm setting\r\n timeUnit tUnit;\r\n uint16_t tMin; \/\/ minimum value\r\n uint16_t tMax; \/\/ maximum value\r\n char tname[7]; \/\/ name to display\r\n};\r\n\r\nconst LcdFont BoldFont{ \/\/ Bold Font for Nokia 5110 Display\r\n [](char c) {\r\n return Bold_LCD_Fonts::kFont_Table[c - 0x20];\r\n }, \/\/ method to retrieve the character\r\n Bold_LCD_Fonts::kColumns_per_character, \/\/ width of each char\r\n Bold_LCD_Fonts::hSpace, \/\/ horizontal spacing array\r\n 1 \/\/ size of horizontal spacing array\r\n};\r\n\r\ntimeElement tElementYear = {YEAR, 2025, 2050, \"Year\"};\r\ntimeElement tElementMonth = {MONTH, 1, 12, \"Month\"};\r\ntimeElement tElementDay = {DAY, 1, 31, \"Day\"};\r\ntimeElement tElementHour = {HOUR, 0, 23, \"Hour\"};\r\ntimeElement tElementMinute = {MINUTE, 0, 59, \"Minute\"};\r\n\r\nRTC_DS3231 rtc; \/\/ create RTC_DS3231 object\r\nDateTime alarmTime = DateTime(2014, 1, 1, 7, 0, 0);\r\nNokia_LCD lcd(10 \/* CLK *\/, 9 \/* DIN *\/, 8 \/* DC *\/, 7 \/* CE *\/, 6 \/* RST *\/);\r\n\r\nvoid setup () {\r\n Serial.begin(115200);\r\n ADCSRA = 0; \/\/ ADC off (would consume current in deep sleep)\r\n\r\n lcd.begin(); \r\n lcd.setContrast(60);\r\n lcd.clear(false);\r\n setBacklight(false); \/\/ Light off\r\n\r\n if (! rtc.begin()) {\r\n Serial.println(F(\"Couldn't find RTC\"));\r\n while(1){}\r\n }\r\n rtc.adjust(DateTime(F(__DATE__), F(__TIME__))); \/\/ \/\/ set rtc to date\/time of compilation\r\n rtc.disable32K(); \/\/ stop signal at the 32K pin\r\n rtc.clearAlarm(1);\r\n rtc.clearAlarm(2);\r\n rtc.disableAlarm(1);\r\n rtc.disableAlarm(2);\r\n rtc.writeSqwPinMode(DS3231_OFF); \/\/ stop signal at the SQW pin\r\n rtc.setAlarm1(alarmTime, DS3231_A1_PerSecond);\r\n \r\n \/* pressing the item or value button will cause a low signal at the corresponding pin\r\n and trigger an interrupt *\/\r\n pinMode(setItemPin, INPUT_PULLUP);\r\n pinMode(setValuePin, INPUT_PULLUP);\r\n pinMode(alarmPin, INPUT_PULLUP);\r\n attachInterrupt(digitalPinToInterrupt(setItemPin), setItemKeyISR, FALLING);\r\n attachInterrupt(digitalPinToInterrupt(setValuePin), setValueKeyISR, FALLING);\r\n PCICR = (1<<PCIE2); \/\/ enable PCINT[23:16] interrupts\r\n PCMSK2 = (1<<PCINT21); \/\/ D5 = PCINT21 for wake up\r\n setItemKeyPressed = false;\r\n setValueKeyPressed = false;\r\n}\r\n\r\nvoid loop() {\r\n\r\n printDateTime();\r\n \r\n if(digitalRead(setItemPin) == LOW){ \/\/ item key = date\/time setting key\r\n goIntoSetMode(DATE_TIME);\r\n }\r\n\r\n if(digitalRead(setValuePin) == LOW){ \/\/ value key = alarm key\r\n goIntoSetMode(ALARM); \r\n }\r\n \r\n if(rtc.alarmFired(2) && alarmOn){\r\n alarmAction(); \/\/ act on alarm\r\n }\r\n\r\n checkAlarmSetting(); \/\/ check whether alarm was (de-)activated\r\n\r\n set_sleep_mode(SLEEP_MODE_PWR_DOWN); \/\/ choose power down mode\r\n sleep_mode(); \/\/ sleep now!\r\n rtc.clearAlarm(1); \r\n}\r\n\r\nvoid checkAlarmSetting(){\r\n if(!digitalRead(alarmPin) && !alarmOn){\r\n alarmOn = true;\r\n lcd.setCursor(77,5);\r\n lcd.print(\"A\"); \/\/ display alarm flag\r\n rtc.setAlarm2(alarmTime, DS3231_A2_Hour);\r\n }\r\n else if (digitalRead(alarmPin) && alarmOn){\r\n alarmOn = false;\r\n lcd.setCursor(77,5);\r\n lcd.print(\" \"); \/\/ delete alarm flag\r\n rtc.disableAlarm(2);\r\n }\r\n}\r\n\r\nvoid goIntoSetMode(setMode settingMode){\r\n alarmOn = false;\r\n lcd.clear(false);\r\n lcd.setCursor(0,0);\r\n lcd.print(\"Press 1 sec\");\r\n delay(1000); \/\/ press 1 second to go into setting mode\r\n clearLineLCD(0);\r\n if (digitalRead(setItemPin) == LOW || digitalRead(setValuePin) == LOW){\r\n lcd.setCursor(0,0);\r\n lcd.print(\"Setting Mode\");\r\n lcd.setCursor(0,2);\r\n if(settingMode == DATE_TIME){\r\n lcd.print(\"Set Date\/Time\");\r\n }\r\n else{\r\n lcd.print(\"Set Alarm\");\r\n }\r\n \/* wait until button is released *\/\r\n while(digitalRead(setItemPin) == LOW){}\r\n while(digitalRead(setValuePin) == LOW){}\r\n delay(50); \/\/ debouncing\r\n if(settingMode == DATE_TIME){\r\n setDateTime();\r\n }\r\n else{\r\n setAlarmTime();\r\n }\r\n }\r\n rtc.clearAlarm(1); \r\n setItemKeyPressed = false;\r\n setValueKeyPressed = false;\r\n}\r\n\r\nvoid setDateTime(){ \/\/ date\/time setting procedure\r\n DateTime now = rtc.now(); \/\/ get current time\r\n uint16_t tItem = 0; \r\n tItem = setDateTimeElement(&tElementYear, now.year());\r\n adjustRTC(YEAR, tItem);\r\n now = rtc.now();\r\n tItem = setDateTimeElement(&tElementMonth, now.month());\r\n adjustRTC(MONTH, tItem);\r\n now = rtc.now();\r\n tItem = setDateTimeElement(&tElementDay, now.day());\r\n adjustRTC(DAY, tItem);\r\n now = rtc.now();\r\n tItem = setDateTimeElement(&tElementHour, now.hour());\r\n adjustRTC(HOUR, tItem);\r\n now = rtc.now();\r\n tItem = setDateTimeElement(&tElementMinute, now.minute());\r\n adjustRTC(MINUTE, tItem);\r\n adjustRTC(SECOND, 0);\r\n lcd.clear(false);\r\n if(alarmOn){\r\n lcd.setCursor(77,5);\r\n lcd.print(\"A\"); \/\/ display \"alarm is on\" flag\r\n }\r\n printDateTime();\r\n}\r\n\r\nvoid setAlarmTime(){ \/\/ procedure to set an alarm\r\n uint16_t value = 0;\r\n value = setDateTimeElement(&tElementHour, alarmTime.hour());\r\n alarmTime = DateTime(alarmTime.year(), alarmTime.month(), alarmTime.day(), value, alarmTime.minute(), 0);\r\n value = setDateTimeElement(&tElementMinute, alarmTime.minute());\r\n alarmTime = DateTime(alarmTime.year(), alarmTime.month(), alarmTime.day(), alarmTime.hour(), value, 0);\r\n lcd.clear(false);\r\n char alarmTimeBuf[] = \"hh:mm\";\r\n alarmTime.toString(alarmTimeBuf);\r\n lcd.setCursor(0,0);\r\n lcd.print(\"Alarm: \");\r\n lcd.print(alarmTimeBuf);\r\n delay(1000); \/\/ Show Alarm Time for one second\r\n lcd.clear();\r\n printDateTime();\r\n}\r\n\r\nvoid alarmAction(){\r\n lcd.clear();\r\n unsigned long alarmStart = millis();\r\n \/* alarm was fired, not key pressed, and alarm did not exceed max. duration *\/\r\n while(!setItemKeyPressed && !setValueKeyPressed && (millis() - alarmStart < MAX_ALARM_DURATION)){\r\n lcd.setCursor(23,3);\r\n lcd.print(\"Alarm!\");\r\n delay(500); \r\n lcd.clear();\r\n delay(500);\r\n if(millis() < alarmStart) { \/\/ if millis() overflow occurred\r\n alarmStart = 0;\r\n }\r\n\r\n }\r\n \/* wait till buttons are released: *\/\r\n while(digitalRead(setItemPin) == LOW || digitalRead(setValuePin) == LOW){}\r\n delay(50); \/\/ debouncing\r\n DateTime now = rtc.now();\r\n if(millis() - alarmStart < MAX_ALARM_DURATION){\r\n alarmTime = now + TimeSpan(SNOOZE_TIME); \/\/ set new alarm (snooze)\r\n rtc.setAlarm2(alarmTime, DS3231_A2_Hour); \/\/ alarm if hours\/minutes match\r\n lcd.setCursor(77,5);\r\n lcd.print(\"S\"); \/\/ display snooze flag\r\n }\r\n else{\r\n alarmOn = false;\r\n }\r\n rtc.clearAlarm(2);\r\n rtc.clearAlarm(1);\r\n setItemKeyPressed = false;\r\n setValueKeyPressed = false;\r\n}\r\n\r\n\/* This functions sets a time element. Pressing the value key increments the value until\r\n the maximum value is reached. Pressing the item key will return. *\/ \r\nuint16_t setDateTimeElement(timeElement *tE, uint16_t currentSet){\r\n char buf[13];\r\n clearLineLCD(0);\r\n lcd.setCursor(0,0);\r\n sprintf(buf, \"%s:\", tE->tname);\r\n lcd.print(buf);\r\n lcd.setCursor(47,0);\r\n sprintf(buf, \"%02d\", currentSet);\r\n lcd.print(buf);\r\n setItemKeyPressed = false;\r\n while(!setItemKeyPressed){ \r\n if(setValueKeyPressed){\r\n while(digitalRead(setValuePin) == LOW){\r\n currentSet++;\r\n if(currentSet > tE->tMax){\r\n currentSet = tE->tMin;\r\n }\r\n lcd.setCursor(47,0);\r\n sprintf(buf, \"%02d\", currentSet);\r\n lcd.print(buf);\r\n delay(SET_INCREMENT_DELAY); \r\n }\r\n }\r\n }\r\n setItemKeyPressed = false;\r\n return currentSet;\r\n}\r\n\r\nvoid printDateTime(){ \/\/ display date\/time\r\n DateTime now = rtc.now();\r\n\r\n char datBuf[] = \"DD.MM.YYYY\"; \/\/ define the date format\r\n char timeBuf[] = \"hh:mm:ss\"; \/\/ define the time format\r\n Serial.println(now.toString(datBuf)); \/\/ print date \r\n Serial.println(now.toString(timeBuf)); \/\/ print time \r\n lcd.setCursor(0,0);\r\n lcd.print(daysOfTheWeek[now.dayOfTheWeek()]);\r\n lcd.setCursor(0,2);\r\n lcd.print(datBuf); \/\/ display date\r\n lcd.setCursor(0,4);\r\n lcd.setFont(&BoldFont);\r\n lcd.print(timeBuf); \/\/ display time\r\n lcd.setDefaultFont();\r\n}\r\n\r\nvoid adjustRTC(timeUnit tU, uint16_t value){ \/\/ adjust year, month, day etc.\r\n DateTime now = rtc.now();\r\n switch(tU){\r\n case(YEAR):\r\n rtc.adjust(DateTime(value, now.month(), now.day(), now.hour(), now.minute(), now.second()));\r\n break;\r\n case(MONTH):\r\n rtc.adjust(DateTime(now.year(), value, now.day(), now.hour(), now.minute(), now.second()));\r\n break;\r\n case(DAY):\r\n rtc.adjust(DateTime(now.year(), now.month(), value, now.hour(), now.minute(), now.second()));\r\n break;\r\n case(HOUR):\r\n rtc.adjust(DateTime(now.year(), now.month(), now.day(), value, now.minute(), now.second()));\r\n break;\r\n case(MINUTE):\r\n rtc.adjust(DateTime(now.year(), now.month(), now.day(), now.hour(), value, now.second()));\r\n break;\r\n case(SECOND):\r\n rtc.adjust(DateTime(now.year(), now.month(), now.day(), now.hour(), now.minute(), value));\r\n break;\r\n }\r\n delay(10);\r\n}\r\n\r\nvoid clearLineLCD(int line){ \/\/ clears on line on the display\r\n char buf[15];\r\n sprintf(buf, \"%14s\", \" \");\r\n lcd.setCursor(0, line);\r\n lcd.print(buf);\r\n}\r\n\r\nvoid setBacklight(bool on){\r\n if(on){\r\n pinMode(backlightPin, OUTPUT);\r\n }\r\n else pinMode(backlightPin, INPUT);\r\n}\r\n\r\nvoid setItemKeyISR(){ \/\/ ISR for pressed item key\r\n if(millis()-lastLow >= 300){ \/\/ ignore bouncing when button is released\r\n setItemKeyPressed = true;\r\n }\r\n lastLow = millis();\r\n}\r\n\r\nvoid setValueKeyISR(){ \/\/ ISR for pressed value key\r\n if(millis()-lastLow >= 300){ \/\/ ignore bouncing when button is released\r\n setValueKeyPressed = true;\r\n }\r\n lastLow = millis();\r\n}\r\n\r\nISR (PCINT2_vect){} \/\/ PCINT2_vect: interrupt vector for PORTD<\/pre>\r\nThe result<\/h3>\n\n
![\"Alarm](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/03\/ds3231_nokia5110.jpg\")
<\/a>Variations<\/h3>\n\n
Minutes instead of seconds display<\/h4>\n\n
PCMSK2 = (1<<PCINT21) | (1<<PCINT20);<\/code>rtc.setAlarm1(alarmTime, DS3231_A1_Second);<\/code><\/li>char timeBuf[] = \"hh:mm\";<\/code><\/li>lcd.setCursor(23,4);<\/code><\/li>printDateTime();<\/code><\/li><\/ul>\n\nUsing the backlight<\/h4>\n\n
millis()<\/code> is off. The pragmatic solution is to only count the waking time and reduce the value for the backlight period accordingly. However, you would then have to leave the light on for at least one minute if you choose the option to display only minutes.<\/p>millis()<\/code> to measure the backlight period. With now.unixtime()<\/code> you get the time since 1.1.1970 in seconds and can use it for convenient calculations. Or you can add another slide switch that activates the backlight and switches off the sleep function. However, there is a risk of forgetting to turn off the light, draining the battery and then oversleeping. But you could also install an upper limit here. Or, or, or… – there are many options you can choose from.<\/p>\n\nAlarm clock – Version 5 with DOGS164 display<\/h2>\n\n
![\"Alarm](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/04\/ds3231_dogs164-1024x415.jpg\")
<\/a>
\n\n![\"DOGS164W](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/04\/dogs164_with_cables-1024x327.jpg\")
<\/a>Connection to the Arduino Pro Mini<\/h3>\n\n
![\"DOGS164](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/04\/DOGS164_connections-1024x387.png\")
<\/a>![\"Alarm](\"https:\/\/wolles-elektronikkiste.de\/wp-content\/uploads\/2025\/04\/DS3231_dogs164_alarm_clock-1-1024x593.png\")
<\/a>