I am trying to program my Arduino Uno board for a 24 hour cycle. I have the Arduino board and two relays. The aim is; every 24 hours relay 1 has to switch on for 5 seconds, and 1 second later relay 2 has to switch on for 15 seconds. I have a sketch for a 12 hour cycle, and that works fine, but when I try for a 24 hour cycle, by changing the const unsigned int totalTime from 43200 seconds to 86400 seconds, everything goes wrong.

24 hour sketch - what is wrong?

//turn relays on and off at certain times.
//when the digital output pin is HIGH, then the relay is off.
//when the digital output pin is LOW, then the relay is on.

enum actioEnum {

struct tableStruct{
  unsigned int time;
  actionEnum action;
const table [] = {
  // remaining 86395 for a total of 86400.
  {0,RELAY2_OFF}, // not needed, it is already off.
  // remaining 86379 for a total of 86400.

const unsigned int totalTime = 86400; // total amount for sequence
unsigned int seconds;

unsigned long previousMillis;
const unsigned long interval = 1000; // 1000 ms; // 1000 ms is one second

const int relay1Pin = 2;
const int relay2Pin = 3;

void setup() {

  // the relay is off when the pin is high.
  // turn output high before setting the pinMode.
  // then the output changes from a floating pin to high,
  // and the relays will not be activated.
  digitalWrite (relay1Pin, HIGH);
  pinMode (relay1Pin, OUTPUT);
  digitalWrite (relay2Pin, HIGH);
  pinMode (relay2Pin, OUTPUT);

void loop () {
  // a one second timer with millis
  if (millis() - previousMillis >= interval) {
    previousMillis += interval;    
    // scan table
    for ( int i = 0; i < int (sizeof(table) / sizeof (tableStruct)); i++) {
      if (table [i] . time == seconds) {
        switch (table [i] . action) {    
          case RELAY1_ON:
            Serial.println("Relay 1 on");
            digitalWrite (relay1Pin, LOW); // low is relay on.
          case RELAY1_OFF:
            Serial.println("Relay 1 off");
            digitalWrite (relay1Pin, HIGH); // high is relay off.
          case RELAY2_ON:
            Serial.println("Relay 2 on");
            digitalWrite (relay2Pin, LOW); // low is relay on.
          case RELAY2_OFF:
            Serial.println ("Relay 2 off");
            digitalWrite (relay2Pin, HIGH); // high is relay off.
            Serial.println ("Error, a bug in the scketch");
    if (seconds >= totalTime) {
      seconds = 0;
  • Why you are not using a RTC module? – ElectronSurf Aug 8 '19 at 6:36
  • The Uno isn't the most accurate in time keeping, and can drift by a few minutes every day. Depending on your application this might not matter. However, if it does, use an RTC like newbie suggested. – Gerben Aug 8 '19 at 9:09

On the Uno unsigned int is a 16 bit number. That means, that it can hold values between (and including) 0 and 65535. If you try to write a bigger value to this variable, it will cut of the higher bits, so that the rest fits into 16 bits. That gives you a totally different value, which is significantly small than 24h.

The solution is to use a bigger variable type for totalTime. You can use unsigned long, which has 32 bits and can hold way bigger values ( 0 to 4,294,967,295 (2^32 - 1)). That should be plenty enough for all your cases.

| improve this answer | |
  • Hi, your solution is to use unsigned long instead of totalTime. how do I change this in my sketch? Regards, Martinus. – Jerry Aug 9 '19 at 21:08
  • No, not instead of totalTime, but as the type of it. In your sketch you wrote const unsigned int totalTime = 86400;, but instead you should write const unsigned long totalTime = 86400;. That changes the type of the variable totalTime from unsigned int to unsigned long. – chrisl Aug 9 '19 at 21:14

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