How to add a countdown timer with pause feature?

Projects General Guidance
25

I don't think you will ever see 1:00 on the display. One microsecond after you start the timer, the remaining time is 59.999 seconds which will display as 0:59.

#include <LiquidCrystal_I2C.h>
LiquidCrystal_I2C lcd = LiquidCrystal_I2C(0x27, 16, 2);

const int TriggerPin = 8; //set pins for the ultrasonic sensor, button, BuzzerPin and LCD
const int EchoPin = 9;
const int ButtonPin = 2;
const int BuzzerPin = 13;

const unsigned long SECOND = 1000ul;
const unsigned long MINUTE = SECOND * 60;

bool TimerRunning = false; // Start paused
unsigned long TimeRemaining = MINUTE;
unsigned long TimeStarted = 0;

void setup()
{
  Serial.begin(115200);
  delay(200); // Allow time for Serial Monitor to connect

  //set pin modes
  pinMode(TriggerPin, OUTPUT);
  pinMode(EchoPin, INPUT);
  pinMode(ButtonPin, INPUT);
  pinMode(BuzzerPin, OUTPUT);

  lcd.begin (16, 2);
  lcd.init();
  lcd.backlight();
}

void pause()
{
  if (TimerRunning)
  {
    TimeRemaining = TimeRemaining - (millis() - TimeStarted);
    TimerRunning = false;
  }
}

void resume()
{
  if (!TimerRunning)
  {
    TimeStarted = millis();
    TimerRunning = true;
  }
}

void timer()
{
  unsigned long currentTime = millis();
  long displayTime;

  if (TimerRunning)
  {
    unsigned long elapsedTime = currentTime - TimeStarted;
    if (elapsedTime < TimeRemaining)
    {
      displayTime = TimeRemaining - elapsedTime;
    }
    else
    {
      displayTime = 0;
    }
  }
  else
  {
    // Timer is paused.  Show the remaining time
    displayTime = TimeRemaining;
  }

  byte countdown_minute = ((displayTime / 1000) / 60) % 60;
  byte countdown_sec = (displayTime / 1000) % 60;
  
  lcd.setCursor(6, 1);
  if (countdown_minute < 10)
  {
    lcd.print(' ');
  }
  lcd.print(countdown_minute);
  lcd.print(":");
  if (countdown_sec < 10)
  {
    lcd.print('0');
  }
  lcd.print(countdown_sec);

  // Time has expired
  if (displayTime == 0)
  {
    pause();
    tone(BuzzerPin, 1000, 200);
    delay(100);
    tone(BuzzerPin, 1000, 200);
    delay(100);
    // Reset the timer
    TimeRemaining = MINUTE;
  }
}

// Speed of sound: 343 meters per second == 0.002915452 seconds per meter
const float MicrosecondsPerCM = 29.15452;
const float MicrosecondsPerRoundTripCM = MicrosecondsPerCM * 2.0;  // ~58.3

void loop()
{
  digitalWrite(TriggerPin, LOW); // send out an ultra sonic sound for 5 microseconds and measure the time it took for the sound to go from the trigpin to the echo pin
  delayMicroseconds(5);
  digitalWrite(TriggerPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(TriggerPin, LOW);
  unsigned long signalDuration = pulseIn(EchoPin, HIGH, 6000);  // Timeout good for 100 cm.
  int plankDist = signalDuration / MicrosecondsPerRoundTripCM; //convert the time the signal took to travel to distance in cm

  //proper posture within the distance range of 10cm to 40cm, timer continues counting
  if (plankDist >= 10 && plankDist <= 40)
  {
    resume();
    lcd.clear();
    lcd.setCursor(0, 0);
    lcd.print("Maintain Posture");
    lcd.setCursor(0, 1);
    lcd.print("Time:");
    lcd.setCursor(10, 1);
    timer();                           //void timer() displays the time here
  }
  else  // if outside of the above stated range of 10cm to 40cm, timer pauses as posture is improper
  {
    pause();
    lcd.clear();
    lcd.setCursor(0, 0);
    lcd.print("Adjust Position");
    lcd.setCursor(0, 1);
    lcd.print("Timer Paused");
  }
}