Reducing a variable every x seconds [Arduino Uno]

Using Arduino Project Guidance
#1

Hi guys, I am new here.

I would just like to ask a question if possible. I used the TimerOne library to create a timer that starts at 0 minutes and 0 seconds, and then goes up by 1 second like any normal timer. For my project, I need to reduce a variable by 1 every x seconds after a certain point, so for example at 0:10 a variable needs to be reduce by -1 every 5 seconds and cannot go below 0. I am totally unsure how to code this, any guidance would be much appreciated.

#include <TimerOne.h>
#include <Adafruit_RGBLCDShield.h>
#include <utility/Adafruit_MCP23017.h>

Adafruit_RGBLCDShield lcd = Adafruit_RGBLCDShield();

int Seconds = -2;
int Minutes = 0;

void setup() {
Serial.begin(9600);
lcd.begin(16, 2);
Timer1.initialize(1000000);
Timer1.attachInterrupt(timer);
}

void timer(){
Seconds++;
}

void loop() {
  if(Seconds==60) {
    lcd.clear();
    Minutes++;
    Seconds=0;
  }

lcd.setCursor(6,1);
  lcd.print(Seconds);
  lcd.setCursor(5,1);
  lcd.print(":");
  lcd.setCursor(4,1);
  lcd.print(Minutes);
  lcd.setCursor(0,1);
  lcd.print("Age");

This is what I have so far for the timer. Any help is much appreciated, thank you in advance.

#2

Where ever in your code the 1 second has elapsed, decrement your variable. If the value is -1, set it back to whatever original value you want it to be.

#3

ieee488:
Where ever in your code the 1 second has elapsed, decrement your variable. If the value is -1, set it back to whatever original value you want it to be.

Hi, thank you for the feedback. I don't really know how to implement that, could you possibly give me an example please?

#4

You can use -- to decrement (count down) just as you can use ++ to increment (count up).

Seconds-- ;

Or,
Seconds = Seconds -1 ;

Then use an [u]if-statement[/u] to stop when you get down to zero.

I am totally unsure how to code this, any guidance would be much appreciated.

I suggest you start with a down-counter that simply counts and displays the results. Then you can add the if-statement. Then add the timing. Then add the code to take care of the minutes and seconds, etc. ...Just work on one little thing at a time and "develop" your program. Don't try to write the whole program at once.

The most common way to make a counter that stops at a certain count is to use a [u]for-loop[/u], but the timing makes it a bit more complicated.

#5 
    static unsigned lastSeconds = 0;
    if (Seconds > lastSeconds )
    {  // Time has changed
        lastSeconds = Seconds;
        // Starting at 10 seconds, every 5 seconds,  decrement the Variable by 1 until it reaches 0.
        if (Seconds >= 10 &&  (Seconds % 5) == 0  && Variable >= 1)
        {
            Variable--;
        }
    }
#6