const int btn_start = A0; // Start button A0
const int btn_stop = A1;  // Stop button A1
const int btn_reset = A2; // Reset button A2

byte hr = 00;  // hours
byte mm = 00;  // minutes
byte ss = 00;  // seconds

bool timerStart = false;  // Boolean to flip between true and false.

void setup()  
{ 
    Serial.begin(9600);

    pinMode(btn_start, INPUT_PULLUP);  //  btn_start as an input into serial
    pinMode(btn_stop,  INPUT_PULLUP);  //  btn_stop as an input into serial
    pinMode(btn_reset, INPUT_PULLUP);  //  btn_reset as an input into serial
} 

void loop() { // Runs in a loop

    if (digitalRead(btn_start) == 0) {    // If statement runs if start button is pressed
        timerStart = true; // Start stopwatch
        Serial.println("Timer Started");  // Print "Timer Started"
        delay(500); 
    }

    if (digitalRead(btn_stop) == 0) {  // If statement runs if stop button is pressed
        timerStart = false; // Stop stopwatch 
        Serial.println("Timer Stopped");  // Print "Timer Stopped"
        delay(500);  
    }

    if (digitalRead(btn_reset) == 0) {   // If statement runs if reset button is pressed
        ss = 00;  // Resets count back to zero
        mm = 00;  // Resets count back to zero
        hr = 00;  // Resets count back to zero 
        Serial.println("Timer Reset");  // Print "Timer Reset"
        delay(500);  // 
    }

    if (timerStart == true) {   // If statement runs if boolean is flipped to true
        Serial.print(mm < 10 ? "0" : "");
        Serial.print(mm);
        Serial.print(":");
        Serial.print(ss < 10 ? "0" : "");
        Serial.println(ss); // Serial prints "mm:ss", formatted
    }

    if (timerStart == true) {  // If statement runs if boolean is flipped to true
        delay(1000);
        ss = ss + 1;
        if (ss > 59) {
            ss = 0;
            mm = mm + 1;  // Timer counts time by rapidly adding a second
        }
    }
}