Arduino Countdown Timer © GPL3+

//7 segement pin number - 7 segement light - arduino pin number

//pin 11 - A = 12
//pin 7 - B = 10
//pin 4 - C = 8
//pin 2 - D = 7
//pin 1 - E = 6
//pin 10 - F = 11
//pin 5 - G = 9

//pin 3 is decimal

/*
12 11  10  9 8 7
|   |   |  | | | 
    A
   ---
F |   | B
  | G |
   ---
E |   | C
  |   |
   ---  . dec
    D
| | | | | |
1 2 3 4 5 6
*/

//pin 12 - first digit = 1
//pin 9 - second digit = 2 
//pin 8 - third digit = 3
//pin 6 fourth digit = 4

const int a = 12;
const int b = 10;
const int c = 8;
const int d = 7;
const int e = 6;
const int f = 11;
const int g = 9;
int p = 0;

int startStopReset = 13; 

const int d1 =1;
const int d2 =2;
const int d3 =3;
const int d4 =4;

long n = 60000; //start time -> CAN CHANGE TO WHATEVER TIME YOU WANT
int x = 100; 
int del = 55; //delay value
 
void setup()
{
  pinMode(d1, OUTPUT);
  pinMode(d2, OUTPUT);
  pinMode(d3, OUTPUT);
  pinMode(d4, OUTPUT);
  pinMode(a, OUTPUT);
  pinMode(b, OUTPUT);
  pinMode(c, OUTPUT);
  pinMode(d, OUTPUT);
  pinMode(e, OUTPUT);
  pinMode(f, OUTPUT);
  pinMode(g, OUTPUT);
  pinMode(p, OUTPUT);
  pinMode(startStopReset, INPUT); 
  digitalWrite(startStopReset, HIGH); 
}
 
void loop()
{
  digitalWrite (p, HIGH);

  clearLEDs();
  pickDigit(1);
  pickNumber((n/x/1000)%10);
  delayMicroseconds(del);
 
  clearLEDs();
  pickDigit(2);
  pickNumber((n/x/100)%10);
  delayMicroseconds(del);
 
  clearLEDs();
  pickDigit(3);
  dispDec(3);
  pickNumber((n/x/10)%10);
  delayMicroseconds(del);
 
  clearLEDs();
  pickDigit(4);
  pickNumber(n/x%10);
  delayMicroseconds(del);
 
  n--; //'n++' for stopwatch
 
  if (digitalRead(13) == LOW)
  {
    n = 60000; //re-start time -> CHANGE TO WHATEVER TIME YOU ORIGONALLY SET (start time)
  }
}
 
void pickDigit(int x) //changes digit
{
  digitalWrite(d1, HIGH);
  digitalWrite(d2, HIGH);
  digitalWrite(d3, HIGH);
  digitalWrite(d4, HIGH);
 
  switch(x)
  {
  case 1: 
    digitalWrite(d1, LOW); 
    break;
  case 2: 
    digitalWrite(d2, LOW); 
    break;
  case 3: 
    digitalWrite(d3, LOW);
    digitalWrite(p, HIGH); //new 
    break;
  default: 
    digitalWrite(d4, LOW); 
    break;
  }
}
 
void pickNumber(int x) //changes value of number
{
  switch(x)
  {
  default: 
    zero(); 
    break;
  case 1: 
    one(); 
    break;
  case 2: 
    two(); 
    break;
  case 3: 
    three(); 
    break;
  case 4: 
    four(); 
    break;
  case 5: 
    five(); 
    break;
  case 6: 
    six(); 
    break;
  case 7: 
    seven(); 
    break;
  case 8: 
    eight(); 
    break;
  case 9: 
    nine(); 
    break;
  }
}
 
void dispDec(int x)
{
  digitalWrite(p, LOW);
}
 
void clearLEDs()
{
  digitalWrite(a, LOW);
  digitalWrite(b, LOW);
  digitalWrite(c, LOW);
  digitalWrite(d, LOW);
  digitalWrite(e, LOW);
  digitalWrite(f, LOW);
  digitalWrite(g, LOW);
  digitalWrite(p, LOW);
}
 
void zero()
{
  digitalWrite(a, HIGH);
  digitalWrite(b, HIGH);
  digitalWrite(c, HIGH);
  digitalWrite(d, HIGH);
  digitalWrite(e, HIGH);
  digitalWrite(f, HIGH);
  digitalWrite(g, LOW);
}
 
void one()
{
  digitalWrite(a, LOW);
  digitalWrite(b, HIGH);
  digitalWrite(c, HIGH);
  digitalWrite(d, LOW);
  digitalWrite(e, LOW);
  digitalWrite(f, LOW);
  digitalWrite(g, LOW);
}
 
void two()
{
  digitalWrite(a, HIGH);
  digitalWrite(b, HIGH);
  digitalWrite(c, LOW);
  digitalWrite(d, HIGH);
  digitalWrite(e, HIGH);
  digitalWrite(f, LOW);
  digitalWrite(g, HIGH);
}
 
void three()
{
  digitalWrite(a, HIGH);
  digitalWrite(b, HIGH);
  digitalWrite(c, HIGH);
  digitalWrite(d, HIGH);
  digitalWrite(e, LOW);
  digitalWrite(f, LOW);
  digitalWrite(g, HIGH);
}
 
void four()
{
  digitalWrite(a, LOW);
  digitalWrite(b, HIGH);
  digitalWrite(c, HIGH);
  digitalWrite(d, LOW);
  digitalWrite(e, LOW);
  digitalWrite(f, HIGH);
  digitalWrite(g, HIGH);
}
 
void five()
{
  digitalWrite(a, HIGH);
  digitalWrite(b, LOW);
  digitalWrite(c, HIGH);
  digitalWrite(d, HIGH);
  digitalWrite(e, LOW);
  digitalWrite(f, HIGH);
  digitalWrite(g, HIGH);
}
 
void six()
{
  digitalWrite(a, HIGH);
  digitalWrite(b, LOW);
  digitalWrite(c, HIGH);
  digitalWrite(d, HIGH);
  digitalWrite(e, HIGH);
  digitalWrite(f, HIGH);
  digitalWrite(g, HIGH);
}
 
void seven()
{
  digitalWrite(a, HIGH);
  digitalWrite(b, HIGH);
  digitalWrite(c, HIGH);
  digitalWrite(d, LOW);
  digitalWrite(e, LOW);
  digitalWrite(f, LOW);
  digitalWrite(g, LOW);
}
 
void eight()
{
  digitalWrite(a, HIGH);
  digitalWrite(b, HIGH);
  digitalWrite(c, HIGH);
  digitalWrite(d, HIGH);
  digitalWrite(e, HIGH);
  digitalWrite(f, HIGH);
  digitalWrite(g, HIGH);
}
 
void nine()
{
  digitalWrite(a, HIGH);
  digitalWrite(b, HIGH);
  digitalWrite(c, HIGH);
  digitalWrite(d, HIGH);
  digitalWrite(e, LOW);
  digitalWrite(f, HIGH);
  digitalWrite(g, HIGH);
}