Because we can. I had a 7-segment display left over from the #Talk Ranking Machine, and this really begged to be done. Yes, it's useless, silly, and potentially even dangerous if someone left it in a public place. But it was quite fun to do too.

Both the display and the battery holder are soldered directly to the Pro Mini -- the pinout just happens to match. There is no off switch, you just have to remove the battery. The code looks like this:

const int GND_PINS[] = {8, 12, 13, A2};
const int VCC_PINS[] = {A1, 11, 6, 4, 3, A0, 7, 5};
const unsigned char DIGITS[] = {
    0b00111111,
    0b00000110,
    0b01011011,
    0b01001111,
    0b01100110,
    0b01101101,
    0b01111101,
    0b00000111,
    0b01111111,
    0b01101111,
};
const int BEEP_PIN = 2;


void show7seg(int number) {
    unsigned char digit;
    div_t result;

    for (int d = 0; d < 4; ++d) {
        result = div(number, 10);
        digit = DIGITS[result.rem % 10];
        number = result.quot;
        for (int s = 0; s < 8; ++s) {
            digitalWrite(VCC_PINS[s], digit & (1 << s));
        }
        pinMode(GND_PINS[d], OUTPUT);
        digitalWrite(GND_PINS[d], LOW);
        delay(2);
        pinMode(GND_PINS[d], INPUT);
    }
}



void setup() {
    for (int s = 0; s < 8; ++s) {
        pinMode(VCC_PINS[s], OUTPUT);
    }
    for (int d = 0; d < 4; ++d) {
        pinMode(GND_PINS[d], INPUT);
    }
    pinMode(9, OUTPUT);
    digitalWrite(9, LOW);
    pinMode(10, OUTPUT);
    digitalWrite(10, HIGH);
}


void loop() {
    static long int last = 0;
    static int count = 2400;
    long int now = millis();

    if (now - last > 500) {
        digitalWrite(10, LOW);
    }
    if (now - last > 1000) {
        count -= 1;
        if (count % 100 == 99) {
            count -= 40;
        }
        last = now;
        tone(BEEP_PIN, 1200, 25);
        digitalWrite(10, HIGH);
    }

    show7seg(count);
}

And that's pretty much it. I might actually make it a game, by adding several wires that you have to cut, that would either stop the counter or make it go faster or something...