Bomb!! (For Airsoft Gaming....)

In this circuit I'm presenting you an airsoft bomb, the circuit is based on the excellent code by ntewinkel at instructables but with some minor modifications. I made it for a friend for mine so he can dress up as an terrorist for the carnival. I think the whole concept went quite well. In the circuit we have 3 wires and one button. Button is for adding seconds as more you press it, time is increasing. As for the 3 wires, One (the correct!!!) is for stopping time, the second is for decreasing time faster and the third one is for resetting time.
Basic changes I've made in comparison with ntewinkel's circuit are :
a) Using common cathode displays and not the LTC clock he suggests.
b) I didn't want my arduino to power the displays directly, so i've added some npn transistors.
c) D13 of arduino is an output which you can connect anything when time is over, I've programmed it so it can make a task for a small amount of time (in my occasion power a siren for 2 secs).
d)Add some resistors for more protection.

In the schematic you can find the connections but I'm writing here the values in case you can't read it well.

button:
5v - Button - A5 - 10k Resistor - gnd

crazy wires:
5v - RED jumper wire - A4 - 10k Resistor - gnd
5v - BLUE jumper wire - A3 - 10k Resistor - gnd
5v - YELLOW jumper wire - A2 - 10k Resistor - gnd

Action:
pin D13 is already hooked up to an LED

7 Segment Display   >   Arduino
           A                          D7
           B                          D8
           C                          D9
           D                          D10
           E                          D11
           F                          D12
          G                          A0
          Colon                   D4
          Digit1                   D2
          Digit2                   D3
          Digit3                   D5
          Digit4                   D6

All resistors at the base of transistors are 1K.
Resistors from the emmiter to 7 Segment Display are 220 Ω.
Resistors for grounding A5,A4,A3,A2 are 10k.
All NPN transistors are BC337.

Arduino Code : https://www.dropbox.com/s/sae7erugq1ibzpf/crazy_timer_with_delay.ino?dl=0