My friend and i built this prop for airsoft/paintball matches. Its sort of like capture the flag only disarm the "bomb".
How it works is there is a predetermined time limit (which u can change in the code) and the opposing team has that much time to find the code and the bomb and disarm it. How they find the code is up to you... capture a person? sure! Find it hidden somewhere? sure! or whatever! When the bomb is turned on it asks for a code and you can enter in any four number combo and that is the disarm code for that round. If the person puts in the wrong code it will add 1min 30sec to the time when the time runs out? BOOM! well thats what it says on the screen and it makes a funky noise = ] This is version 1.0 hopefully in the future we will add wires to unplug to disarm it and some other neat things but for now this is perfect and works well for what we want! I hope you enjoy the instructable!

Step 1: Supplies

You will need the following electronic components:

1 Arduino (We use duemilanove and you can buy these everywhere online)
1 Arduino Proto-Shield (or if you want just a breadboard or protoboard)
1 LCD Screen 16x2 ( we used this one from sparkfun! http://www.sparkfun.com/products/255)
1 Numeric KeyPad ( we used another one from sparkfun! http://www.sparkfun.com/products/8653)
1 Potentiometer ( we used a 50k i believe! bought from raidoshack awhile ago)
1 8Ohm Speaker ( tore one out of an old toy, but you can find them EVERYWHERE!)
1 Green LED
1 Red LED
4 1k Ohm resistors

Other Materials:

Some sort of case ( we found ours at a thrift store but i think they sell them at walmart)
a chunk of wood ( this is to bring the arduino off of the bottom of the case and up towards the top)
Screws (to attach the arduino to the chunk of wood)
Heat shrink tubing

Tools you'll need:

Soldering Iron
Solder
Wire (we used alot of old computer cable so it would stay kinda together)
Wire Cutters
Hot Glue Gun
Hot Glue
Saw
Dremel
Screw driver

Step 2: Cutting Up Your Box

This is basically up to you how you want the layout to be ... you could have everything inside so when u open it up u can see all of the nice hardware and it looks more low budget (as if this isn't low budget enough!) or you can make it clean where you only see the keypad and such.... anyways its all up to you its really just a style thing.
We cut the square out for the lcd screen and then drilled the holes for the LEDS, then cut out the keypad and we just eyed everything as far as positioning. We did all our cutting with a dremel and razor blade.

We then cut the slots in the back: First we cut a hole for the potentiometer to fit through, then we cut another hole in the back just for the power adapter hole and then also a hole for the USB cable just for programming if we needed to we wouldn't have to take the arduino out.
all of these holes and such are your call for style so just do a layout of everything until you get it how you like it then make the cuts.

Step 3: Placing the Arduino

This again will depend on your case height and such as you can see here we had to use a piece of scrap wood to pull the arduino off of the case. We glued the wood to the case and then screwed the arduino into the wood. You'll notice the way the wires sag in this photo ... thats a good thing you'll want them to be longer than you need in order to open and close the box without pulling the wires or breaking connections!

Step 4: Wiring and Such

You'll have to just follow the schematic we unfortunately didn't make a step by step on this.
Heres a quick outline to go along with the schematic
Arduino pins :

DPin 0 = Nothing
DPin 1 = Nothing
DPin 2 = Pin 8 on keypad
DPin 3 = Red LED
DPin 4 = Green LED
DPin 5 = Nothing
DPin 6 = Nothing
DPin 7 = Pin 4 LCD
DPin 8 = Pin 6 LCD
DPin 9 = Speaker +
DPin 10 = Pin 11 LCD
DPin 11 = Pin 12 LCD
DPin 12 = Pin 13 LCD
DPin 13 = Pin 14 LCD
APin 0 = Pin 2 Keypad
APin 1 = Resistor to Pin 3 Keypad
APin 2 = Pin 4 Keypad
APin 3 = Resistor to Pin 5 Keypad
APin 4 = Resistor to Pin 6 Keypad
APin 5 = Resistor to Pin 7 Keypad

LCD Screen Pins:

Heres the missing pins on the LCD
LCD pin1 = Potentiometer 5v
LCD pin 2 = Potentiometer GND
LCD pin 3 = Potentiometer Middle
LCD pin 5 = GND
LCD pin 15 = 5v
LCD pin 16 = GND

That should be all of them .... on the spark fun website if you follow the links on the supplies page there are some really great tutorials that explain how to hook up the pad and the lcd.

Step 5: Uploading the Code

Heres the code you will need to get it to work the way ours does as always feel free to make it better!

/*Airsoft Bomb
Version 1.1Creators:
Chase Cooley&&&&&&&&&&&&
Joey Meyer*/

#include <Keypad.h>#include <LiquidCrystal.h>
#include <Tone.h>
#define pound 14
Tone tone1;int Scount = 0; // count seconds
int Mcount = 0; // count minutesint Hcount =0; // count hours
int Dcount = 0; // count daysint val = 0;
long secMillis = 0; // store last time for second addlong interval = 1000; // interval for seconds
char password[4];int currentLength = 0;
int i = 0;char entered[4];
int ledPin = 3; //red lightint ledPin2 = 4; //green light

LiquidCrystal lcd(7,8,10,11,12,13);const byte ROWS = 4; //four rows
const byte COLS = 3; //three columnschar keys[ROWS][COLS] = {
{'1','2','3'},{'4','5','6'},
{'7','8','9'},{'*','0','#'}
};
byte rowPins[ROWS] = {18, 2, 14, 16}; //connect to the row pinouts of the keypadbyte colPins[COLS] = {17, 19, 15}; //connect to the column pinouts of the keypad

Keypad keypad = Keypad( makeKeymap(keys), rowPins, colPins, ROWS, COLS );
void setup(){
pinMode(ledPin, OUTPUT); // sets the digital pin as outputpinMode(ledPin2, OUTPUT); // sets the digital pin as output
tone1.begin(9);lcd.begin(16, 2);
Serial.begin(9600);lcd.clear();
lcd.setCursor(0,0);lcd.print("Enter Code: ");
while (currentLength < 4){
lcd.setCursor(currentLength + 6, 1);lcd.cursor();
char key = keypad.getKey();key == NO_KEY;
if (key != NO_KEY){
lcd.print(key);password[currentLength] = key;
currentLength++;delay(200);
}}
if (currentLength == 4){
lcd.noCursor();lcd.clear();
lcd.home();lcd.print("You've Entered: ");
lcd.setCursor(6,1);lcd.print(password[0]);
lcd.print(password[1]);lcd.print(password[2]);
lcd.print(password[3]);delay(3000);
lcd.clear();currentLength = 0;
}}

void loop(){
char key2 = keypad.getKey(); // get the keylcd.setCursor(0,0);
timer();if (key2 != NO_KEY)
{while (key2 == NO_KEY)
{key2 = keypad.getKey();
}if (key2 != NO_KEY)
{lcd.clear();
lcd.setCursor(0,0);lcd.print("Enter Code: ");
while (currentLength < 4){
lcd.setCursor(currentLength + 6, 1);lcd.cursor();
char key2 = keypad.getKey();if (key2 != NO_KEY)
{lcd.print(key2);
entered[currentLength] = key2;currentLength++;
delay(200);lcd.noCursor();
lcd.setCursor(currentLength + 5, 1);lcd.print("*");
lcd.setCursor(currentLength + 6, 1);lcd.cursor();
}}
if (currentLength == 4){
if (entered[0] == password[0] && entered[1] == password[1] && entered[2] == password[2] && entered[3] == password[3]){
lcd.noCursor();lcd.clear();
lcd.home();lcd.print(" Defused ");
currentLength = 0;delay(2500);
lcd.setCursor(0,1);lcd.print("Reset the Bomb");
}else
{lcd.noCursor();
lcd.clear();lcd.home();
lcd.print(" Wrong ");lcd.setCursor(0,1);
lcd.print(" Add - 1:30 ");if (Mcount < 14)
{Mcount = Mcount + 1;
}if (Scount < 59)
{Scount = Scount + 30;
}delay(1500);
currentLength = 0;}
}}
}}

void timer(){
if ( Mcount >= 15 ){
while ( Mcount >= 15){
lcd.noCursor();lcd.clear();
lcd.home();lcd.print(" !BoOm! ");
tone1.play(NOTE_A2, 200);digitalWrite(ledPin, HIGH); // sets the LED on
tone1.play(NOTE_A2, 200);delay(10); // waits for a second
digitalWrite(ledPin, LOW); // sets the LED offtone1.play(NOTE_A2, 200);
delay(10); // waits for a seconddigitalWrite(ledPin2, HIGH); // sets the LED on
tone1.play(NOTE_A2, 200);delay(10); // waits for a second
digitalWrite(ledPin2, LOW); // sets the LED offtone1.play(NOTE_A2, 200);
delay(10); // waits for a secondchar key4 = keypad.getKey();
if (key4 != NO_KEY){
while (key4 == NO_KEY){
key4 = keypad.getKey();}
if (key4 = '#'){
lcd.clear();lcd.print("Reset the Bomb");
}}
}if ( Mcount == 60) // if Mcount is 60 do this operation
{//delay (32); // good place to fine tune timing
Mcount = 0; // reset McountHcount ++;
}if (Hcount> 23)
{Dcount++;
Hcount = 0; // have to reset Hcount to "0" after 24hrs}

lcd.setCursor (0,1); // sets cursor to 2nd linelcd.print ("Timer: ");
lcd.print (Dcount);lcd.print (":");
lcd.print (Hcount);lcd.print (":");
lcd.print (Mcount);lcd.print (":");
lcd.print (Scount);
if (Scount >59) // if 60 do this operation{
Mcount ++; // add 1 to McountScount = 0; // reset Scount
//delay (58); // changes ms per min}
if (Scount < 60) // do this oper. 59 times{
unsigned long currentMillis = millis();//delay (988); // changing by one = 60 ms a min
if(currentMillis - secMillis > interval){
tone1.play(NOTE_G5, 200);secMillis = currentMillis;
Scount ++; // add 1 to ScountdigitalWrite(ledPin2, HIGH); // sets the LED on
delay(10); // waits for a seconddigitalWrite(ledPin2, LOW); // sets the LED off
delay(10); // waits for a second}
}}