Here's an idea for you, I'll make the bomb in my head and explain, I suck at drawing schematics so you may want to print this out.
Stuff you'll need to acquire.
Get a few Estes Model Rocket engine packs from your local hobby shop.
Pull out the ignitors, and a piece of paper and make a 2"x2" square. Open one of the rockets, and scrape about a teaspoon full of powder from the engine (The Dark Grey/Black Powder, should be the longest one, DO NOT SCRAPE OFF THE STUFF FROM THE TOP!!!, this is high explosive and will most likely do serious damage to one or more of your cadets), pour it into the paper and place one of the ignitors inside. Roll it into a ball so the ignitors are still outside the paper ball. (Make one of these for each cadet)
Okay, get ahold of a cheap computer, something around $50 from ebay or something, don't worry about specs, you'll only need a few parts.
Take out the hard drive and gut it. (You can use the disks inside as target practice if you want, they break in a rather nice way)
Take the IDE cable and set it aside.
Now unsolder the socket the IDE cable was attatched to and remove two of the pins that are parallel to each other (Only the ones inside the socket, you're still going to want to connect these to your PCB later) this is where the real "bomb" meets the dummy, make a small mark on the inside with something bright, like some orange paint
Now take out the memory and unsolder the sockets (these are purely for show, no actual use in the "bomb")
If the computer has a sound card with a cable attatched to a CD Rom drive, remove that (this will be your fake connection between the real guts and the fake)
Unsolder the socket and set it aside.
Now take care not to get yourself killed on this next one.
Remove the power supply and cap the plug, open this baby up and discharge the main capacitor, unsolder the capacitor and close it up.
Strip the connector coming from the CPU Fan (the female connector), remove some of the insulation from the ends, and cut two of the wires from the top of the connector, bridge these two (mark them with a piece of tape so you won't forget which ones they are) and hide them so you won't be able to see it from a casual glance.
Now get out something that can cut PCB with little damage (probably a hacksaw) cut around the CPU and sand down the edges, now get out a drill (not sure of the measurements) and drill 4 holes large enough to fit a spacer in around the corners.
Now take a look at the schematics. See where it says instant detonation, here's where one of your parts is going to come into play.
Remember that connector from the fan you stripped, those two wires you bridged are going to be where this part of the bomb is. (The current travels up one wire and down the other, out to the "real" bomb)
Take the stripped ends and connect them to the pcb, making sure the two that are bridged are maked, this is probably where your cadets will start as this connector goes to one of the plugs on the dummy power supply.
Now the CPU will be mounted somewhere where it looks important, doesn't matter where so long as it doesn't actually connect with any of the "real" bomb components, once you have found a place for it, drill 4 holes onto the bomb PCB in line with the 4 you drilled into the computer pcb around the CPU.
Mount the memory sockets next to this (also artificial)
Okay now the hard drive and IDE socket, mount the hard drive on the side of the bomb, and place the IDE socket where "cut here for maximum timer speed" is, remember those two pins you removed from the inside, here's why, when you solder in the socket, don't clip these two pins on the underside, instead, push them together and solder them together, replicating the above idea, if they seperate the hard drive from the "bomb" by pulling out the IDE cable, nothing happens, but if they see the orange paint, they may inspect further, see the missing pins, look underneath and try to seperate them, or unsolder them, resulting in maximum timer speed.
Now that that's mounted connect the hard drive to the socket via the IDE cable.
Remember the "Cut here to stop wire on the schematic" well pull out your sound card connector, and if you only have one socket, strip one of the ends.
Mount the socket next to IC input, and connect the cable to it, making sure the connector you use corresponds with a wire that has a contact on the connector (one of the pins on the socket has no function, thus it also has no contact in the connector, so no connection between the wiring.
Now follow the wire used to connect to the IC input and solder this one down next to set switch 5.
So if they pull this connector, they just disarmed the bomb.
Now Where it says bomb output, just rig one wire to each end of the ignitor ball I told you about earlier. Place this inside a film canister (LEAVE IT OPEN!!!) Mount this into a corner where no parts are near and the cadets face is away from. This is the BOOM!
If they fail, pop, out shoots some flames, and burning cinders. Make sure they wear a face shield, or at least safety goggles.
OR, if fireworks are legal in your state, consider making your own low voltage ignitors, and hooking up a smoke bomb to it, and put it inside a film canister, close her up, and drill a hole in the top, and you can also put it parallel with a buzzer.
:twisted:
Let me know if I didn't properly explain anything before you try to do this one.
I know if I was better at schematics, you'd understand a whole lot easier.
Also, when all is said and done, make sure you weave all the wires together, so it makes a confusing braid, that way they'll pour over this, probably waisting valuable time.
Oh... And NO RED OR BLUE WIRES