There are a bevy of good (some great), detailed, extensive Instructables showing you how to make a near-screen-accurate chest arc reactor for a Iron Man/Tony Stark costume.
I appreciate and enjoy all those but they can be a little time-consuming/expensive/requiring skills you may not have.
So I made this one for under $15 and in under 15 minutes. It's perfect if you need to throw together a costume but don't have a lot of time and/or money (or, in my case, craftsmanship skill).
Here's all you need:
1. A battery-powered, short string of blue LEDs (example: http://www.littlebrightlights.com/site/1435548/product/AC-100B). My string was Philips brand, 3 feet long (if I remember correctly) with 18 lights but really, any brand and any amount of lights under 20 would probably work just as well -- I got mine at Target for $8. Each of my individual lights are just about 1/2-inch long and 1/8th of an inch across. Try and find lights that are on a string of a shorter length, with as little space between bulbs as possible (this information should be on the box). The lights are the most important thing, not the length of the cord.
2. A hockey puck-shaped piece of white Styrofoam (1-inch thick, just under 3 inches in diameter). I got my piece by cutting the bottom off a conical piece of craft Styrofoam I already had. Packing Styrofoam will probably be too brittle to cut and it also has less opacity/reflectivity. I'd recommend using the lighter, craft Styrofoam that has the crystallized look and is made to be cut (Michaels is a good option).
3. A pen or marker.
4. A toothpick.
5. Tape (glue would work, too). I used 2-inch wide clear packing tape but any kind would probably work.
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Step 1: Mark Dots on Styrofoam Piece.
However many lights are on your string, you should make that many marks on one (or both) sides of the Styrofoam piece (i.e., 18 lights = 18 marks). I used a Sharpie. It might help to practice on a piece of paper, first, to get the spacing down.
I made one in the center and then worked my way outwards. It doesn't need to be an exact science, just make it look presentable and in some semblance of a concentric circle or two. The lights will be so bright that most people generally won't want to stare at it long enough to count whether or not there are an equal amount of lights in each quadrant.
Step 2: Punch Through Styrofoam.
With a toothpick, poke straight through the Styrofoam, using the marks you made in Step 1 as a guide. You're going to be pushing the LEDs through these holes so make sure the holes are all the way through and straight as an arrow.
I wouldn't advise using something larger than a toothpick or else the lights may not be snug inside the Styrofoam and you may risk cracking the Styrofoam between two holes.
Step 3: Insert LEDs.
Stick the LEDs through the holes (try to coordinate it so that the wire connecting the LEDs does not cross over itself a bunch of times -- do it clockwise or counterclockwise).
If you end up having an extra light or two (i.e., if you have 18 lights like I did but marked 17 holes and realized there was no place for another until now, put the excess light(s) in the center). You may need to poke through the center a few extra times with your toothpick to make the hole a little larger, to accommodate an extra light or two.
I kept my lights just below the surface on the front so that the light would reflect throughout the Styrofoam and the front would be flat/smooth. If you think it would look better to have them poking out of the Styrofoam a little bit, go for it.
Step 4: Tape the Wire Down.
The back of the styrofoam is going to look like a garden of weeds with all the wires bundled together.
Test out how to best flatten it out without tape, first (hold it down with your hand). See what direction works best -- the flatter you can get the wires, the thinner the completed reactor will be and the better it will look/fit. My final reactor was about 1.25 inches thick.
Using the tape, flatten out the slack wire, trying to keep it from hanging off the sides as much as possible (although it really won't matter if it does hang off the sides). I kept the tape from overlapping to the front, in front of the lights but if the tape is clear, it might not matter whether or not it falls in front of the lights.
Step 5: Hang It Over Your Shoulder.
The easiest way to "install" the reactor behind your t-shirt or undershirt is to just hang the battery pack over your shoulder so that it balances its weight on its own.
Taping the reactor to your shirt is troublesome (wrinkles, your shirt will sag, and it could fall off) and taping/fastening it to your chest would probably be extremely uncomfortable/ineffective after a while. Also, it's better to have the reactor separate from the shirt so that it more accurately creates the illusion that the reactor is part of your body (i.e., your shirt will ripple when the wind blows but the reactor won't move; if you stretch your arms, the shirt will move, not the reactor).
Step 6: Put on a Shirt.
Simply put a t-shirt on overtop of the reactor currently hanging over your shoulder.
(I did not cut a hole in any shirt but that might work better for you.)
The LEDs are so bright, though, that they can clearly show through most regular t-shirts.
It will stand out best through a lighter grey t-shirt (as pictured). Black is a little too opaque and white diffuses the light a lot so that your whole chest looks blue, which can look odd. I used a regular Hanes "crew" shirt I got at Target.
Step 7: Done!
If you're wondering about how to effectively turn the reactor on and off if the battery pack is under your shirt, on your back, I'd say don't worry about it. The string of lights I use runs on 4 AA batteries and I went to a party with the reactor turned on for the entire time and it never died (at least four hours).
Anyway, with this reactor, you can be casual Tony Stark and just throw on some nice slacks and a watch to go with your shirt.
Or you can be Prisoner of War Tony Stark and dirty yourself up a little bit, get a green army jacket, wear a drab-colored beanie (grey, olive, brown) grow out a little beard, and wear some beat up pants/shoes.
Depending on how thick your finalized reactor is, it might behoove you to NOT hold your shoulders back as much as you probably do normally (otherwise your reactor will stick out against your shirt and look a tad strange). Bad posture can make the reactor look more like it's a part of your chest rather than sitting on top of it so -- for once in your life -- it's perfectly fine to slouch.