Introduction: The 15-dollar, 15-minute Arc Reactor.

Picture of The 15-dollar, 15-minute Arc Reactor.

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: 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.

Step 1: Mark Dots on Styrofoam Piece.

Picture of 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.

Picture of 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.

Picture of 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.

Picture of 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.

Picture of 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.

Picture of 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!

Picture of 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.


AmberS89 made it! (author)2016-09-12

This is great! Thanks for the ideas! There are really great posterboard LEDs available for this and you can use the included dowel to make the holes a perfect size. I also used the idea to overlay the foam with the arc reactor printout. I went a step further and cleaned up the sides and back with gaff tape in order to make the light a little more isolated.

JustinBrown (author)AmberS892016-09-13

This looks great!

AmberS89 (author)JustinBrown2016-09-13

Thanks! I couldn't have done it without your great tutorial to inspire me.

Tribalhero made it! (author)2016-07-30

Let me just say, .... Awesome project dude!!! My only problem was using the toothpick to poke the holes. They ended up being to small and I had to start over. The next time I tried, I used a Phillip's head screwdriver to make the holes. Sure they were a little big, but it was easier to stick in the leds. I also used a Mark III arc reactor printout to give it a little shape. But overall, I think it is an awesome, spectacular, and amazing project to partake in!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

JustinBrown (author)Tribalhero2016-07-30

Wow, that is a really cool and simple way to improve the design big time! Thank you for the kind words and for your excellent idea!

creed-1 made it! (author)2015-10-31

Will go great with the last minute mask I put together for Halloween. Thanks! (I ended up poking from the other side so the sharpie marks don't line up in the pic.)

Mark 42 (author)2015-10-29

It looks like something that could be done using the screen of a smart phone.
Just a video, or even a picture on the screen.

Gaige Kerns (author)2015-10-29

If you really Wanna Bump up your game, take some surface Mount LEDs, a Copper Breadboard, some etching solution, a couple wires, and your battery pack, and build it thinner! No having to slouch! If anyone wants the print out sheet for the etching, let me know!

jvarga1 (author)2015-10-29

bruno13069 (author)2015-10-29

I made one using a 3 light press lamp. I mounted it to a piece of cardboard that I could hang around my neck at the right height. Cheesy, but I just needed it for a few hours at work.

BeckiW (author)2015-09-14

Ok I am having trouble finding these lights online. Anyone have any recommendations? Or do you think this would work?

MorganV1 (author)2015-05-15

Did the battery pack already come with the lights?

JustinBrown (author)MorganV12015-05-17

Yes, they were attached! I didn't do any rewiring or anything requiring electrical know-how.

lisa.rimke (author)2014-10-31

Made this for my 4 year old son's costume and he loves it!
It's large, but whatever, still cool looking :)

Grafx4Life (author)2014-10-22

Finally done. Probably took closer to 4 hours due to changes I had to make, but I'm pretty happy with it. Thanks for the Instructable!

Grafx4Life (author)2014-10-09

I have everything I need but I'm too tired to build it tonight. Bout a 9 ft string with 36 lights but they are really small, so I might double them up. Or maybe I'll check Target. The link you provide for you lights is no longer working.

ehhdean (author)2013-10-22

Not bad. I may add this to my suit next time. I need a more powerful light.

sequret96 (author)2012-06-13 heres mine

missmaggy2u (author)2012-02-09

One question. When you hang it over your shoulder, how does it stay in the center of your chest?

mhkazemi (author)2012-01-22

that is good idia
reality good
you can develop it
think about!!!

carellano1 (author)2011-11-03

Loved this instructable and did my own. Could only find a 10 led string so my arc reactor looks more like the prototype he first made in the 1st movie. The cord was too short though so i cut it and and soldered longer wire to the battery pack so i can put it in my pocket. Used the leftover wire to hang it around my neck. Lemme know what you guys think :)

JustinBrown (author)carellano12011-11-04

This looks rad. Great work, sir.

kunfumaster (author)JustinBrown2012-01-10

hey man thats awsome!

dominicgrz13 (author)2011-10-20

Hey Justin I have a question, I don't know if its because you made this a year ago, but i could not find the right lights that you used from target also would the lights from ( ) work just as good without me having to change the size of the foam circle?

And finally, what color lights would be more noticeable under a suit?

I'm going to be Tony Stark :D

JustinBrown (author)dominicgrz132011-10-20


Those LittleBrightLights look good to me, you might just have a bit more slack wire, with a six foot string. I used blue lights and those seemed to work very well but I think white would generally be fine, too.

Thanks for the question.

dominicgrz13 (author)JustinBrown2011-10-21

Oh, OK cool thanks bro.

JacSpark (author)2011-09-15

Would it be possible to use the LED "throwies" idea? but just stick them inside the foam?

JustinBrown (author)JacSpark2011-09-15

Sure, I don't see why not.

tparikka (author)2011-05-16

looks great

acardboardllama (author)2010-12-30

pretty cool brother

Doctroid (author)2010-10-30

There's a ton of arc reactor instructables but this is the one I was most inspired by. I decided to do it Wednesday night, bought the materials and made it on Thursday, and wore it on Saturday. Being short on time (at the same time I was scrambling to finish up my son's bounty hunter costume), the simple behind-the-shirt approach seemed the way to go.

I did it differently though. I used a plastic jar lid with holes poked in it, covered with clear plastic with wax paper behind it to diffuse the light some, and with electrical tape in a circular grid over that to make the light pattern more like what was in the movie. I used 12 lights -- 10 in a circle on the outside and 2 in the center. (I cut the others off. Turns out with the LED strings sold at Target you can cut off an even number of lights and have the rest still work -- if you cut off an odd number, the last remaining one won't light up.)

For clothes I wore an a-shirt, a brown military shirt, and a military style cap, all reasonable approximations to things Stark wore in the cave in Afghanistan in the movie though not in that particular combination. I hot glued string to the arc reactor to suspend it from my neck and secure it around my chest, with the batteries inside my waistband. Worked pretty well and was easy to put together.

The arc reactor is too low on my body in the picture, I know -- as soon as I saw it on the camera screen, I went and re-adjusted it higher!

JustinBrown (author)Doctroid2010-11-13

Wow, Doctroid -- this looks great. I'm glad I could inspire you and help with a costume.


smallstrangegod (author)2010-10-26

I just wanted to thank you! I just made this. It was so easy. It looks great and my boyfriend is going to love it! :) you saved our Halloween! I used LED micro lights from Target. They were 5.99 and they're battery operated. The foam discs I bought at the dollar store in the fake flower section. YAY! :)

Whoa, awesome! I'm glad I could help and it looks like you improved my design. Kudos, m'lady.

Green_Primus (author)2010-05-06

Not to rain on the "simple-parade" but wouldn't a dollar store round closet led disk work just as well?  they compress in the middle and light up (so all you see under the shirt is light)  and the back is adhesive (if you're into that, my chest hair isn't so much)   I used this exact set up for the 1st movie and it work out perfectly...just putting this out there.

anibioman (author)Green_Primus2010-10-17

i did that but i taped some of it of so it had the arc reastor pattern of light

JustinBrown (author)Green_Primus2010-05-09

Yes, I fiddled with the closet disk idea. But I found it to be relatively dim and didn't quite have the same effect.

Burnsides (author)2010-05-09

 Where does one purchase the lights needed for this? What section of the store would these be in?

JustinBrown (author)Burnsides2010-05-09

 I got mine in the Christmas lighting section at Target. If it's not around Christmas time, I imagine that some hardware/outdoor stores like Lowe's or Home Depot may have some. Otherwise, the Internet is probably the best bet.

But they're just simple LED lights that you would put on a Christmas tree or to decorate your house around a holiday.

Burnsides (author)JustinBrown2010-05-09

 Ok, thanks!

andrewreases (author)2009-04-07

sorry dude but the arc reactor lights are like a wheel with lines going through it with a circle in the center

JustinBrown (author)andrewreases2009-04-07

15 dollars, 15 minutes. Close enough for hand grenades.

Apollo707 (author)JustinBrown2010-02-26

i agree on the hand grenade policy ;)
the others are prettier but couldn`t be done in the short time frame
i think it looks just fine for the application

JustinBrown (author)Apollo7072010-02-26

Here's a dude who gets it.

Apollo707 (author)JustinBrown2010-02-26

thank you ladies and germs, don`t applaud, throw money ;)

winterfresh (author)Apollo7072010-05-06

 Awww. You don't except credit cards?


mg0930mg (author)JustinBrown2009-05-18


cprocjr (author)2010-05-06

 Not as realistic looking as it cool be, but good enough to work! 
I thought about making some of the other (more complex/accurate) ones, but I didn't want to spend the time. I might actually make this one! I just need to find some light...

XWXAXDXEX (author)2010-01-22

AWESOME!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! I'll get right to it.

itak365 (author)2009-09-19

That's a pretty awesome jacket, by the way. Good job with the arc reactor

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