This instructable demonstrates how to make a simple but attractive replica of the Arc reactor used by Tony Stark in the Iron Man films and comics. I developed this prop because my finacee and best bro Julie wanted one she could wear for immersive cosplay. The goal was to make one which 1) looked good under a shirt; 2) Looked good uncovered; and 3) was flat enough and light-weight enough to be stuck onto to skin comfortably all day without concern.

The design consists of a simple LED circuit built onto a printed circuit board and then covered with a few pieces of cut plastic. It is then attached magnetically to a thin piece of steel that can be stuck to skin with costume glue. My first attempt was a failure, which I outline for instructive purposes. Items marked with an asterisk are optional.

I'm thrilled with the final product. One 3V coin cell battery will give at least 6 hours of intensely bright illumination that shows through even heavy shirts. After that, the battery runs down a bit and so the light becomes less intense. This level lasts for over a day of continuous use (like the original Iron Man!). You may prefer at times to use partially drained batteries, as the fresh ones can be too bright for dark settings like a movie theater.

If you don't have access to a laser cutter, you can try printing the aesthetic components using 3D Hubs, or, you can get the benefit of the under-the-shirt glow by omitting these superficial elements.

Parts: Electronics

White SMD LEDs (23x) -- $5

Battery holder for 20 mm coin cell battery -- $1

A 100 ohm resistor -- $1

Surface mount right-angle switch -- $1

Solder Paste -- $17

Steel sheet -- $1

Neodymium magnets (x10) -- $3.70

A printed circuit board (PCB) -- $45 for 3x - Special thanks to James Irvine, who designed a ready-to-manufacture PCB file found in step 3

*Copper clad board

*Ferric chrloride

*Assorted etching supplies: Gloves, a disposable bowl, a plastic spoon, a measuring cup, a sharpie, rubbing alcohol, and papertowls

Parts: Aesthetic components

1/4" thick acrylic for cutting or 3D printed parts

Copper tape -- $13

Hot glue

Tools:

A ruler

A band saw or jigsaw

A hot glue gun

A skillet or a soldering iron

A small stylus

*An X-acto knife (craft knife)

*A thin plastic sheet

Step 1: Get a PCB - the Worst Way

The lighting is simple in design but looks complicated. The original inspiration was a product Julie bought online. It was made of clear plastic disks with white LEDs inserted into holes and soldered in parallel. When it wore out, I just planned to build a new one, but make a few changes: instead of using regular LEDs, I thought I'd use Surface Mount Device (SMD) LEDs. Also, If figured I could laser cut some pieces to make it look a little more faithful. I based these off of another prop she'd found online.

Surface Mount Devices are the electronics components used in consumer electronics. Before this, I was only familiar with the chunky LEDs used in most hobby projects, but I'd previously built Leah Buckley's light up bike jacket, which was how I discovered SMD components. They're designed to mount directly on a circuit board, so they don't have any extra bulk. The downside is that you can't plug them into a breadboard, but the plus side is that once I learned how to mount them the end result was amazingly minimalist. I want to emphasize that I do not have much experience with electronics or soldering. I actually found this process easier than conventional soldering.

Instead of wires, SMD components have pads. My 'brilliant plan' was to glue them to an acrylic disk, glue down stiff wire, and then connect the SMD pads to the wire with a conductive glue I found online called Wire Glue. The idea was perfect except for the fact that Wire Glue is a scam. It should market it'self as "poorly insulating glue". It doesn't matter how much you use or how close together your wires are. It's resistance is hundreds of ohms per mm. It doesn't work and it's completely worthless. Below you can see the process I used to assemble a product that looked cool but failed miserably. No matter how much I globbed on, it didn't improve, it just got uglier. Eventually I accepted that it wouldn't work.

Step 2: Get a PCB - the Way I Did It

Once my first attempt failed I was forced to figure out a new way to wire the LEDs, so I began reading up on how they're actually designed to be soldered. I'd bought the Wire Glue because I'd failed at soldering in the past, so I assumed that there was no way I could solder components this small. Turns out I was wrong. If I can do it I think almost anyone can, provided you get the right tools.

I'll go into soldering details in step 5. First, we need a PCB to solder. In this step I outline how I made mine, but in the next step I'll outline how I wish that I'd made mine.

I made my PCBSt by reading several instructables on etching circuit boards. All you do is buy copper clad prototyping boards, cover the places you want to have connections, and etch the rest away.

The professional way to cover the board is by printing traces onto transfer sheets, but sharpie works too. That's what I did. I used several cups as stencils to draw an inner and an outer ring and then added in the pads where the LEDs would sit.

This part is important: It's critical to layout the LEDs precisely because any asymmetry will be very noticeable when you're done. I discovered this because I placed LEDs on my first attempt just by eye. When I tried to add the diffuser with the copper bands I found that there was no position in which the LEDs weren't askew. In my second attempt I made a guide using an iterative approach. I segmented a circle and then cut a triangle as close to 18 degrees as possible and traced it. Depending on whether it was too big or too little I repeated the process using the last stencil as a guide until I had a circle with reliable demarcations every 18 degrees. I then used straight edge to connect three LEDs in the middle in the shape of a triangle. The goal is to provide some illumination to the center of the arc reactor. Depending on your preference you can add more than three or leave them out entirely.

I cut my copper clad board with a band saw. I then cleaned it thoroughly with rubbing alcohol and then copied the design onto it with the same stencils I used before, all while wearing nitrile gloves. You can erase stray lines with a cotton swab soaked in rubbing alcohol.

I flipped over the board and drew my design on the back side. It just needs a pad that connects to the battery holder, a pad that touches the under side of the battery, and a way to connect to the other side. A connection through the two layers is called a 'via'. It's just a small hole with solder in it that connects to copper on either side.

I added a switch and a resistor so that I could change the brightness. This is totally optional.

Once I had both designs drawn on, I filled a bowl with warm water, dissolved ferric choloride in it according to the instructions, and then dipped my board in. After ten minutes it had dissolved away all the copper that was showing. I rinsed it to quench the process and cleaned off the shapie with rubbing alcohol to see a beautiful circuit board. You can find more detailed instructions on etching PCBs here and here, but I recommend that you don't even bother, because in the next step I'll explain how to just order a professionally made board online.

Step 3: Get a PCB - the Right Way

Etching a PCB at home isn't really necessary, because designing a custom PCB and outsourcing the manufacturing is actually remarkably easy and affordable. There are a variety of programs which can be used to design PCBs. Eagle is a popular one. I've recently used Autodesk's 123D Circuits online app with good results for a different project. It has it's limitations, though. Fortunately, a DIYer on reddit offered to make a clean PCB design for me after seeing the homemade prototype I posted in r/diy. James Irvine (reddit user u/Iambecomelumens) designed this PCB in Altium. The .rar file contains all the files necessary to edit this in Altium. The .zip file is packaged and ready to submit for manufacturing to OSH Park. As of this writing they charge $45.30 to print three of these boards, which is their smallest order. Be aware, you'll still need to cut the board into a circle when it arrives. I recommend a band saw if you have it or a jigsaw if you don't.

For questions or high-fives, James can be reached at james@irvine.org.za.

Step 4: Assembling the Board, Front Side

Attaching SMD LEDs was easier than I expected. One needs to lay down solder paste, place the LEDs, and then heat the board to reflow the solder paste. I deposited solder paste using a very small screw driver to dab it where I wanted. The correct way is to cut a stencil and then smear the paste over it. I've since tried using a stencil on another project, and both ways worked on the first try, to my surprise. I cut my stencil using a craft knife and a piece of plastic film cut from the front of a binder. If you have access to a laser cutter I would definitely recommend the stencil method. If not, dabbing seems to work fine.

Once the solder paste has been laid, the LEDs can be placed with tweezers. The paste is designed to lightly stick the components in place, and it works great.

Once placed, the board must be heated to melt the solder paste. I followed an excellent set of instructions provided by SparkFun. The key thing to know is that there is no mistaking whether the solder paste has reflowed. I was very concerned about damaging my board or my LEDs, so I was eager to use the lowest heat and the briefest heating possible. I was unsure initially if the solder had reflowed, but I obeyed the instructions and waited. If you're unsure if it's reflowed than it hasn't, because once it has it turns from dull to shiny, and the change is unmistakable.

After letting it cool, check the connections with a multimeter before proceeding to attach the components on the back side of the board. Touch up any spots as necessary with a soldering iron.

Step 5: Assembling the Board, Back Side

I was worried about the LEDs falling off if I used the skillet again, so I used a soldering iron to reflow the solder paste on the back side.

I included a switch because I don't understand electronics well. I wasn't sure if powering the LEDs without a resistor would burn them out, but I worried that if I added a resistor to be cautious I wouldn't get full intensity. I split the difference and included a switch that can run the LEDs either directly or through two 100 ohm resistors in parallel (so 50 ohms).

Firstly, while there was some debate on reddit.com/r/diy, running LEDs from a coin cell battery is generally considered safe. Several people insisted that resistors are ALWAYS necessary, so if you want to play it safe, include 10 ohms or something.

Second, the 50 ohm circuit is actually nice for reducing the power if you want a subtle effect instead of a distracting one. I thought it might save power and make the battery last longer, but someone pointed out that it actually draws the same power either way, the resistors just turn it to heat.

Third, you can see that I don't know how to solder resistors. I laid them flat. I should've drilled little holes for them, but I honestly have so little experience with PCBs that I just pretended they were surface mount components like a dummy. Well, it worked.

Lastly, if you want to outdo me, replace the power supply with a joule thief. It's a special circuit designed to step up voltage from a battery, so you can power LEDs from batteries lower than the driving voltage of the LED. A joule theif has a cool little torroidal magnet which works well thematically with the Arc reactor, so I think it'd look absolutely sick if you cut a 1/2" hole in the PCB and mounted the torroidal magnet in the center, then powered it with a watch battery. I bought the parts, but never got around to trying it out. If you do, let me know so I can see how it worked out.

Step 6: Attach the Aesthetic Components

I cut out rings of acrylic using a laser cutter, although you could 3D print them too. I had four parts: two outer rings of the same size which I stacked to give the outer ring a bit more height and two inner rings of slightly different sizes that overlap partially. Download the attached files to either cut or print the rings.

This is where much of the size come from. I wanted it to feel right in your hand, so I made it about 0.4" thick overall. You can make it thinner if you want it really flat against the skin.

Step 7: Attach Using a Backplate

If you're interested in wearing the arc reactor, the best way I've found is to use a costume prosthetic adhesive of your choice to affix a steel disk to the skin, then glue rare-earth magnets to the back of the Arc reactor. The reactor can then be stuck to the plate. If you attach the plate and the magnets correctly the whole assembly should be extremely reliable.

Constructing the mounting plate

I purchased thin sheets of steel. I tried to cut a disk using a jigsaw, but it bent to badly. I was lucky that tryinga friend had access to a band saw, as he was able to cut the disk, and notch the outside so that I could fold the edge up to give a sense of depth. A tin can lid would likely work as well. Just be cautious to file the edges or cover them in electrical tape or something. I recommend covering the plate with electrical tape where the magnets will contact, as the magnets can actually be hard to pull off, and you may see magnets come off or the glued ring of the reactor come loose while trying to pull the whole assembly off the plate.

Attaching the magnets

I glued the magnets with hot glue because I didn't have any epoxy on hand. I would epoxy the magnets. If you hot glue, be careful: you can weaken magnets very quickly by touching them with a the hot tip of a hot glue gun. Just lay a bead of glue down and then press the magnet down quickly. I had to stack two of the thin magnets to make them extend out farther than the battery compartment. Demagnetizing probably ins't a problem, though, since there are so many of them and they're so strong to start with.

Put it on

In the past, my brother, my fiancee, and myself have become increasingly fond of using Scotch double-sided tape for attaching costume prosthetics. It's deceptively strong, and yet it comes away painlessly when you want it to. If you lay a piece of floss or string under the tape, it helps when removing it from the backplate.

I've tried spirit gum and liquid silicone adhesive, and both were disappointing. I've heard brands matter, so feel free to experiment. That said, I can vouch for one reliable product, and that's 3M double-sided Scotch tape.