Wireless Witch’s Cauldron Prop Build

This is what happens when you’re volunteered to do a prop build for your daughter’s dance team, and you just happen to dabble with electronics, lasers, and 3D printing 🙂

The purpose of the prop was for usage in a Disney “Villains” dance, where it could be rolled around on stage by the dancers during their routine. I decided to do a multi-piece design where the cauldron is a shell that sits on a rolling base, and the “brains” can easily be removed for transport and charging. What follows is what I came up with for a 3′ diameter cauldron, and it all starts with some Fusion 360 modeling and laser cutting 1/8″ plywood. There are many ways you could do this part by hand, but it was nice to visualize the product in Fusion 360 before even starting on it.

The Cauldron Body

The cauldron body consists of a laser cut skeleton wrapped with bird netting, cheese cloth, and drywall mud. Great stuff foam is used to add rigidity. Drylok and latex paint finish it off. I had to segment the design since my laser cutter isn’t large enough to do the rings in one piece.

The plywood skeleton is then glued up and a pool noodle is used to fill out the rim. It is then wrapped in bird netting and cheesecloth using spray adhesive before the layering of drywall mud begins.

This is where I made mistake #1 in this project… It was my intention to add great stuff foam to the internals for rigidity, but I should’ve done it before starting the mud. The end product would have required less mud and been much easier to shape. Also, when it comes to mud, make sure to use the stuff you mix yourself (hot mud) as it sets via chemical reaction and not evaporation like the premixed stuff.

After filling the walls with great stuff foam and many (more than I care to count) layers of mud and sanding, a coat of Drylok masonry sealer goes on to give it some texture. This will be getting a nice thick coat black latex paint.

The Base – Just roll with it

The base was cut from some scrap OSB that I had from a previous project (reason it’s green) and 3 casters added to the bottom. I cut some risers from 3/4″ plywood and stapled pipe insulation to them to create a cradle for the cauldron to sit in. It then gets 96 WS2811 bullet nodes wrapped around it with the help of some pixel strips.

The pixels are then wrapped in plastic and a temporary shield made from poster board so that the foam can be applied. This way, the pixels should be serviceable down the road if need be.

Great stuff foam is then liberally applied around this ring. Mistake #2 – I should have laid down some black paint first. It didn’t matter in the end, but would’ve made things a little cleaner.

The shield is cut off, cauldron test-fitted, and foam trimmed to fit. It then gets some very light coats of red and black spray paint layered for effect.

The cauldron has its paint and now we’re getting somewhere. Time to focus on the top.

The Cauldron Top – Pixels and Misters Oh My!

The top consists of 88 12V WS2811 bullet nodes inserted into a 1/8″ plywood cutout, topped with an acrylic sheet and 5 24V ultrasonic misters in 3D printed holders. This is where I noticed mistake #3… I should have made my top ring a little wider inward so that the top could sit on it. A little modelling and 3D printing rectified the problem with some brackets for the top to rest on. The top of course is bigger than my laser cutter, so it was time for me learn about using registration marks in Lightburn for doing multi-cut alignment. The magnets work great for keeping everything flat and the Lightburn camera support helps make me make sure they aren’t in the cut path of the laser.

Ahh, the fresh smell of burning plywood.

The stock is then rotated and re-aligned with the registration marks to perform the second half of the cut.

And now I’m up to mistake #4 – spend a little more time making sure the laser and alignment laser are actually aligned with each other. Oops, nothing some sanding won’t take care of.

88 WS2811 bullet nodes ready for action. The silver cans you see are the misters, we’re getting to those next.

The top holds 5 of these misting assemblies which consists of a dollar tree Tupperware container and 3D printed inserts to hold the mister at an optimal height in the water. The design includes a splash guard as well to help keep the mess down. The ultrasonic misters run on 24V and are typical of what you would find in an essential oils diffuser.

The top is finished off with a piece of acrylic resting on some PVC standoffs. It also houses a waterproof switch to turn the prop on.

The Brains

This is what makes the cauldron come to life. It consists of Lithium-Ion battery packs built from recycled cells, some buck converters, and an ESPixelStick V3 running 4.x alpha firmware playing back a local xLights sequence from the SD card. Recycling batteries into packs could be a whole article of itself. There are even sites and channels dedicated to it, so I’ll quickly gloss over my process. Take note however, that Lithium-Ion cells are nothing to play with. They can and will burst into fire if over-charged, discharged too hard, or physically damaged. For me, it all started with a deal on some modem packs from Battery Hookup.

I even made a short video showing the process that I used to crack these open and pull the cells out.

Once the cells are harvested and cleaned up, they are all capacity tested on an Opus BT-C3100 charger and labelled appropriately. Kapton tape is used to patch any cells which may have damaged heat-shrink from the disassembly process.

The best cells are then entered into rePackr for the construction of two 4s5p packs. To hold the cells together, I designed a parametric cell holder which I made available on Thingiverse.

.15mm Nickle strip is used to spot weld the connections and Kapton tape used for protection as needed.

An eBay spot welder with a lawn tractor battery was used to perform the welds. These cheap welders do need some modifications though so that they won’t self-destruct. If you choose to use one of these, make sure to watch this video first about adding a capacitor and beef up the solder traces on the MOSFETs. I measured right at 600A of current on mine when doing test welds.

A 4S Lithium Protection Board, balance leads and XT60 connection is then added to the pack. These packs will be charged and balanced on a RC charger.

Finally, the pack is wrapped with fish paper and heat-shrink. One completed 4s5p pack, two of which will be used in series on the prop to provide 29.6V.

The two packs fit nicely into a plastic field box from Wal-Mart and are connected in series.

On top of this, separated by a piece of EVA foam and attached to plywood are two buck converters and an ESPixelStick V3. One buck converter runs at 24V for the misters and the other at 12V for the ESPixelStick and pixels. A fused link to the switch in the lid turns everything on. All wires pass through cable glands for strain relief and the rear of the box is drilled for venting.

Finished Product

Unfortunately, I don’t have any video of the prop being used in the dances as they don’t allow recording or photos at competitions. We plan on using it in our Halloween show this year though and will get some better footage then. Here it is rolling around the garage to give an idea of the effect.

To top it off, they’ve won a few awards with the dance. One in particular was “Great Use of Prop”, which the team allowed me the honor of posing with 🙂