Magic Cauldron, Blowing Glowing Bubbles!




About: I love tinkering with about anything in the rare spare time I have, and when my wife and three daughters let me. I'm particularly interested in 3D printing (DIY Kossel printer), electronics (amateur with a l...

This magic cauldron was built in preparation for the birthdays of my two older daughters -- like many children they're both extremely fond of the Harry Potter novels and they both want a Harry Potter theme party. The cauldron has a very magical effect: inside, there's a bubble machine that blows bubbles upwards, as well as a line of UV LEDs. In combination with UV bubble fluid, in the dark you will see glowing bubbles emerge from the cauldron. As often, the effect is hard to catch on a smartphone camera, but it actually is really beautiful.

Now, there is another story to how I came to building this, and I think it's worth telling: last year, I constructed my bicycle bubble machine and published it. On thingiverse, a user commented that this machine would be really cool using UV bubble fluid and corresponding LEDs. I had never heard of UV bubble fluid before, I was intrigued by the idea, but my budget for tinkering was completely exhausted by my first 3D printer I had built that year. So I answered: I'll go for it somewhen, but, frankly, I'm broke. This read another thingiverse user, overflo from Vienna. He had a spare bag of 50 UV LEDs lying around, liked my machine and obviously likes to give back to others -- two weeks later, the LEDs arrived by mail, complete with corresponding resistors, a wired example and a helpful letter, because I had told him I'm rather an electronics beginner. A year passed by and I didn't get down to building something with the LEDs, until now. Well, it's not for the bicycle, I did face a lot of problems, I did a lot of mistakes, but I like my magic cauldron, despite all difficulties and shortcomings. And: I finally put overflo's LEDs to good use.

Please note: This instructable makes use of electricty and lots of fluids, two things that don't mix well. Really take care that you don't have the high voltages in the places where it get's messy, and make plenty use of heat shrink tube, isolating tape and similar things.

Step 1: How It Works, and What Didn't

There was one huge obstacle to my magic cauldron idea: there didn't seem to exist any vertical bubble machine. Every machine I could find was working horizontally, which is due to the way nearly 100 % of bubble machines are constructed: you have a wheel with bubble wands, which dip into the bubble liquid when they're low, and pass a fan when they're up high. So what I needed was a vertical bubble machine. I could think of two ways of building this: my first idea was to use a belt system, with holes in the belt, instead of bubble wands. Using four rollers the belt could pass low through the liquid; the fan would be mounted in the middle of the belt.

Some things seemed complicated to me and I didn't have much time before our vacation and the wizarding contest deadline, so I setttled for a different approach: a classical Ferris wheel, again with the fan mounted in the middle. The vertical bubble machine uses a simple 80x80mm fan for blowing the bubbles and a small gear motor for turning the wheel.

The magical glowing effect is realized by UV-A LEDs that sit just under the edge of the cauldron, directed upwards, and by using a UV active bubble fluid, like its sometimes used at partys or discos. The effect is of course most impressive when it's getting dark.

While up to here everything worked like expected, my decision to make a cauldron of my own, first trying it with papier maché, then with plaster, really wasn't worth it. If you want to build your own magic cauldron, please consider the easy alternatives I'll mention when it comes to this step! Also for the LED stuff, there are easier ways which I'll mention.

Step 2: What You'll Need

If you want to build this cauldron, please first read the whole instructable before buying things needed -- while mine works, you'll want to do some things differently from me. I'll talk about this in the following steps.

Let's begin with the bubble machine. You'll need:

  • a 3-6V geared motor like this
  • a 80x80mm 12V fan -- you'll find lots of this in PCs or printers, but the one I scavenged from a printer didn't perform that well. I then bought this (80x80x38mm Lüfter M-FAN DC DZ08038B12HA) which clearly seemed to be overkill. But it had the advantage that it did a just right air flow when driven at only 5V and this way I could use just one stepdown switching regulator for both motors.
  • a few small M3 screws and nuts to attach the fan to the fan mount.
  • Super Glue
  • water-tight foil (for alternatives, see later)
  • the printed parts. Files are attached to this step: You'll need the following:
    • the base plate
    • two sidewalls
    • one geared wheel
    • one simple wheel
    • one pinion
    • one fan mount
    • the two parts of the electronics box
    • 8 bubble wand connectors -- you choose which, for the cauldron I liked the two with the smallesT rings (BubbleWand1.stl and BubbleWand4.stl), but print and mix them as you like
    • start without the funnel part -- I used it when trying to get a PC fan to work but then went for a different fan; it might work for you, for me it didn't

Now for the lighting, like I did it, you'll need either:

  • UV-A LEDs -- the more the better, but 8-16 fine; mine were best driven at 3.5V, using 20m;
  • a corresponding constant current source for driving LEDs at 20mA, like this: these little chips make the whole soldering much easier, as you won't need resistors: just solder them in a daisy chain, put your 32V on one side, and thats it :)

Or, if you think about my advice in Step 5:

  • a simple UV LED stripe, which needs no or at least less soldering and is less of a hassle to mount.

And for making it all live:

  • some old scavenged power supply -- mine gave 32V DC which is more than enoug; if you take my advice and go for an LED stripe, use a 12V one
  • a stepdown switching regulator, I used these

Tools and stuff:

  • wire cutter
  • soldering iron, solder
  • strip board (or something similar)
  • standard pin strips
  • possibly gaffa tape or similar
  • heat shrinking tube

For the cauldron: I'll talk about this in the corresponding step.

Step 3: The Core: a Vertical Bubble Machine

The vertical bubble machine is the part of this project I'm really proud of -- it's a nice bubble machine, even when not needing it for a magic cauldron, and for many occasions it's suited better than a typical bubble machine: the horizontal ones you always have to put in some higher place, so the bubbles can be seen; this machine just blows them upwards. I embedded a video with only the bubble machine to show this nice effect.

Building it is straight forward. Take a look at the pictures. On a WebGL enabled browser (not in the instructables mobile app, sorry) you can also consult this interactive rendering, which also allows you to gradually change to an exploded view:

For those without WebGL, I attached a PDF drawing.

First, print all the pieces -- dont' care much about infill, as everything's pretty flat anyhow. I normally use three perimeters, but nothing will be strained very much in this machine. It's up to you, which of the bubble rings you use -- I like to mix the smaller two types for the cauldron, but mix all types of rings when using it as a normal bubble machine.

See the assembly drawing I made in Fusion 360 as to how to put it all together. First mount the side pieces on the base plate; put the wheels on the side pieces. Now, first mount the fan and the motor on the fan mount plate, the fan with a few M3 screws and nuts (you don't have to take long ones and go through upper and lower holes, if you only got short screws, just go through the lower holes of the fan only), the motor just press-fit; please note that in the end I decided to stick the fan mount up-side-down, as compared to the interactive design and the PDF drawing: it doesn't really matter, but I thought this shields the little motor and its connections better from splashing liquid from above. However you do it, when mounted, the fan should be on the upside of the fan mount plate. The fan mount plate is pressed into the hollow shafts in the side pieces. The small geared motor has to be mounted so the small pinion meshes with the larger gear. Last step, push the bubble rings in the wheels, connecting both wheels.

The wires from the motor and fan go through one of the hollow shafts to the outside (see the photo).

Step 4: My Personal Waterloo: the Cauldron

A word of caution: you don't need to do what I did. In fact, only a complete moron would do what I did. Take a look at the first photo -- a simple planting trough, doesn't cost that much at some gardening store; make sure your bubble machine fits into it (mine would have fit, if only I had had the idea before); spray it bronze or gold. That's it. So, all you need here is:

  • a planting trough
  • bronze spray

If you happen to have a bronze cauldron at home, that's of course fine, too.

Now, not for you to copy this, but only for laughing about what a dumbass like I am has to go through when making something like this for the first time:

I decided to build one on my own, starting with the good old papier maché balloon, inflating a large party balloon to the right size for my bubble machine. I then spent a lovely evening with my kids, tearing newspapers, dipping them in wallpaper glue and putting it on the balloon. We let it dry and repeated. Let it dry and repeated. Let it dry and repeated.

Decided this won't work out and especially not get stiff enough. Bought plaster strips. Dipped the strips in water, put it on the papier maché balloon. Let it dry and repeated. Let it dry and repeated. Covered some cable channel with plaster strips for the edge, as well as printed feet. Let it dry.

Cut open the balloon where the edge of the cauldron later would be and noticed the papier maché was still wet on the inside. Scratched it all out of our plaster balloon. Decided the structure of the strips came through too much. Went over it with normal plaster. Tried to sand it to make it more smooth. Forgot that I should do this outside, not in the living room. Tried to withstand my wife's nervous fit. Went outside to spray it bronze. Decided not to try anymore but just get going.

Today by chance found that perfect planting trough in some store. My turn for a nervous fit.

Believe me: just buy some sort of cauldron form and spray it bronze. It should have some hole to let the cable out.

Step 5: The Magic: Wiring the LEDs

First of all: I had these single UV-LEDs that Flo sent me, but you may want to take it easy and just buy a UV LED stripe -- that's easier to mount and you don't need to solder as much. In the end, you just want UV LEDs on the inside of your cauldron's edge, pointing slightly upwards. I using an LED stripe, where the single LEDs obviously can't easily be directed somewhere, I'd just mount it to the upper part of the round edge, so the whole stripe points upwards. If you don't have a bunch of UV LEDs lying around, there is no point in doing this complicated, use a stripe instead!

While Flo sent me the corresponding resistors, I went the easy way: I had these constant current source chips designed especially for LEDs needing 20mA. They work like this: connect your DC input (in my case 32V) to one side, connect a daisy chain of LEDs to the other, without resistors; according to the documentation, with the 32V I had I could connect up to 8 LEDs per source, which I did. As the chips come as little SMD parts, I soldered little pin strips to the input side, so I could then solder them in parallel to a little stripe board, which again I connected to the 32V power supply.

If you buy LED stripes, you'll use a 12V power supply anyhow, and don't need any separate resistors as they're included.

Use shrinking tube to isolate as much as possible of the bare wires.

Step 6: Powering It Up

To make it alive, the wires from fan and motor, routed through the hole in the middle of one of the side walls, have to be connected to a stepdown regulator (5V), as well as the LEDs connected to the power supply. The electronics should be enclosed, so they don't get any splashes of bubble liquid. I printed this enclosure with a round hole that can be glued to the side wall where the wires come out. Just route the wires through it before soldering everything together.

Use heat shrinking tube for all connections where possible! You can use hot glue to close the outer hole of the enclosure where the wires come out.

Take the stepdown switching regulator, adjusted to an output of roughly 5V using a multimeter, and connect the motor and the fan to the output; I soldered it onto a strip board for this. Connect the input to your power supply, routing the power supply cable through the hole in the other half of the enclosure.

The LEDs, on wires long enough to later attach them to the edge of the cauldron, I also connected in this box -- I connected the constant current sources directly to the 32V input. If you took my advice and went for a simple 12V LED stripe, you'll also use a 12V power supply and can connect the stripe directly.

The dc input I later routed through a hole in the cauldron to the separate power supply.

Take care that the high voltage side of your power supply is far enough from your cauldron!

Take care that the electronics don't get splashed in liquid! We don't talk about deadly voltages on the 12V side of your power supply, but it's electronics nevertheless!

Step 7: Putting It All Together

This part was really a bit awkward. As my cauldron wasn't water tight, I put a water tight foil into it, routing my DC input wire on the side of it through a hole to the outside, and used gaffa tape to fix the foil. You might have a watertight cauldron (much better -- just drill the hole for power in somewhere in the upper part or let the wire exit from the cauldron opening, if you don't mind), or might want to experiment with a decent water tight coating. I was fine with the foil, although it does look pretty makeshift.

I now carefully placed the bubble machine inside the cauldron and fixed the LEDs with gaffa tape -- if you do this, cover the wires as much as possible with the gaffa tape so they don't get easily shortened. But you of course took my advice and went the easy way with an LED stripe -- in this case you might want to put clear tape over it to avoid contact of the LED stripe with liquid.

Now there's another caveat: If, like mine, your cauldron has a round bottom, you might be surprised by how much bubble liquid it will take to fill it up to a point where the bubble machine can grab it -- the machine has in the current version a flat bottom and thus will not stand on the base of your round cauldron, but higher. In the end, I threw a lot of small pebbles into the liquid to make its level rise. A good idea would be to design a round base for the bubble machine, or a base with an integrated liquid tank; I might get down to this some time after my vacation where I write this down now.

Step 8: Magic!

If all went well, try it when it get's darker: It's a wonderful effect! Position the cauldron at some elevated place where people don't watch directly into the LEDs and where they don't trip over the power chord or otherwise mess up things.

If you build this, I hope you'll benefit from the mistakes I made -- using a different cauldron, LED stripes and a tighter liquid reservoir I think I would do this much better next time. But it was fun anyhow and the kids love it :)

One idea for making this better I'd still like to share: using an IR sensor, a small arduino (plus motor shield and a shield for powering up the LED stripe) and that Magic Wand Remote Control, it would be possible for the cauldron to light up and start bubbling only with a certain gesture of your magic wand. Combine this with a good incantation ...

Have fun!



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    14 Discussions


    10 months ago

    I love the vertical bubble machine concept. Great Job! UV LEDs are a super idea!

    I understand the challenge of cauldron building ! Thanks for sharing!


    1 year ago

    I finished first version. To be honest I was looking for pomper machine. Now I have to improve the tracción. Thanks for sharing this project with all details. It is so amazing and my twins are so excited!!!!!

    1 reply

    Reply 1 year ago

    Wow, it looks impressive! But it certainly can't compete with your wonderful kids :)


    1 year ago

    Hallo Heinzdrei,

    May I ask you what is the DC level for both engine (3-6V geared moto and 8x8 fan 12V)? 3DC volts for gear and 5 volts for fun ???

    Vielen Dank :)

    1 reply

    Reply 1 year ago

    I used these adjustable step-down converters and started with 5v for the geared motor and 12v for the fan. Especially for the fan, whether you want to keep this at 12V or adjust this to less depends on the model you use -- I used a very strong 12V-fan, so I gradually adjusted the voltage down until I had the sort of bubbles I wanted. For the geared motor it doesn't matter that much -- just try and see how you like it best!


    Reply 1 year ago

    cool! It makes me very happy to see its first build by somebody else :)))


    1 year ago

    This is so awesome proyect. I´m gonna try to build one, I will let you know how is going. Wish me good luck ;)

    1 reply

    Reply 1 year ago

    Glad to hear you like it, I'd really love to hear about the resuts! Thinking back, by all means go for a ready-made UV LED strip instead of daisy-chaining LEDs and you'll have much more fun than me :)


    2 years ago

    This is such a cool project. Very well done, all around! :)

    2 replies

    Reply 2 years ago

    Thank you -- and thanks very much for posting again after I somehow lost the first one, the feedback on instructables is really something that makes my life more beautiful :)


    Reply 2 years ago

    Glad to offer feedback!

    I love getting it myself - that really is the heart and soul of this creative community! :)


    2 years ago

    Great idea! It sure looks magical : )