Introduction: Witches Cauldron

About: I am a contract software developer, mainly web based these days. My hobbies are fixing stuff that wasn't broken until I got my hands on it, electronics, Arduino and Modern Jive Dancing.

Being an Instructables fan, I read a lot of Instructables, and like many other people, get inspiration to do something similar for myself.

I wanted to create a Cauldron.

I wanted to create some animatronic eyes.

I like to play with LEDs.

I like to use Arduinos.

So, as usual I decided to do something a little 'over the top and time consuming' and build a cauldron with eyes and coal effect and smoke.

This is not a tutorial on exactly how to do everything in fine detail, although most parts will be explained.

Pictures were taken at random times, so, again, not step by step instructions.

The whole project was completely not thought out enough and I added or took away bits as necessary.

My suggestion to anyone attempting to do something slightly over the top, is to try and plan it first.

Main Parts List:

Gym ball.


Power supply.

Gap filler foam.

Coal lumps from an old gas BBQ.

Short length of PVC waste pipe.

Meccano pieces.

Cheap packet of snakes.

PVA adhesive (2 parts water, 1 part PVA).

Glue gun.

Filler of some kind.

Various paints.

Lots of waste paper.

3D printer.

Smoke machine.



Step 1: The Cauldron

This was made papier-mâché style (ish). I used PVA as the adhesive mainly because I already had some.

The paper supply came from any old paper documents that were stored in the loft and were at least ten years old and not required to retain anymore.

I started with an inflated gym workout ball and covered it with cling film to stop any adhesion and therefore not ruin my ball.

The air stopper was placed at the top so that I could deflate the ball when I was done with it.

The paper was torn into strips and layered onto the ball with the PVA. Basically I just kept on layering it until I ran out of paper.

The layering went on for days. I added 2 or 3 layers at a time then took the whole thing indoors in the warm to dry properly overnight, then the next day I repeated the process.

Obviously I left a reasonable gap at the top because cauldrons are made that way.

When the paper ran out, I found that the walls were about 8 to 10mm thick, so I thought that would be enough; and it was plenty. I now have quite a rigid product.

The ball was deflated and then easily removed and I was left with a cauldron shape, sort of.

The top was trimmed, softened as best possible and turned outwards to create the lip. More papier-mâché was added to make a tidier rim.

This created big problems for me further down the line because I changed my mind and decided that I wanted a bubbling look to the top.

The problem was that the top (hole) was not uniform and smoke leaked everywhere. More on this later.

The cauldron was stood onto an old tray, mainly to hold the coals, but it also made it easier to carry.

Once the eyes were made I could see roughly where the holes had to be for them and gently cut them out.

A couple more holes were also required for the power inlet and smoke tube.

Papier-mâché is not the easiest material to cut neatly so I made some of the holes larger than needed and tidied up the edges with more paper and a bit of filler.

Step 2: The Animatronic Eyes

I do own a 3D printer but it was very cheap and it is not very good, but I did manage to print the main parts from a Polymaker_3D item on Thingiverse.

I didn't manage to successfully print the base but I was able to get accurate dimensions from a couple of failed prints and cut it out of thin plywood.

The only other part that I had to improvise was where the connection rods joined to the eyes.

Polymaker_3D (the original designer) used a flexible plastic (PolyFlex) which I didn’t have. I used some 3mm tubing that I had lying around and hot glued it both ends to the connecting rods of the mechanism. It needs to have a reasonable amount of flexibility to it otherwise it will not work successfully.

The eye balls needed a lot of rubbing down, but I managed to get a reasonably smooth surface.

A bit of filler was used to give a raised effect in the very centre for the pupil.

Painting was somewhat trial and error as I am not very artistic at all, but the end result was not too bad.

For the veins, I scrapped a scalpel along some red cotton thread until it split into finer strands and just used PVA to fix them roughly and randomly in place.

To fit the eyes into the cauldron I created a bit of a framework of Meccano and bolted them to the front of the cauldron. I was a bit worried that just fixing the front would not be enough so I used a bit more Meccano to support the rear of the board to the bottom of the cauldron.

Once the eyes were fitted, papier-mâché shaping was applied around the eyes and to create the nose and eyebrows. Any fixing bolts were also covered up at this time. The eyebrows were finished off with an old paintbrush, with the bristles cut up and randomly placed.

I initially wanted the eyes to follow movement in front of the cauldron using ultrasound sensors, one for right and one for left. When the eyes were not installed, the mechanism worked really well (at least as well as these sensors can). They were installed with the sensors just above the eyebrows as shown in some photos. I didn’t like the look of them but they gave the desired effect, so I just went for it.

They didn’t work. I think that when I plugged them in for the first time I fried them.

To remove and/or replace them now would be a bit of a major exercise so I decided to do away with them, cover them up and modify the code so that the eye movement was just based on random calculations.

Step 3: The Coal Effect

I had an old rotting gas BBQ behind my greenhouse, which had lumps of coal (coke), so I decided that prior to taking it to the tip I would re-purpose the coals.

I created a string of 18 red and yellow LEDs and wrote some Arduino code to make them flicker at random times.

The LEDs were placed randomly around the base of the cauldron and I then proceeded to hot glue the lumps of coal around them.

This left gaps where some of the LEDs were visible, which looked a little too obvious.

Like I said before, I didn’t think this out completely and therefore making it up as I went along.

This is where some gap filler foam came in. I gently squirted foam into the obvious gaps and a few more. Once the foam had cured, I broke the lumpy bits off of the front and dug a bit more out for good measure.

If you attempt something like this yourself, don’t squirt too much foam into the holes as you may just cover up some of the LEDs completely (like I did). I think that I have hidden 4 or 5 completely!

Once I was happy that I’d removed as much foam as I needed to I then proceeded to make those areas look a little like glowing embers with various coloured acrylic paints. I did say that I was no artist!

I finished the bottom of the cauldron with a watered down black acrylic in a spray bottle to give some kind of sooty effect.

Step 4: The Smoke

In a previous life, I was a DJ. Not a good DJ, probably one of the worst DJs ever, but it did pay its way for a number of years.

A few years ago I decided to give it up, but I knew that if I kept the equipment I would end up doing gigs for friends and family so I gave most of my equipment away to the local Boy Scouts, but kept the CDs, a couple of the gantry lights and the smoke machine.

There is no way that the smoke machine can go inside the cauldron so the smoke has to be piped in from the outside.

A hunt around my shed turned up a short length of black waste pipe. I drilled some reasonable sized (10mm ish) holes along it and fitted it into the cauldron. The open end of the pipe inside the cauldron was taped up otherwise all smoke would just push itself out of that end.

Because most smoke machines use a water based mixture, the pipe will get condensation build up inside of it, so I decided to insert the pipe at an angle so that any moisture can run to the outside world.

The smoke machine itself does not need to be physically attached to the pipe because the smoke is forced out, it just needs to be as close as possible. By not being attached, any water vapour in the pipe can just drip out without actually touching the machine.

My machine has a remote button that needs to be pressed when the machine is ready. The supply to the remote switch is 5 volts and I did think that I could easily replace the switch and control the machine with an Arduino but I ran out of time. The plan was to just let the Arduino turn on the smoke for about half a second every five seconds, or something like that.

Step 5: The Bubbling Effect

This is where the rim of the cauldron proved to be a bit wrong.

Unfortunately, by this time, most major parts are now complete and I have now changed my mind on what I want to do. Again.

Back out into the shed, I find an old cheap wok type thing. Not sure where it came. Another previous life I expect.

I looked at parts of the wok and think, ‘this might just fit’.

Well, it was close, just not close enough. The rim that I had created was not perfectly round and it was nowhere near flat either which meant that too much smoke would escape where I didn’t want it too.

The rim now needed some re-work, so I set to with glue gun, filler and finally papier-mâché to blend it all in again.

I drilled a number of holes in the wok.

I drilled holes, tested the smoke, drilled more holes, tested the smoke etc, until I was happy with the result. What I ended up with is one hole almost half the size of the wok. I found that the pressure behind the smoke was not enough to force it through the holes but enough to find every weak spot around the rim of the wok and through the eye sockets.

During the testing, I set off the smoke alarm, which is internet connected and immediately got a call from my landlord asking if I had burnt the sausages, which took some creative explaining. I didn’t have any sausages and now I’m hungry and want some.

The wok also came with a grill/grid/thing which fitted just below the rim. I got out my filler foam and squirted gently onto the grill and kept releasing the trigger to create ‘balls’ of foam. I covered the whole grill.

I used the grill as a template really as it was the right size and shape, I originally thought to leave it attached to the foam for stability but it meant that cleaning out the excess foam underneath was almost impossible so I eventually removed it.

Once the foam had cured, I went around the edges with a scalpel and trimmed the foam to the same level as what would be the top of the grill when inserted. That way it fitted nicely into the wok.

Smoke needs to be able to penetrate through the bubble effect, so I made lots of small holes through the foam in places where they cannot be seen too easily. A bit of trial and error as usual but we get there eventually.

I wanted the bubbling smoke stuff to look green so a number of green LEDS were placed into the foam and wired to a USB lead and plugged in as required; and it was rubbish. I used 20 green LEDs but through the foam they looked yellowish and not very bright. I found some different green LEDs and added another 20 to the mesh. It was better and green this time but still not very bright.

I didn’t want to use a mains spotlight even though it would most likely have worked very well. I wanted to use just the low voltages from the power supply.

I found a 3 meter strip of green LEDs and stuck them just under the rim of the cauldron. These are 12v, but as I am using an ATX power supply (see below) anyway, 12v was easy.

Again not as bright as I would have liked but you have to draw a line somewhere.

The surface of the bubbles was painted green with a very weak mix of green acrylic and water in a little spray bottle. A few very wet coats of this gave the light on the surface and darker on the lower parts effect.

Step 6: The Power Source

The power source is part of another project that will most likely eventually be put on Instructables. I know that it has been done by many others but they are all different and one more just helps inspire others to do their own thing in their own way for their own needs.

You have probably guessed by that remark that it is an ATX power supply from an old computer.

What I did for this is removed the main 24pin power socket from a dead motherboard and fitted that to the rear of the cauldron.

By doing it this way I can just use any ATX power supply that I can get my hands on, although that is only two at this moment in time.

Batteries, were never an option and as it turned out, I needed 5v, 6v and 12v for this project.

This was probably the most organised part of this project.

A switch and two LEDs were fitted to the rear of the cauldron.

One LED to show that the power supply was plugged in and powered up, the other is activated when the switched is pressed (green wire to GND) to supply power to the Arduino etc.

The pins on the 24 pin socket were broken out onto a piece of PCB. This Power Supply used the more common 20 pin plug but this still fits the socket and because of the pin shaping, ie. only fits in one place. I can now power my cauldron with either 20 or 24 pin ATX power supply.

I soldered three USB sockets onto the PCB as well as a small buck converter (to give me 6v).

The USB sockets will run the Arduino and the Bubbling effect the buck converter is wired into one of the 12v lines and reduces the output to 6v solely for the servos.

When I linked the boards to the power socket and the animatronics, I used jumper wires rather than hard wire everything, that’s how I fried the ultrasound sensors; and I used standard USB leads to connect things. My reasoning behind this is so that I can modify it to my hearts content without any major upheaval. Plus I can just remove the bubbling effect as it is on a USB plug as well as reuse the Arduino if I ever need to.

Step 7: The Snakes

I was walking past the local toy shop and in the window they had some really cheap stretchy snakes. So I bought a pack.

They come very brightly coloured, so once I had them all hot glued around the rim of the cauldron I painted them to show off my complete lack of artistic skills once more.

Step 8: Severed Hand

I found, somewhere on eBay, a severed hand, which I thought could be a nice addition although not sure where I would use it at the time.

The idea of making a finger move seemed like something doable so I set to with another Arduino, a servo and some more Meccano. Plan A.

After pulling the stuffing out of the hand and inserting the new finger into it I found that the servo was not that great when trying to move the rubber(ish) finger. It moved but not by much.

I didn’t want the finger to move much, just a bit of a random twitch really, but this proved to be more problematic with my limited engineering skills and even more limited supplies.

I searched Thingiverse for prosthetic hands and found something that maybe would work, so I set out with my next to useless 3D printer to create the parts required. Plan B.

I now had a finger, printed in three rough parts and pivoted at the knuckles.

I managed to attach it to the Plan A servo base and control it with strong string. It worked great outside of the rubber hand but once inside the hand there was no movement at all. The servo was just not powerful enough.

As this was my last servo and running low on time to get a bigger one, I resorted to Plan A. I only wanted the finger to twitch at random times and I did have movement so I called it quits and left it at that.

The paint job that came with the hand was about as realistic as an honest politician, so again I set to with my not so artistic skills and did what I could.

The rope was a length of blue nylon rope that was located in the shed and painted brown and smeared with different shades of blood ish colour.

Step 9: Finished

I hope that

this Instructable has been interesting for you and maybe inspire you to do something yourself.

Be gentle in the comments :)

Step 10: Documents

Reclaimed Contest 2017

Participated in the
Reclaimed Contest 2017

Halloween Contest 2017

Participated in the
Halloween Contest 2017

Make It Move Contest 2017

Participated in the
Make It Move Contest 2017