Introduction: Mini Fogging Cauldron - No Dry Ice, No Fog Fluid

About: There are some things you should just NEVER do.....

Self-Contained Mini Fogging Cauldron - No Dry Ice, No Fog Fluid

Want a tiny, completely self-contained fogging cauldron only suitable for the smallest of witches? Well, read on...

While walking through a store just before St. Patrick's Day I saw some miniature cauldron 'pot-of-gold' party favors. I thought, "I'm not that into St. Patrick's Day, but these would be great for Halloween!"

Why put gold coins in these mini cauldrons when they would be great for brewing a bubbling potion?

Then, as serendipity would have it, while looking for some electronic atomizer boards, I saw a very small battery-powered, self-contained atomizer circuit board. These atomizer boards are like mini versions of the larger ultrasonic atomizers I have used to create fog before; see my Water Only Fog Machine.

The main build of this Instructable is essentially a small Fog Module that just drops into the mini plastic cauldron. This Fog Module is ideal for this mini cauldron but can be used anywhere you want a mini source of fog/smoke.

The electronic atomizer board is really the wonder of the whole project. It contains the electronics to drive the piezo atomizing disk along with a charging circuit for the lithium ion battery, LED indicators and operation timer.

Warning: Technobabble follows about why this is so cool. You can skip to the next step if so inclined.

The piezo disk (transducer) is different than the ones I have used in the past. In the past I have seen solid disks that vibrate at ultrasonic frequencies that are placed under water that cause the surface of the water to break up into fine mist (fog) droplets. These new disks have tiny (probably laser drilled) holes in them that pump super fine sprays of water when the piezo disk vibrates at ultrasonic frequencies. The disks get the water to spray from a fiber wick saturated with water that is contacting the back of the disk.

Going from seeing the plastic party favor cauldron in the store to the finished Mini Halloween Cauldron took a bit of experimenting.

I had to make sure all the components would physically fit.

I had to test the atomizer to see if it provided appropriate fog - both volume and flow. I had originally planned that I would need to provide a mini fan in the bottom the cauldron since I have seen fog stagnate and not move when using other ultrasonic atomizers (foggers). This new type of disk was so energetic that a fan was not needed.

Most installations have the piezo disk mounted horizontally and the water mist spray pointing vertically. This orientation had several problems:

A horizontal disk took up a large amount of area of the cauldron opening.

I would have to provide a deflector over the disk since I did not want the fog spray to be vertical.

If water fell back onto the disk the disk would stop spraying fog.

All of these issues were solved by mounting the disk vertically on the inside surface of the cauldron (Water Cup) and spraying the fog horizontally. It greatly simplified the design and worked great!

Now on to making a Mini Fogging Cauldron....

SAFETY CAUTION: This project includes a Lithium Battery. This project should not be left unattended while operating or unattended while charging. Lithium batteries can overheat and cause personal injury and/or fire and death. If the battery gets hot it can be disconnected by cutting the foil battery tabs with scissors. Try to avoid shorting the foil tabs of the battery together with the scissors while cutting.

Use CAUTION and have FUN.

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You can see most of the supplies needed in the photo. I've also included a close-up photo of the electronic board and a cutaway diagram of the Fog Module (for lack of better terminology).

Supplies Needed:

Alternate fog illuminating LED connection:

  • CR2032 Lithium Coin Cell Battery
  • CR2032 Battery Holder

Step 1: Build the Water Cup

The Water Cup holds the water (go figure) and is the main structure.

I didn't use the plastic cauldron to hold any liquid, but I did build a Water Cup out of PVC pipe and sheet to slip inside the plastic cauldron. The Water Cup is constructed of thin-wall PVC. This is not Schedule 40 pipe. This is thinner wall that is available at some big box hardware stores. I used the thin wall so it gives a greater diameter for the opening, no other reason. If you want to use Schedule 40 you can.

I wanted all of the joints in the water cup to be PVC solvent bonds (solvent melts the two pieces of PVC together) to prevent leaks. So the bottom is from PVC sheet. This is a foamed material. I would use solid sheet if I could find it, but this works OK. Cut out a piece of sheet large enough for the pipe to be glued to it.

Make sure you have cut the end of the pipe nice and square. Cut a piece of sheet PVC large enough to accommodate the pipe. Apply PVC cement to the pipe and glue the two together. After the cement has dried, cut off the corners of the bottom sheet and sand the bottom sheet to match the shape of the pipe.

Place the circuit board (with the battery folded under it) into the plastic cauldron, place the PVC pipe with the sheet glued onto the bottom into the cauldron on top of the circuit board and mark the height of the future water cup (at the top lip of the cauldron) on the PVC pipe with a pen.

Cut the PVC pipe to length. This is the Water Cup that forms the main structure of the Fog Module.

Step 2: Construct the Ultrasonic Disk Mount

The Ultrasonic Disk Mount holds the vibrating transducer disk that generates the mist (fog). The transducer disk is held oriented vertically so the mist sprays out horizontally toward the side of the Water Cup (PVC Pipe).

The Disk Mount is constructed from thin solid PVC sheet that I finagled from the thin PVC pipe by making some calculated cuts and heating it (best done outside) and flattening it between two flat surfaces. I did not want to use the thicker PVC foamed sheet because of space limitations. This flat sheet was probably not flat enough for the bottom of the Water Cup, but was acceptable for the Disk Mount.

I glued a stiffener to the top and bottom edges of the mount to give it strength. I drilled two holes in the Mount; one the diameter of the transducer opening and the other to accommodate the solder connections since they sit proud of the disk.

The end cuts of the Mount were sanded so that they matched the curvature of the Water Cup pipe.

The Mount was glued in place with PVC cement to the wall of the Water Cup about 5/16 inch down from the top lip of the Water Cup.

Step 3: Create the Circuit Board Mount

The Circuit Board and Battery are mounted to the bottom of the Water Cup. The Circuit Board Mount consists of two walls made from 0.2 inch (5mm) thick PVC sheet scrap were added to keep the Circuit Board from moving from side to side. In order to get the spacing between the two walls correct I wrapped wax paper around the Circuit Board to keep the cement off and used it as a gauge to get the spacing right. I glued the side walls in place with PVC cement.

Step 4: Create the Bottom Cover

The Bottom Cover is a piece of 1/8 inch PVC sheet that captures the Circuit Board between the two Circuit Board Mount walls.

There are two holes drilled into it. One is to accommodate the small coil that sticks up above the surface of the circuit board. The other hole is to allow access to the push button that turns the board ON and OFF.

The holes are drilled, the sheet is cut out and then taped into position on the bottom of the Water Cup so that the corners can be cut off and rounded to match the profile of the Water Cup (PVC pipe).

After the Circuit Board is placed into position the Bottom Cover is again taped into place to capture the Circuit Board. Remove the Bottom Cover for now.

Step 5: Install the Disk and Fiber Wick

In order to install the Transducer Disk you will need to unsolder the Disk wires from the Circuit Board. You can see HV+ and HV- markings on the Circuit Board where the wires attach. I used black pen to color the HV- lead wire before unsoldering it to keep polarity identified. Remove the silicone 'donut' gasket off of the Disk prior to mounting.

The Fiber Wick, which carries the water to the back of the Transducer Disk, is cut to length to go from the bottom of the Water Cup to the top edge of the Disk Mount. The Fiber Wick is cut in two down the length as the wick is too large in diameter to fit behind the Disk Mount.

The Transducer Disk is positioned behind the Disk Mount with the Disk center and solder joints lining up with the appropriate holes.

The Fiber Wick is slid behind the Disk. I found it helpful to cut a small angle on the bottom edge to help it get past the Disk when starting to insert the Wick. Push the Wick all the way down to the bottom of the Water Cup. The Transducer Disk may want to slide down the mount as the Wick is pushed into place. I found it helpful to make a small hook out of a small paperclip to grab the bottom edge of the Disk and hold it while the Wick was pushed into place.

Step 6: Install the LED and Disk Wires

Holes for the LED and the Disk wires are drilled through the bottom of the Water Cup, paying special attention to where the wall of the Water Cup meets the bottom.

I recommend coating the LED lead wires with a thin layer of epoxy glue or other glue as they will be underwater at the bottom of the Water Cup. LED lead wires are actually plated steel so they will eventually rust if not protected.

Insert the LED leads through the appropriate LED holes. Take special note of the polarity of the LED. I marked it on the side of the Water Cup where the leads exited the bottom. The long lead wire is positive (+) and the shorter is negative (-). If you accidently cut both leads the same length without noting which is which you can also identify the negative lead by a flat spot on the flange at the bottom of the LED.

Wires need to be soldered to the LED lead wires very close to the bottom of the Water Cup. Use different colored wires so that you can keep track of the polarity.

Insert the Disk wires in the appropriate holes. After all the wires are located in the appropriate holes cover the holes at the wires with silicone sealant to make the holes are water tight.

Step 7: Modify Circuit Board for LED Connection

I wanted the LED illuminating the fog to automatically turn on when the cauldron was running. The Circuit Board has a blue surface mount LED that lights when the atomizer is operating. I wanted to use this fact to light the green LED in the Water Cup when the unit operated. Not knowing the ability of the integrated circuit to drive two LEDs I had the thought of disconnecting the blue LED and connecting my green LED in its place.

This process is not for the uninitiated or faint of heart ;-). So if you are not comfortable soldering on small traces you can opt out and use the alternate method to power the LED described in the next step.

The one side of the blue LED goes to the battery (+) bus through a 330 ohm resistor. The negative LED connection goes to pin 7 of the integrated circuit as shown in the modification photo. Follow the modification photo to cut the appropriate trace to disconnect the blue LED and solder the negative Green LED wire to pin 7 and the positive Green LED wire to the battery (+) bus through a 330 ohm resistor, scraping off the green solder resist to expose bare copper.

Note: Not shown is a 330 ohm resistor in series with the positive Green LED wire inserted between the positive Green LED wire (red wire in photo) and the battery (+) bus (scraped circuit board area). I forgot to replace this resistance as the blue LED had a 330 ohm surface mount resistor in series with it.

The Disk wires are soldered back to the solder pads they came from. Be careful to have the negative Disk lead wire go back to the HV- pad and the positive wire to the HV+ pad.

Step 8: Alternate LED Power Method

If soldering LED wires onto small traces is not your thing, you can use an external battery source to power the green LED from a coin cell lithium battery, a CR2032 to be exact.

You will need to solder the positive and negative LED wires to the matching (+) and (-) battery holder terminals. This will allow the green LED to glow continuously. You will have to turn the LED on and off by inserting and removing the battery as desired. Make sure to insert the coin cell in the proper orientation to make sure the negative battery terminal (smallest, the button) goes to the negative terminal of the battery holder so the LED is connected to power correctly.

You also have to be careful to tuck the battery and holder into the cauldron before inserting the Fog Module and make sure it is located in the free space to the side of the Water Cup.

Step 9: Secure Circuit Board

Place the Circuit Board between the Circuit Board Mount side walls and place the Bottom Cover over it aligning the holes with the transformer and power button. Tape the Bottom Cover tightly into place with electrical tape.

Step 10: Blacken the Water Cup

The white PVC pipe of the Water Cup just does not work well with the black cauldron. So it's time to take out the permanent marking pen and color the top edge of the pipe black. Black electrical tape is applied to the circumference of the PVC pipe to blacken the wall of the PVC pipe.

Step 11: Drill Hole in Cauldron for Switch Access

Drill a hole in the bottom of the plastic cauldron so that the power button can be accessed.

Optionally a button extension of miscellaneous plastic rod or a small dowel can be glued to the power button to make it easier to turn the Fogging Cauldron ON and OFF. Make sure none of your glue runs down onto the button. (This happened to me and it was a pain to fix!)

You can charge the battery by plugging a micro USB cable into the charging port. The red LED will flash as the battery is charging and stay steady when it is fully charged.

NOTE: Do not charge this lithium battery unattended as you need to be present to intervene if it overheats. Overheating batteries can cause personal injury and/or a fire. See final step "Safety Improvements". If the battery overheats you can disconnect the battery by cutting the foil tabs with scissors. Try to avoid shorting the two foil tabs together when cutting.

Step 12: Test the Fog Module

For the test, instead of filling up the Water Cup I decided to add water directly to the Wick with an eyedropper. Use Distilled water to reduce the likelihood of the Disk micro holes from being clogged with minerals. Saturate the Wick and then push the power button to turn the unit on.

The unit should turn on, generate fog and the green LED should be glowing nicely.

Step 13: Final Assembly

If everything is working OK carefully slide the Fog Module into the plastic cauldron (mind the battery holder if you used the alternate LED power), add distilled water to the Water Cup up to the bottom of the Disk Mount and start getting SPOOKY!

Enjoy your Mini Fogging Cauldron!

Step 14: Safety and Other Improvements

When I received my first two Circuit Boards I tested them both. The first one worked for a few minutes and then a transistor on the board started smoking and then the lithium battery started getting hot. I had to disconnect the lithium battery by cutting the foil tabs with scissors.

This project should not be left unattended while operating or unattended while charging.

Lithium batteries are nothing to take lightly. They can overheat, explode and start fires. If the battery gets hot it can be disconnected by cutting the foil battery tabs with scissors. Try to avoid shorting the foil tabs of the battery together with the scissors while cutting.

I strongly recommend the following two additions, which I will soon document:

  1. Add a fuse in series with the battery. A 250mA (0.25A) pico fuse (looks like a resistor) should be added between one of the battery tabs (unsolder it) and the circuit board battery connection.
  2. Add a micro slide switch to fully disconnect the battery when the cauldron is not in use so that the power is fully disconnected and there is no small trickle drain keeping the push-on, push-off logic operating.

As a functional improvement, I noticed that the circuit board turns itself off after about 7 minutes operation. I intend to add a MOSFET transistor tied to the operation indicator LED (blue, now green) to trigger a simulated button press when the LED turns off, thereby turning the unit back on. This modification would require the slide switch, addition #2, in order to turn the unit off.

Halloween Contest

Seventh Prize in the
Halloween Contest