Unity Candle

Taking the idea of a unity candle to the next level

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Some good friends are getting married, and have asked various people to help out with the wedding. They came to me with one request. To build them a "Unity Fireball" for the ceremony. This project involves creating a controllable flame effect, with focus on safety, and reliability. The flame effect portion of this project portion of this project is not encouraged, see disclaimer below. Other elements of this project can certainly be used in many other projects without issue. Specifically, the sequencing controller, prop construction and robust electrical control over long wires.

This project includes what I believe is a new innovation in the design of a propane poofer, in the use of a sequenced pre-release of propane which is not ignited. The project logs explain more about this.

A "Unity Candle" is sometimes used in a wedding to symbolize the two people coming together. Each participant uses a small lit candle, and together, they light a larger Unity Candle. Continuing this symbolism, but representing the background of this particular couple, they sought a much larger flame. A control box is located in the ceremony space to allow the couple to light the candle. It has a large firing button on the top, and key switches on the sides. They each have their own key to symbolize their joint efforts coming together. Once armed with the key, the large button becomes active to fire the candle. When fired, a 30 foot high fireball comes out of the candle itself.

Front Room Photography took this fantastic photo during the ceremony:

How the ceremony came together:  (this is not the THP entry video, see link on left for that)

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If you have any questions or comments, please use the comment box below.

Project log index:

More details
How it all worked out
Installation and setup of the system on site
Wiring the system on site and being a connected device
Construction of the firing button and keyswitches
Discussion about electrical robustness in this system
Rain shield for the hot surface ignitors
Estimating the heat output and costs of operation
Additional testing of various sequence parameters
Building the decorative shell
Code running on the Arduino Micro
Added circuitry, schematics and design notes
Modifications to the controller
Reverse engineering the existing hand controller
Notes on getting the parts
Initial manual testing of the propane poofer
Mounting the ignitors
Building the relay board to drive the poofer
Assembling the propane poofer
Initial discussion of safety design
Researching details of a propane poofer and possible improvements
Researching methods of making a fireball

Disclaimer: This project should only be attempted by those who fully understand it and have appropriate training. This project information is not appropriate training which must be sought out elsewhere. Anyone attempting must comply with their local ordinances and codes.


  • 1 × Liquid Propane 20lb Tank will lasts approx 50 bursts
  • 1 × Unfilled propane tank Preferably never filled with propane
  • 2 × 3/4" solenoid valve with Viton Seals, F-NPT Rated for propane. 12VDC used, 120VAC is an alternative.
  • 1 × Pressure regulator, 100psi Rated for propane. Substitute 60psi if desired.
  • 1 × POL fitting Full bore

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  • THP Entry: System Design Document, Video and Requirements

    Quinn08/21/2014 at 03:27 0 comments

    This log is intended to make clear all the entry requirement specifics in one place. Other log entries provide more details on the project and design.

    A block diagram of how it all connects is shown here: (System Design Document)

    The controller schematic is shown in this project log.

    The first prize video is also linked on the main page links, and here.

    Some comments on the contest requirements:

    This is hardware and software project where a completed functioning system will be built.

    The system is connected in that is uses a variety of interfaces to connect to a stage and lighting control system. Interfaces supported are DMX, MIDI, RS-232 serial and USB. This allows a show operator to sequence the firing with other lighting and sound effects in a performance.

    This project is being documented here, in build logs, parts lists and eventually build instructions. Schematics, software and images are provided here. Please see the extensive documentation provided in project logs.

    This project is fully open hardware and open software, with everything needed to recreate it located here on

    While the system as implemented here has safety concerns that make it an advanced project, several of the elements described are quite usable by many hackers, and can be used in a wide variety of other projects. Specifically, the sequencing controller, stage control system interfaces, prop construction and robust electrical control over long wires. Some examples of other projects could be a model rocket launch system, DIY dance lighting, and haunted house effects.

    The dual stage propane poofer is an innovative technique which can produce more voluminous fireballs with a smaller outlet size than existing poofer designs. The smaller outlet allows more economical and easily available plumbing parts to be used. No prior instances of this technique could be found in the burgeoning fire art or theater pyrotechnic communities.

    See the details section of this project above for an index to the topics covered in the many project logs.

  • More details

    Quinn06/13/2014 at 23:47 0 comments

    The ceremony control box:

    The candle in this case is 3 feet high and 12 inches wide on top of a 3 foot stone pedestal, located about 15 feet beyond the ceremony space. The pedestal is made of insulation foam, cut and screwed together, with decorative paint to make it look like stone. The backside is open to allow installation and access. The candle on top is made of a section of galvanized steel ducting with an opening in the back for installation and to allow air to flow up through the shield.

    This is actually only a decorative shell to a two stage release propane poofer. A common propane poofer releases the contents of an accumulator tank past a flame to ignite it. To increase reliability, and prevent the visibility of the open flame pilot, Hot Surface Ignitors were used. These are off the shelf parts used in the electric ignition on many gas furnaces and water heaters. A pair was used for redundancy, though in testing, a single ignitor always worked. The ignitors were mounted in a wind shield at the top of the primary accumulator output pipe which served prevent the wind from cooling the ignitor, as well as to shield the ignitors from any propane coming out of the second accumulator output pipe. The most noteworthy difference of this from most poofers is the use of a second output path that is not ignited. This allows the release of propane into the air above the poofer prior to ignition to create more of a burning fireball effect than a jet.

    The basics of a propane poofer involve a propane tank with regulator the passes "high" pressure gaseous propane and fills an accumulator tank. In this case, the accumulator is filled to 60psi.(a standard regulator for a propane grill is 0.5-1psi) The accumulator tank has a high flow path for the propane to release under the control of a solenoid valve and out the top to be ignited. A 3/4" pipe path is used here as it is the maximum output size of the easy to acquire accumulator tank, and can empty the entire tank in about 1.5-2 seconds. The poofer is fired by opening the solenoid valve which quickly releases the stored propane in the accumulator to create the fireball burst. This system of using an accumulator is required because a standard liquid propane tank has a limited rate of evaporating the liquid into gas to be released and burned. This allows high volume bursts to be released very quickly. Further discussion of the principals of a poofer is left to the reader to research elsewhere.

    The remaining part of the system is a controller which implements safety features as well as runs programmed sequences to correctly time the two solenoid valves. This was built from a surplus hand controller, with an arduino micro, additional buttons, and IO drivers and protection built in. This was designed to be very tolerant of electrical noise, spikes and electrostatic discharage without damage or incorrect operation.

    During operation, there are several safety features. At the accumulator tank, there is a ball valve which can quickly close off the output as one emergency shutdown. At the controller, removing the cable functions as another emergency shutdown. The rest of the safety factors are for the flame effect operator and located at the controller. The primary is the positive manual enable. For the system to fire, an arming button must be held, which allows the operator to continually watch for site safety, and release if there are any issues. Releasing this button will prevent firing, as well as stop and shut down any sequence that is running. This arming button only makes the system active, as the keyswitches and main button for the couple actually fires the sequence. This allows the operator to be responsible for operation and safety of the system, while allowing the couple who are not responsible for the system to fire it. The controller also implements password protection, with it locked at powerup, as well as a single button to lock it.

  • Success!

    Quinn06/13/2014 at 20:00 0 comments

    The wedding was this past weekend, and was a great success! I'll have a video, a summary and maybe some more pictures up soon, but wanted to get this out there.

    I setup bright and early in the morning, getting everything installed and ready before returning later in the day better dressed. The minister gave a great lead up to the ceremony, explaining that this unity candle was a bit different, reflecting the background of the bride and groom. The guests were mostly unaware of what exactly the unity candle was. Actually, let me just quote D:

    "To symbolize the joining of families and communities, we use the metaphor of a unity candle. Lit together, it joins two flames into one, representing lives and passions joined into one. Of course, given our bride, and our groom, our candle is a little more elaborate than some, and holds some special significance. It is represented by this box, which you probably can't see from where you are, but it has two key holes, one on each side. The key is somewhere in this room... no.(laughter) The keys represent the complex array of life experiences, of personality, of desires, and strengths, Maggie and Scott each of you bring to this relationship. Once the keys are in place, the bride and groom will, together, press the button at the center of the box, right on top. A symbol of their joint and unified purpose in their coming lives."

    The bride worked at it, and eventually managed to find the key in her dress to much chuckle from the guests. The groom simply pulled the other out of his pocket, to even more laughs. Listening to the recording is great, because from the guests you could hear quite a few shocked reactions the moment it went off, followed by an energetic round of applause, while in the video you can see the couple grinning overjoyed, chuckling to themselves :)

    Sorry it isn't a great picture, it is a screen capture from a video which wasn't the best. I'm collecting another video or two, and will see what I can put up. The photographer knew all about this, so should have some good pictures as well. I'll share some of those as well if I'm able. Actually, funny thing about that. This was kept mostly a secret from the guests, though obviously I've been putting up logs here for some time under the assumption that no one was likely to come across this. Apparently when the couple had emailed the photographer that they were going to have a "unity fireball" along with a description, she was doing her own prep research and stumbled across this log. Sure enough, "unity candle fireball" in google points right here ;)

    Overall, the bride and groom were overjoyed at how it all worked out, and the lasting memories they, as well as their family and friends will have.

    Special thanks to M&S for asking me to do this for them, to A, K and J for helping create and decorate the decorative shell, to A for hand painting the box, A and J for code reviews, to D who helped bury wires, helped with multiple setups and tear downs, and was my safety backup in case of my illness, as well as to S who helped me take everything down after the ceremony.(formal wear is not so conducive to this sort of work)

    To answer a question that was frequently asked, yes, the bride and groom did actually fire the candle flame. I stood in the wings with the controller to unlock and configure the system, as well as to hold down the positive manual enable button as long as the area was safe to launch, but the couple pressing the button actually did fire it.

  • Putting it all together

    Quinn06/11/2014 at 13:48 0 comments

    All the pieces were made with some flexibility in setup, as grass is never even, and at this site, there is a notable slope towards the pond. One of the primary techniques used was to drive stakes into the ground, then attach pieces to them at the right point to make things level.

    The shell was installed first, by driving three stakes into the ground, and using some helpers to hold it level. Once it was determined in the correct spot, screws were driven through the stakes into the foam. The stakes are in the inside, so not seen. This robustly held the shell in place. A similar method was used for the tall table the button box was setup on.

    The accumulator tank obviously couldn't have screws attached, so instead, three stakes were driven into the ground around it, and a ratcheting tie down strap(in orange) was wrapped around and tightened. This allowed me to tip the tank to be vertical, despite the uneven ground. Once the strap was tightened, the tank was rock solid. Here is a picture of the backside of the setup from the rehearsal:

    Behind the shell was a large industrial deep cycle battery(grey). This powered an AC inverter(black and grey, up front of the large battery) to power the hot surface igniters. I didn't measure the HSI's, and they didn't come with a datasheet, but both were powered fine with a 400w inverter without causing an overcurrent on it.

    Also placed behind is the box with the relay card and switched AC outlets(at bottom, up front of the battery). There is a smaller 12V battery(grey, on top of larger battery) to drive the control system as I didn't want to tap into a battery capable of driving that much current for the long, smaller gauge wiring.

    All of that is simply tucked behind the shell. The guests were all in one direction, so the workings were not visible to any of them when seated. I put up some stakes and a rope to keep anyone who arrive early away from the candle, and took it down prior to the ceremony. Here is how it looked from the front during the rehearsal:

    The propane line is shown in those pictures from the rehearsal, but for the actual ceremony, the accumulator tank was filled during setup, and this line as well as the LP tank were removed so they wouldn't be in pictures. This meant only one blast could be fired, which is all that was needed.

    I only got one picture from the setup prior to the ceremony, though this does show where it was all located in relation to the guests.

  • Installation wiring and connectedness

    Quinn06/10/2014 at 16:09 0 comments

    To keep unsightly wires out of pictures, the wires between the button, controller and candle were surface buried. This was done by sinking a spade about 4" into the ground, prying at an angle, pushing the wire into the slot, then pulling the shovel out. Takes a bit of time, but with two people worked fine and made for a much nicer look. The wire was scrap, a 6 pair 22 gauge solid shielded outdoor rated cable. Not technically rated for direct burial, but plenty fine for several days.

    Because of this, wires were run, cut to length and wired in place. To make this easier, all three points had screw terminals, so the ends of the cable were just stripped and wired in, without needing to worry about connectors.

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  • Ceremony Button

    Quinn06/10/2014 at 02:37 0 comments

    The control for the couple was made out of a simple wooden box, like the sort found at craft stores. It was painted flat beige, and A. drew on a lovely design in the style of wedding and invitations. The box will serve as a memento for the couple afterwards, and I'm sure they'll wire it up to activate something in the future.

    The key switches were mounted with simple holes in the side. I originally got the small key switches sold at Sparkfun, but found that they were just too cheap. I knew they were not real tumbler key switches, but was ok with that given these are more symbolic than for security. However, the physical feel and size of them just felt too much like a toy. I ended up ordering a set of real key switches from C&K.

    The firing button on the top is one of the "massive arcade buttons" from Adafruit, though Sparkfun also sells them. I wanted to reduce the black rim around it, so instead of just drilling the smaller 1" hole for mounting, I used an adjustable hole cutter, and mounted it flush. I simply used a file to create the two notches which prevent the button from spinning.

    The box was placed on top of a tall table(a plant stand I believe) in the ceremony space. This picture was from the rehearsal. For the actual ceremony, the table was covered in a table cloth matching the wedding colors, and the wire was buried. The table was also attached to a pair of stakes to make it level.

  • Robust Electrical Control

    Quinn06/09/2014 at 21:57 0 comments

    This project includes electronic control of things which are inherently dangerous. While there are a number of physical elements which improve safety, it is still controlling a large open flame. Because of this, there is little to no room for electrical issues causing accidents, and elements were incorporated to support this. Further, the use of long cabling introduces potential issues due to EMI. Plus ESD damage to chips really puts a damper on a project. This whole topic is commonly ignored in electrical projects because it doesn't always seem necessary. When it works, you'll not even notice it, when you don't include it, you'll likely notice the failures when you least want them.

    Electro-Magnetic Interference can affect any electrical circuit, but longer wiring can work as an antenna which exacerbates potential issues. These lines can have spikes of voltage that seem to appear out of nowhere which can falsely trigger things. I incorporated a number of elements to combat potential issues.

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  • Rain Shield

    Quinn06/09/2014 at 15:37 0 comments

    Given the nature of needing this to work, and the fact that it is constructed for an outdoor wedding, I needed to prepare for possible rain. The electronics could all be wrapped up in plastic, but I was concerned that rain falling on the hot surface ignitor could cool it, and prevent it from getting hot enough to ignite the propane. Or possibly worse, and crack it due to rapid change in temp. I cut a simple rain shield of of scrap sheet metal which just clips over the top to cover one of the HSIs. I ran this through testing, and though it does deflect the flame a little at an angle, it ignited fine.

  • Heat and cost estimates

    Quinn06/05/2014 at 16:36 0 comments

    I ran some estimates in a prior log about the heat output from this. The accuracy of this was restricted by some assumptions, namely that the pressure in the accumulator remains constant, and it assumed that only 1 valve was open. I had calculated this by using the flow rate of the valve from a datasheet. Instead, with some test data, we can recalculate based on the before and after pressure of the accumulator tank to determine the propane actually released.

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  • Parameter adjustments

    Quinn06/04/2014 at 18:52 0 comments

    With the electronics done, they needed to be tested with the system, and I was now able to see timing affects of various delays, and tank pressure.

    To simplify this description, we need to remember that there are two outputs of the tank. One vents straight up with nothing in the way, and no ignitor. This will vent the propane, and not ignite it. I will refer to this as vent. The second output has hot surface ignitor's at the opening which will ignite the propane that leaves. I will refer to this as ignited.

    The sequence I started with, opens the vent, then after a 750ms delay, opens the ignited. Both are left open for another 750ms, then both shut off. The purpose of this was to get a volume of propane in the air above the candle before igniting. Being able to do this was a design feature from the beginning, which I came up with from researching. All of the examples I found on the web produced higher velocity flames, some even approaching "flame thrower" type looks. I've never found any examples of this pre-ignition venting technique, so as far as I know I came up with it just from thinking of various techniques for achieving the appearance I was after. This is part of the reason I wanted to log this project, to share the idea to others.

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[this comment has been deleted]

Quinn wrote 07/14/2023 at 23:33 point

Are you just advertising?

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Fred wrote 09/18/2017 at 20:53 point

Hi Quinn,  I am confused by there apparently being 2 projects with differing parts. I hope I can ask a few questions as I build a similar project. First, one parts list shows a 25' LP hose with quick disconnects on both ends. What is this for? All the pictures show standard screw on connectors, except for one image that seems to show a quick disconnect on one end of the feed line. Are these things reliable for sealing gas lines? I've used them for air, but they often stop working and leak, no?

What I want to do is have 1 large 80lb LP tank to feed 2 gas grill accumulators inside two 7' columns separated by about 8 or 10'. I want them to burn pilots that look like tiki torches and then do a random sequence of effect like on unit poof out a ball, then the other. Or they alternate a few short bursts. Or they both blow a 2 second pillar of fire simultaneously. I like the rolling cloud effect and the short puff and of course the large column of fire. I don't need more than perhaps 5 to 10 feet of flame. How do I feed two accumulators from the same source? I was worried that if the second one T's out from the first, that when the first one goes off, it would drain the second one a bit or that there could be some other issue I am not considering.

What pipe do I want from the accumulator to the exit tube? Do I use 1/2" coming out of the accumulator to a 1/2" exit? Or do I need 3/4" all the way from the accum. past the solenoid? I saw an image with one unit with a small bottle type accumulator that was 1/4" from accum. to solenoid and then something bigger? But the twin stack you created looks like 3/4" for all the pipe after the accum. Can you help? Thank you!

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Adam Fabio wrote 06/24/2014 at 06:01 point
Thanks for a excellent and well documented project! I'm not sure if connecting two lives will cover the "Connectivity" portion of The Hackaday Prize, But I'm happy to have your unity candle in the competition!

  Are you sure? yes | no

Quinn wrote 06/24/2014 at 21:55 point
Hi Adam, and thank you for the feedback! The connectivity aspect is probably better covered under the controllers ability to load and save firing sequences from a computer via RS232, as well as being able to integrate with a theater/DJ style lighting control system.(instead of the interconnected nature of the various elements to this project.) I had considered this portion of the prize goals, but decided to submit after Brian Benchoff's comment about what "connected" means on the blog which included the example: "A 3D printer is considered ‘connected’, even though it’s just a serial connection. The simplest answer is some sort of digital I/O."

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