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Hoverlay II

open hardware interactive midair screen

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Hoverlay II is a low-cost, interactive, scaleable, open source vapor screen. It allows projection of images, videos and games right into midair and comes with it's own selection of interactive applications.

Short explained, the device creates a thin, stable film of vaporized water, continuously travelling upwards in midair, which then acts as a floating rear projection canvas.

It works great with Microsoft Kinect and we've also successfully tested it as a telepresence display device using Skype.

See the video:

Even though one Hoverlay II unit is still quite handy (326 mm wide), up to 5 units can be plugged together to form a badass big 1630 mm wide, floating screen of >1 m height. Bigger screens are possible by choosing more high-end components.

Hoverlay II is open hardware licensed under GPL V3.


Vaporization is done by off-the-shelf ultrasonic atomizers, either really cheap ones (the black ones in the picture below, 2.50 $ each) or some more rigid ones (the silver one, a Fogstar 300, 290 € each), air is moved by an array of strong and silent pc case fans, laminar flow is formed by 3D printed flow formers. Rear projection works with any kind of video or still image (even dia-) projector.

The housing

The housing is CNC-milled out of black PVC hard foam plates ("Palight", not to be mixed up with PVC soft foam plates) but can be made from pretty much any waterproof material that can be glued or welded, i.e. acrylic.

If you choose Palight, you can theoretically build this just with a paper template, patience, a steel ruler, some drill bits and a box cutter. All the DXF files for milling, lasercutting or box cutter samurai are available in the GitHub repo. Never put PVC in a laser cutter though (super carcinogenic). The BOM list in the repo tells you, how often you need to mill or cut a certain file. A single file may contain more than one part.

Operating principle

The diagram below shows an axial cross section of a Hoverlay II module, it's probably self explaining:

As you can see, there are three channels. The fans suck in air into the left and right channel, which then travels upwards while beeing compressed and accelerated by the convergent channel walls. The air then is pushed through the 3D printed flow formers, which is just many, many small, parallel channels packed into a honeycomb grid. When the air flow exits the flow formers, its mostly laminar, meaning it contains almost no turbulence anymore (such as rotary turbulence from the fans) and can continue travelling upwards almost turbulence-free for quite some distance (> 1 m).

The two fast laminar air flows from the left and right air channel then carry off the fog from the center fog channel, an effect that is known as "Venturi Effect", both accelerating the fog upwards and shaping it into a thin film layer right between them, as you can see in the picture below.

This thin layer of fog has the ability to diffuse incoming directional light (such as light from a projector), which then can be perceived as if it was diffusely emitted by the fog layer itself.

A perforated pressure compensation tube was meant to help delivering fresh air into the fog channel and lifting out the fog homogeneously. A few words about this tube: It's just a perforated (manually, on a drill press) ø 25 mm PVC tube, cut to length of one Hoverlay II module (326mm) that slides in though the corresponding cutouts in the side panels. The pressure compensation tube can be dismissed (the cutouts still have to stay there!) when only using one or two Hoverlay II modules in series. It was meant to help homogenizing the fog density over the full width of larger (3+ modules) installations. However, we found a way better method for that, by using some sort of "brightness booster" in form of two additional fans, blowing air into the pressure compensation holes on both ends of the Hoverlay II array. This both helps homogenizing the fog density over larger screens as well as visibly increasing the fog density (thus brightness). Here you can see the brightness booster in action, it's a true enhancements over all previously posted videos:

I will soon release a nice and tidy, 3D printable "brightness booster" that can be snap-fitted onto the existing pressure compensation tube holes for easy installation. Until then, just ignore the pressure compensation in the BOM, because it just turned out to be a intimidatingly dull solution to the homogeneity problem on larger screens.


A Microsoft Kinect is sitting right on top of the projector, requiring minimal calibration and mapping. It turns out that the fog layer does not disturb the pattern recognition of the Kinect at all, so it works just fine.

Some more technical details

I spent a...

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  • 1 × AC power supply (fused!) depending on your choice of foggers, 24 V or 36 V AC, about 120 W
  • 1 × DC power supply (fused!) 12V DC min. 200W
  • 8 × PC case fans 12V DC, approx. 40 CFM each
  • 1 × Set of printed parts from the repository
  • 1 × Set of milled parts from the repository PVC hardfoam plates is a good material for building enclosures, easy to cut, easy to glue
  • 4 × Ultrasonic atomizers, alternatively one single (strong) one smaller ones have about 24V AC, approx. 14 W each, while stronger ones go beyond 36 V AC, approx. 120 W each
  • 12 × Female power connectors 3,5 mm gold plated rc power connectors female
  • 6 × Male power connectors 3,5 mm gold plated rc power connectors male
  • 1 × tons of screws size depends on your build
  • 1 × plastics glue for putting together the housing

View all 20 components

  • Hoverlay @ C31C3

    Moritz Walter12/20/2014 at 17:29 0 comments

    The Hoverlay will be part of the Chaos Communication Congress 2014 (C31C3) from December 27th to 30th, 2014 at the CCH Congress Center Hamburg, Germany, Earth, Milky Way. So if you're in Germany, Hamburg these days, come by for a visit. We'll be glad to see you! And to everybody who cannot come: Merry Christmas and Happy new year!!

    Also, besides that there'll be many other cool projects and an assembly by the Attraktor Makerspace. Don't miss it :)

  • THP Semifinals Video Released

    Moritz Walter09/27/2014 at 00:03 0 comments

    Find out more about the internals and future of the Hoverlay in the THP semifinals video:

  • Full CAD model released

    Moritz Walter09/25/2014 at 15:53 0 comments

    Both Hoverlay I and II were completely layouted in Adobe Illustrator, a surprisingly flat 2D drawing tool I just happen to work with a lot. This is how it looks like, basically:

    Making all the parts fit without using 3D CAD required some imagination and a few trigonometric vitamins. Now, since I am getting more and more into OpenSCAD, it's possibilities (like "programming things", yay), and limitations (like no programatical generation of polyhedrons, ouch), I can finally release a full model of the Hoverlay II.

    Mind that this is not meant as a 3D printable version of the Hoverlay II, but it's an important step to get there with a potential Hoverlay III. In order to really get this out there on the printers of the makers, splitting up the Hoverlay module in even smaller segments might be as ineviteable as shinking down the module as a whole.

    However, in theory, and if you're crazy enough, this is completely 3D-printable. In the screenshot above from Repetier Host you can see that my build platform is completely covered by the object, which means my Prusa i3 is just too small for that. If you plan to threw this onto your FDM machine, make sure your support material spool is full and your printer has the massive build envelope of at least 326 x 271 x 240 mm. Also make sure you print the housing and the vat separately.

    All the updated files are commited and synced to the GitHub repository by now.

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[this comment has been deleted]

Moritz Walter wrote 03/26/2016 at 10:22 point

Hi, thanks! Yes, that is exactly the power class that gives good results.

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Moritz Walter wrote 03/26/2016 at 13:33 point

@Sergio I don't think 6 l/h is too much, but 3 l/h will certainly suffice. I'm not manufacturing these, but I'm currently willing to sell my ready assembled Hoverlay as a whole, including foggers and power supply, because I'm moving to Canada and transport is tedious.. Just send me a PM me if you're interested!

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alexdw1 wrote 02/22/2016 at 10:42 point

Hi,  Great project.  I was wondering the max height you have achieved for projection and noise levels.

I'm looking for something to have in a classical concert hall with just one piano.



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Moritz Walter wrote 02/22/2016 at 11:20 point

The useable height is about 1 m with a 7000 ansi lumen event projector.

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Analog wrote 09/03/2014 at 20:07 point
While I'd personally like to see the laminar sheet have a smoother flow- the project is still quite well done. I liked version 1 and I'm happy to see you've made some great improvements with version 2. Keep up the great work, I'm looking forward to a version 3 =)

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Moritz Walter wrote 09/28/2014 at 14:47 point
Hey! Thanks for the feedback! A smoother fog layer can be achieved, by increasing the width of the flow formers. This will basically increase the width of the buffer air flow, which makes it less prone to turbulences created in the interaction with the ambient air. However, you'd at least need to double it, and doubling that means also doubling the flow rate in general, so you either double the number of fans or you'll need 80mm fans with ~100 CFM which would be very loud and also don't really exist I think.

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Jasmine Brackett wrote 08/06/2014 at 20:59 point
Hello iamnotachoice, this looks really impressive. We'd love it if you could add a few more details to your project to give it the best chance of going through to the next round of The Hackaday Prize.

By August 20th you must have the following:
- A video. You've got several, so pick the one (or make a new one) which best describes your project. It should be less than 2 minutes long. Put it on YouTube (or Youku), and make it the only video in the 'External Links' section on your project page.
- At least 4 Project Logs
- A system design document
- Links to code repositories, and remember to mention any licenses or permissions needed for your project. For example, if you are using software libraries you need to document that information.

You should also try to highlight how your project is 'Connected' and 'Open' in the details and video.

There are a couple of tutorial video's with more info here:

Good luck!

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Moritz Walter wrote 08/06/2014 at 21:16 point
Thanks for the instructions, a better video and the other stuffs are on the way :)

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Jared Sanson wrote 08/05/2014 at 06:39 point
Wow pretty cool!
Do you have problems with things getting damp in your room after running it for a while? I would assume spraying water mist into the air can't be that great for computers in the room!

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Moritz Walter wrote 08/05/2014 at 07:57 point
Thanks! This is dry fog, so your hand does not get wet when you touch it. However, it does increase overall humidity in a (fully closed) room with about 250 ml per hour. That seems to be less than if you'd fill the room with transpirating programmers. So far no computers near it have failed, even after 20 hours of continuous operation.

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Gabriel wrote 07/23/2014 at 18:33 point
Does the air speed have influence on image quality?

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Moritz Walter wrote 07/23/2014 at 22:11 point
I tested different flow rates by pwm'ing the fans down, finding out that a high flow rate of air results in a better image. Though, increasing the flow rate means decreasing the fog density (thus perceived brighness of the image) and needs to be compensated by adding more foggers (increasing fog density again).

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Rocket501 wrote 07/22/2014 at 23:25 point
Awesome project! I am interested in making this. How much does it cost? Do you recommend any of the ultrasonic atomizers?

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Moritz Walter wrote 07/23/2014 at 18:03 point
Thanks! My recommendation would be: use the strongest and most rugged foggers you can find or afford for this application. I honestly cannot recommend the ones I used, still looking for better ones. Will be glad to see another build, feel free to contact me if something's unclear.

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Adam Fabio wrote 07/15/2014 at 05:20 point
Hoverlay is looking better and better every time I see it! Thanks for submitting it to The Hackaday Prize! Your superstring video is just incredible! Don't forget to keep the updates rolling in for the contest - you might just find out how well hoverlay works in space!

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Moritz Walter wrote 07/15/2014 at 10:05 point
Thanks a lot! Be assured I keep you guys updated!

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anfractuosity wrote 06/01/2014 at 22:16 point
Very cool! I'm curious if you could use multiple projectors somehow to get 3D?

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Moritz Walter wrote 06/01/2014 at 22:35 point
There is a nice effect you can get with this by shooting onto the screen with two projectors from back and front. That setup could at least show you an object from --- wait for it :) -- back and front. I also tested shooting in there with two projectors from slightly different angles. That however does not yield good R/L channel separation for the spectator (actually you wouldn't call that "channel separation" anymore), the fog is just too diffusing. Using a regular 3D projector with active shutter glasses gives good results though.

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Mike Szczys wrote 03/29/2014 at 18:29 point
Early prototype is fantastic! Can the ultrasonic atomizers be used continuously for long periods of time?

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Moritz Walter wrote 06/01/2014 at 22:38 point
Depends on your definition of "long periods of time". It works for many hours (gets warm though). The membrane wears out due to mechanical stress and scale sooner or later, but can be replaced in most foggers.

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akohlsmith wrote 03/02/2014 at 22:06 point
Trèz cool. Is the top of the laminar flow distorting due to the ambient air not being still? I don't know enough about laminar flow to know the limitations.

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Moritz Walter wrote 03/02/2014 at 23:25 point
Thanks! The quality of the fog wall is best in still air. Though the laminar flow is quite robust against slight ambient air streams, it's steadiness will decrease a little, but it still works good enough for getting a picture.

  Are you sure? yes | no

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