The Return of the Secret-Knock Gumball Machine

Fun & Games Technology
The Return of the Secret-Knock Gumball Machine


As a new engineering intern here at Make, my first big project was to rebuild a popular MAKE magazine project – the Secret Knock Gumball Machine by Steve Hoefer. It’s a gumball machine with a twist: if you want a gumball, you have to knock on the machine with a secret rhythm (and guess it right), and then you win a confection. This project is very popular amongst children, because hey, what kid doesn’t like free candy? I utilized a variety of new materials and design techniques to make my version more durable, visually appealing, and rewarding.

New and Improved

I wanted to put my own spin on this classic, not only to optimize its performance, but also to make it more exciting and — most importantly — to make it as durable as possible. The previous machine that was built by Make Labs is made entirely of wood, which is coming apart and fraying away. And one of the downsides of the wood case is that it prevents people from seeing how the machine works and the components inside. That’s no fun! To solve these problems, we decided to use acrylic, which is clear, so everyone can see the inner workings of the machine, and it’s also very durable. Although the original article featured an acrylic machine, Make Labs had never made an acrylic version of our own, and it was about time we did.

The most troublesome part of the old design was the dispensing mechanism. Although it did function properly, it was more complex than it needed to be and would often jam up. I spent some time studying dispensing mechanisms and came up with a new design that uses a horizontal dispensing dish, like many common gumball machines have today. I then created a new 3D-printed dispensing mechanism that so far has been working really well.

A cardboard laser-cut prototype being assembled.

Tougher and Laser-Powered

The most challenging part of this build was creating a new case out of acrylic that can snap together and can also be bolted, for extra protection from the crazed frenzy of children who would try to grab the machine and run away with it. The original plans are great for cutting by hand, but since we have an Epilog laser cutter, we wanted something optimized for the tool. The case was first drafted in AutoCAD, then tested multiple times by laser cutting cardboard, to make sure everything fit correctly. The next step was to transfer the design over to be cut in acrylic. This stage required a lot of trial and error, and really taught me how to operate a laser cutter.


The new machine features a brand new dispensing mechanism, a clear acrylic case, upgraded components, and an RGB LED strip that gives the user a very rewarding experience. I had a great time working on this project, and hope to do many more like it in the future. Come check out our acrylic Secret-Knock Gumball Machine this year at Maker Faire Bay Area, May 17–18, at the MAKE booth!

This is what happens if you enter the code incorrectly.

Please Try This at Home

If you would like to make your own Secret-Knock Gumball Machine using a laser cutter, you can download the .dwg and .stl files here. Please note that you may have to make some adjustments to the tabs due to different tolerances, which vary with your laser cutter. This file is optimized to use 3/16” acrylic. Using smaller sizes will also affect how the tabs fit together. Some filing may be necessary for a proper fit.

7 thoughts on “The Return of the Secret-Knock Gumball Machine

  1. Ozelf says:

    Hi, I am making this project and was wondering if you could please post the arduino source code. Thanks

    1. Sam Scheiner says:

      Hey Ozelf!
      All the files I used including the source code are included in the link provided! Good luck with the build!

      You can also use this link –

  2. MR. JAMES says:

    Hi Sam,

    What servo did you use that you designed the case/3d printed part around? Does it come with a horn that matches the holes on the dispenser? Thanks!

    1. Sam Scheiner says:

      Sorry for the late response. As far as the servo horn goes I just used a standard doubled sided servo horn which I slightly enlarged the holes to fit the screws. Good luck!

  3. manganlabs says:

    Sam, can you tell me what size screws & nuts you used in this design?

  4. manganlabs says:

    MR. JAMES I don’t understand the 20°, 160°, 20°, 175°, 90° dance that you have the servo doing. What is the reason for this?

  5. Troy Alesi says:

    Is there an updated list of parts and sources, so I can price this one out?

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I love engineering! I like to think I am a pretty creative individual who loves to tinker. No project is too big or too small. Oh did I mention I love Making?

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