Infinity Cube

Summary

I saw one of Adam Savage's One-Day Builds, where he created an “Infinity Rhombic Dodecahedron” by using two-way mirrors to form the faces of a solid, with LED lights running along the interior edges. I thought I could do something similar. However, a rhombic dodecahedron is a complicated shape, so I started with a cube.

New Challenges

• Learn about laser cutting and physical assembly.

• Practise surface mount soldering.

Physical Design

Differences from Adam Savage's Design

1. Created a Cube instead of a Rhombic Dodecahedron, to manage scope by keeping the shape simple.

2. Adam had a hard time shoving the LED light strips into the dodecahedron and an even harder time making all the electrical connections because of mixing up the VCC/GND when three ribbons meet at a point. I solved this problem by creating a custom PCB for each edge, with terminals radially symmetrical so that GND goes down the middle of the PCB and VCC travels along the edges. Then you can flip the PCB any which way and it will still line up with its neighbours.

   • My implementation could have been better but it was an effective solution.

3. To avoid the tricky part of sealing everything inside and having it be tight as well, I installed the lights in a wooden frame and then set the mirrors in the frame after.

4. Initially the frame was going to be 3D printed but this turned out sloppy-looking and I found out my printer needs much better calibration.

Modelling

Modelling took place as an iterative process of drawing things up in Blender, trying to laser-cut (or 3D print) the box, inspection, and adjusting the model. Eventually the following design emerged.

Materials

• 6mm Baltic Birch

• 3mm Acrylic

• Black PLA

• Two-way stick-on film that you get to make windows block sunlight.

• Glues: Wood Glue, Hot Glue, Epoxy (2-part Gorilla Glue)

• Spray paint, black.

• PCBs, assembled.

• LIttle jumper wires to stick the PCBs together, and solder.

• Power connectors, VCC 5 Volts

Dimensions

• The cube will have a nominal dimension of 10cm but actual outer dimensions are subject to change.

• The inner dimensions will be 95mm x 95mm x 95mm.

• The plexiglass windows will be 90mm x 90mm x 3.175mm.

• There shall be some kind of lip for the window to rest on when it’s seated in the window.

• PCBs are 85.09mm long, 7.62mm wide, and 1.6mm deep. The points of the PCB have an angle of 109.5 degrees, so that three PCBs can be joined at right angles to each other with no gaps.

  • OSH Park requires that PCBs are minimum 0.25in wide.

  • The smallest possible cube that can enclose the PCBs is 92.7409mm ^3.

  • PCBs will thus have approximately 0.5mm wiggle room between each other and the wooden wall.

• Plywood will be 5mm birch and have only one outer cube.

• The prism rest provides a triangle 6.5mm tall, 6.5mm wide with a 9.2mm diagonal, ensuring enough space for both lip and board support.

• Rest of geometry was calculated carefully to ensure a close fit, but allow for about 1mm of wiggle room.

Concept of Assembly

1. Put the wood box together.

2. Seat in these angled prisms, which will

   • hold the PCBs at 45 degrees to the cube faces and 90 degrees to each other, and

   • provide a lip for the windows when they are placed in the cutouts in the frame.

3. Drill a hole in one of the corners to run a 2-conductor power wire.

4. Install the PCBs, solder and test.

5. Place the mirrored windows in the frame, and fasten with epoxy glue.

PCB Design

PCB design followed suit from the schematic. Some comments:

• The VCC pad was placed behind the GND pad at each point. This gives the PCB rotational symmetry and makes it easier to join 3 of them at a corner.

• In future, there could be 2 VCC pads; one on each side of GND, and that would make it even easier.

• The PCBs are 3.35 inches long and 0.3 inches across, since I couldn't figure out how to work in mm.

• I can't remember how I worked in the 109.5 degree angle at the point. It may have been an approximation.

OSH Park requires that PCBs are 0.25inches minimum.