Ten Tetrahedra Tree Topper

Last crawled date: 1 year, 10 months ago

*Ten Tetrahedra Tree Topper (TTTT)*

It turns out that you can inscribe ten different tetrahedra in a dodecahedron. This project illustrates this polyhedral compound in a 250mm diameter, two-piece model. (Links in references below.)

It was modeled and prepared for printing in SpaceClaim, then printed in PolyJet Vero Clear on an Objet 260. The prepared version is included for those with a similar (255x252x200mm) build envelope. Also included are SpaceClaim and STEP files with the solids before merge for assembly. That should make it easy to adopt the design to a printer with a smaller envelope or so it can be made without pressure-washable supports.

As prepped, it requires a 3/8-16 tap for the upper blind hole and a 3/8 through hole at the bottom. It could be direct modeled or drilled to accept 9 or 10mm tap and through drills. The two halves are assembled with all thread, as well as a custom "pentanut" washer, a normal washer, and a nut.

It uses 50 individually addressable LEDs (WS2801) from Adafruit. Originally, I was going to drive them with the same Arduino that I was using for the tree, but the clever Lady Ada gives you a free Trinket mini Arduino-like CPU when you order a bunch of stuff. I recommend you get a few extra strands, because if you are like me, you will kill a few LEDs during assembly and you will want some leftover to play with. (Silicone lube ended up being the best way to not destory LEDs.) Final assembly was straightforward, as it was easy to be tidy with the LED cord by tucking it in between the LED receptacles. That said, it would be tricky to fit more than a Trinket in there.

How to drive the LEDs was up in the air when I was ready to print, so I left a hole off of the model. PolyJet Vero machines like soft Plexiglass, so you might consider a drill with a dubbed tip (ie, grind the cutting edge flat so there's no rake). A good location for the hole right is where the vertex of the pentanut meets a concave edge. I had no problem running USB, 2-18GA stranded wire for 5V power, and 5-18 solid wire for reset (needed for programming) and two pins plus 5V and ground for interactivity. (See photo of base of the TTTT.)

It turns out that the Trinket is really tiny. I quickly ran out of memory and ended up storing all the topology of the model in flash memory, which was fun to learn, but eventually I wished I had more memory. It deserves something beefier, and, in hindsight, I wish I had the board outside the TTTT given that the Trinken needs to be reset to be programmed. Source is up on
Github. The index data for the LED strand and all of the pre-computied topology was turned to data structures a Google Sheet.

The tree itself recieved 300 LEDs, which, all together, are bright as all get-out but smell a bit like injection-molded silicone. Those are controlled by a conventional Arduino (an old Lilypad, to be precise). It turns out that the green is very strong on these strands, so I ended up halving that channel for warmer whites. The tree and topper lights are all powered with some old 10A 5V supply of unknown origin.

References:
* GrabCAD (lastest info): https://grabcad.com/library/ten-tetrahedra-tree-topper-1/
* Video: https://youtu.be/B6gWAr850ew
* https://en.wikipedia.org/wiki/Compound_of_ten_tetrahedra
* Adafruit
* LEDs: https://www.adafruit.com/products/322
* Trinket 5V: https://www.adafruit.com/products/1501
* Github: https://github.com/bcourter/Ardunio-Projects/tree/master/tttt
* Google sheet: https://docs.google.com/spreadsheets/d/1bSkJw6RBlLszQwCV98VSWpaGMqbUV6SY9e-ySOjWa3M/edit#gid=0