[Tech] Eve Pendant, Phase 2

For an upcoming Disney-themed event that my wife and I will be attending, I decided that she needed something really really special to wear. One of her favorite Disney characters is Eve, from Wall-E (YouTube link), and, well, I realized that her face was an awful lot like OLED screens I'd seen.

Once I found the Adafruit tutorial for an OLED necklace, I was hooked. I was totally doing it.

Problem One: Screen Selection

The Adafruit tutorial, naturally, suggests Adafruit products. I wanted fast and easy, so I went for the parts list as given--with one exception. The Adafruit OLED screen is white on black. To match the movie, it would have to be blue on black.

Eventually I settled on the Sparkfun OLED screen and breakout board. Cheaper options are available on eBay, but much more time and board fabrication were required to use them.

Then the order went in and I had to wait for the parts to come.

Problem Two: No 3-D Model

With parts on their way, I got help from some coworkers to start drafting the necklace itself. The first dozen or so people immediately told me, "Hey, I'll bet this thing is on Thingiverse!!1!" Well, Eve certainly is. A good Eve is not, nor is a flattened Eve ready-made for stuffing with electronics and hanging around someone's neck.

I settled for taking a screenshot of a YouTube video, pasting it to a Canvas in Autodesk Fusion 360, manually sketching tangential line segments over it, then extruding and filleting to taste.

Once the parts arrived, I measured, drafted, and inserted them until I was confident everything would fit. It came out rather thick, but OK:

I am hoping to make a Rev B that packs all the circuitry on one board and is a more..aesthetically pleasing geometery.

Problem Three: Level Shifting

Well, it turns out that the list of parts on the Adafruit tutorial is geared towards the Adafruit OLED screen, which is 5V tolerant, so it calls for the Trinket Pro 5V. I ordered the SparkFun OLED screen, which is not so forgiving. Its logic demands a strict diet of 3.3V. A simple test with my o-scope showed that the I/O on the Trinket Pro 5V is, indeed, close to the input voltage. (A multimeter would've sufficed, but wasn't handy.) Looking at the schematics (on GitHub) revealed that the amount of kludging required to convert the board to 3.3V was too great for my taste. (Resoldering a part or two is OK, but rerouting traces by hand?)

With a week to go before the event, I got a little desperate. I ordered the Trinket Pro 3V, but it won't come in until time for final assembly.

It turns out Vetco Electronics is located near me. Their website said they were all out of NTE4504B six-channel high-to-low level shifters, so I thought wonder in and see what they had. Most places like that have folks with experience in the field, so I thought I'd just ask for a level shifter and see what they said.

That failed, so I sat there with my phone in the shop looking for an NTE part that did what I wanted. I'll simply note here that part shopping on a phone--which necessarily includes datasheet inspection--sucks. I landed on the NTE4049, bought them, got them home, and realized that they're inverting level shifters.

Curses.

Some part of my subconscious must have been hard at work for me, because I had bought two of them. This meant that I had enough inverters to hook up each microcontroller output to two sequential inverters. I nearly ran out of wires, but I successfully made a rat's nest:

Note the OLED screen and breakout hovering off to the left; the dark shape towards the bottom is the battery charger, suspended in mid-air by its hookup wires; the Trinket Pro is the circuit board on the breadboard close to the center of the pic; the level shifters are obscured by wires; and the Spartan 3 Dev Board by Digilent is acting as a highly glorified 3.3V power supply.

(Why not use the Spartan 3's I/O to do the level shifting? I didn't feel like doing the schematic-datasheet deep dive on it. I should sometime.)

This angle shows the level shifters behind the Trinket:

Not A Problem Four: The Darned Thing Works!

Here's a video of it running the SparkFun "cube" demo. Please feel free to ignore the fairly uninformative audio I was inspired to add while filming, and do forgive the focus issue with the screen.

Phases 3 and 4 will be programming the board and hewing the pendant out of 1" polyethylene kingboard (after some 3-D printed prototypes, I think).