We all love to show off our hacker chic. Some of us go for the store-bought set of Bluetooth-based Personal Area Network devices. Others might spend a weekend (or several) assembling a pre-designed IoT smartwatch to adorn our wrists.
So far, these are pretty conventional approaches! You might not cast them a second glance among a crowd of commuters these days.
But what if you really want to stand out from that crowd? Well, Sawaiz Syed has released a set of digital earrings, the design of which is sure to do just that.
Based around an old favorite of ours, the diminutive Microchip ATtiny85-20, these jazzy bits of digital jewelry are anything but dinky when it comes to aesthetic!
Taking the form of a trimmed, slim sliver of flexible polyimide PCBA, some 39mm by 4.2mm, the layout of these earrings places the MCU, an accelerometer and an LDO up at the top of a hanging pendant, leaving a litany of eight LEDs streaking downwards towards the ground.
From left-to-right, the load-out is as follows:
- LSM6DSM tri-axial accelerator
- Microchip ATtiny85-20MU: At 4mm^2, this is a good choice of function against footprint area!
- A Microchip MIC5365 LDO regulator lets this lightning rod run at a light load, with typical usage sitting somewhere around 25mA during moderate movement.
- 8 x WorldSemi WS2812B-2020: everyone's favorite NeoPixel, but much, much smaller — just 2mm^2!
The pattern displayed by these LEDs is likely to be completely unique to the individual wearer, as instead of being predefined, and sacrificing valuable bytes of RAM in the ATtiny, is instead seeded by the accelerometer readings, suitably sampled and scaled, so as to reduce sensitivity below potentially "fast strobe" modes of operation!
The absolute change in the sampled acceleration data for each of the three axes is calculated with respect to the last sampled readings, at which point, six lower bits of data are shifted away, before being cast to an uint8_t, ready to be written to the RGB values of the LED.
Once this has process has been performed for the first LED in the strip, the LED index position is updated, and the process repeated, in something akin to a FIFO. This could lead to some very cool subject material for light painting!
The main loop, shown below, really does lay out what's going on. It's simple, but effective code!
From looking through the GitHub repo of the project, it's clear that Syed has some professional background in electronic design. From the general, well reasoned component choice, to the sectioned schematic, to the clean lines of the layout itself, the documents are a pleasure to read.
The TC-2030 is a common programming fixture in many a board fab house, but for what it is, it's not exactly at a hobbyist-friendly price level.
We are sure that anyone who feels like building a batch of these trinkets could probably figure out a basic set of pogo-pins in a 3D-printed jig! At worst, anyone tagging along on the build could temporarily tack some flying leads on to the programming pads located on the rear of the board!
Shown above on the left hand side of the rear face of the board, we can see the six-pin programming header, and the requisite alignment hole system, that is thoughtfully located outside of the board edge.
As it serves no purpose once the firmware is loaded, it is simply trimmed away. A neat trick, which also means the fab can deal with the drill spots intersecting with the board outline.
A further plated hole is located at the top of the earring, to allow a wire earring hook, with the plating hopefully providing just at touch more mechanical robustness than bare FPC alone, but time will tell on that one!
You can pick up a set of the flexible boards direct from the shared OSHPark project here. Weighing in at a a touch over $2.50 a set, it's worth grabbing a few if you're up to the challenge of that QFN-20!
It's really neat seeing the possibilities being opened up by the recent trend in miniature serial RGB LEDs. Tiny trinkets such as these earrings are now possible, with the LED parts freely available for cents a piece!
What's more, with variants such as the -EC15 packaged sk6812 (seen above with it's 2020 brother, and long lost APA102-5050 cousin) taking up a whole 1.5mm^2 of board area, there's surely a great deal more to come yet in the way of miniature matrices, pico-sized pendants and brooches bristling with beautiful lit patterns.