The Adafruit GEMMA has three available digital I/O pins, but thanks to the technique of Charlieplexing you can drive up to six LEDs with those pins, one at a time. In general, charlieplexing allows n tri-state pins to control n^2-n LEDs. My example Arduino sketch in github/charlieplex generates 8-bit PWM for each of the six LEDs, although it does require the full CPU to do it. Here is a short video of it in action, showing the smooth fades on and off.
My sewable sequins are made from 0805 surface mount LEDs and small metal beads from a local shop. Becky Stern of Adafruit has a great tutorial on making LED sequins — I used smaller LEDs than Becky, so I had a harder time putting them together and had to build a small jig with a piece of wire to support the bead while soldering it under a microscope. If you don’t need the small size, Adafruit’s premade LED sequins are a much easier way to add them to your project.
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