In 2019 Nathan Petersen will graduate with a degree in computer science, and like many of his fellow classmates, he plans to decorate his graduation cap. While others normally use low-tech methods like glitter and stickers, he will instead use his hardware skills to light up the room — or at least his head — with an 8×8 LED matrix.
His Gradled Mini prototype (pronounced “grad-LED”) uses a Silicon Labs Tiny Gecko EFM32TG110F32 Arm Cortex-M3 microcontroller for control, along with a column and row drivers to activate individual LEDs. These drivers source current via the column driver, and sink through the row driver, allowing each LED to be selected. The MCU displays each column in turn, refreshing each at 60Hz to make them appear to stay on using persistence of vision. Currently, the device exists as a very well-made prototype. After proving out some of the concepts here, it should make a great decoration when put into the proper form.
You may wonder why Petersen chose individual LEDs instead of addressable RGB or RGBW units. The answer ultimately came down to cost, as 64 red LEDs are available much more cheaply than the same number of RGB units. While it’s not exactly an apples-to-apples comparison, his writeup does a good job of explaining his design choices and analysis throughout the project. It’s a good example of how one probably should make design choices — rather than just grabbing whatever happens to be handy in your spare parts bin!