I've wanted to build something for the square inch project since the original challenge announcement, but most of my ideas have involved the ESP8266 or ESP32, and were a bit too grandiose for my first foray into PCB design. So with a few weeks left, I thought I'd build a blinky memento for my upcoming mission trip. The basic idea is a square inch flag-themed LED grid with 1 LED for each month I'll be gone.
My wish list after that includes:
- Lasting the full 2 years (preferably without recharging, and on a coin cell for assembly convenience)
- General robustness, so that these can be carried/worn if desired - Leads me to think of a wooden/acrylic case and a capacitive touch sensor to trigger an MCU wake and the animation
- Keeping track of time accurately enough for at least to-the-month granularity over the 2 year runtime (without resets or other functionality-breakers)
- Relative ease of assembly and programming (time's running mighty thin!) - Arduino compatibility and parts bigger than grains of sand are preferable (though I'll likely be hot-plate reflowing)
I considered several options for how to achieve the functionality I was going for, including consolidating processing and low-power capacitive touch interrupts with the ATmega328PB, but this didn't seem to be well-supported in the Arduino environment, and required a Windows-based proprietary tool to setup the touch sensing.
Instead, I decided to use a plain-old ATmega328P and an AT42QT1010 momentary capacitive touch sensor - the board was starting to look a little tight! For 3.3V power from a 3V CR2032 coin cell, I decided to use an MCP1625 boost regulator. Things look to be coming together!