The initial idea for this project spawned off of another that is using an ESP32 for continuous decibel monitoring. While the beefier ESP32 allows for wireless reporting of measurements, there's a price to pay in energy: 20mA or so of current consumption while recording from the I2S microphone, and a whopping 80mA while connecting to WiFi.

As a lover of ultra-low-power systems, I knew that microphone monitoring and decibel calculation could be done for a fraction of that energy budget. Searching for a viable low-power, low-pin-count microcontroller, I came up with the STM32G031J6: a tiny 8-pin SON package with a Cortex-M0+, I2S connectivity, and promising low power characteristics. I ordered its development board, the STM32G0316-DISCO, and began testing it with the microphone breakout board I had on hand.

After getting the system running with some code, it was clear that this could be very low power: down to 2mA at 1.8V -- less than 4mW! This was achieved through low clock speeds, optimizing calculations, and using an RTOS with sleep modes and interrupts.

Now with the rise of Hackaday's business card challenge, I knew it would be no problem to stick these two ICs onto a PCB. With such little power required though, I wanted to push the limits and run the circuit entirely off of solar power; no bulky battery, just a thin, light card that could be used anywhere (where there's light).

So, the next step is to design a PCB...