I've been working on a revision to the NoiseCard PCB in the pcb-rev2 branch of the project's repository. I'm fairly happy with its current state, so I'd like to share how the board looks and what's been changed:

Energy Harvesting

An energy harvesting circuit has been added based on the NEH2000BY. This means that an energy store can be built up while the NoiseCard is in any ambient light conditions, as opposed to the previous design which practically required constant direct sunlight.

This chip is optimized for solar cell harvesting and can store energy at a rate of up to 2 milliwatts, almost reaching the power requirements of the NoiseCard while it's powered on. Paired with the new capacitor storage, the NoiseCard should be able to always provide enough energy for general usage even in low sunlight or indoor areas.

Larger capacity capacitors

After a thorough search, I came across small surface mount supercapacitors like the CPH3225A. They're much more dense than the current tantaulum capacitors, with a smaller physical size and a larger capacity of 11mF at a rated voltage of 3.3V. This should lead to a 12x improvement in possible run-time over the previous setup! The cost increase for this is less than a dollar per capacitor, very acceptable.

0.1" IO header and breakaway section

To make the NoiseCard repurposable, I routed all microcontroller pins through a 0.1" header and added a breakaway strip. This lets you remove the "business card" part of the PCB, including the on-off button and LEDs, leaving you with a solar-powered microcontroller with 10 GPIO pins. All you need from here is an SWD programmer (present on nearly all STM32 development boards) and a quick solder (shorting R5 to keep the system "on", or add some other on-off control) and you're free to start tinkering!

Pushbutton on-off control

The previous design used a slide switch... I don't like it. I just found it to be very bulky and rough on my finger. With a thin tactile button, all you'll need to do is lightly press down and the NoiseCard will power up.

Want one?

If low power and solar harvesting interests you, then maybe you'd like to have a NoiseCard for yourself :)

I may sell a batch on Tindie if there's enough interest, let me know if you are. I'll at least order a small run of this design sometime soon and see how the card's performance improves.