Re: My floating balloon plans
The one I got from Amazon is micro USB. It also has a jumper to change voltage between 5v and 3.3v. Sparkfun has a mini USB version (https://www.sparkfun.com/products/9873) and a micro USB version (https://www.sparkfun.com/products/13263).
I'm using a breadboard for prototyping. I also have an Uno, but the voltage difference would complicate things.
Yes, I may do this. I'm using the onboard LED to indicate status (lit if there's a GPS lock and flashing while transmitting), but that won't be necessary at 40,000 feet. :-)
The biggest weight savings would be in going with smaller supercapacitors. Going from 5F to 3.3F would save 1.2 grams. But having two in series to make 5.4 volts also cuts the capacitance in half. I'm going to test a lot to see how much of a difference it makes, but think I'll probably go with the smaller ones.
The Si5351A Clock Generator board is also rather heavy - I think ~4 grams. It has voltage switching/regulation and 2 extra outputs that aren't necessary. I'm going to look into another option or maybe trying to design a small custom breakout board with only one output. That would save around 2 grams.