A Tiny Balloon Computer

Another PCB design gets shipped off to China for manufacture!

This is a tiny balloon computer.  Without battery or GPS, it weighs less than 40 grams.  It can transmit RTTY or CW at 25 mW on the ham 2M and 70cm bands, and will last up to 16 hours on a single AA.

This board will be Arduino compatible, and the primary goal will be to support White Star‘s superpressure balloon experiments.  A superpressure balloon maintains some pressurization above ambient in order to maintain altitude.  In order to figure out how to design the balloon envelopes, and to verify our math, we need to know what’s going on inside the balloon.  It’s difficult to pierce the balloon without generating leaks. The ideal solution is to place a second balloon computer inside.  Both will transmit pressure to the ground, and we’ll be able to compare pressure inside the balloon to ambient pressure for model verification.  The pressure sensor can sense a change as small as 4×10-4 PSI, so this should work.

This is my first attempt at real RF design.  Fortunately, operating within the ham radio bands means I don’t have to worry too much about spurious transmissions or leaky harmonics.

The output filter is the most poorly designed section.  It’s surrounded by vias, made up of the linear arrangement of inductors and capacitors.  I wouldn’t be surprised if I ended up with RF coupling across the inductors, bypassing the filter entirely.  This design is *probably* good enough for 434MHz operation, but not 800MHz.

More information, and design files can be found here: http://projects.meatandnetworking.com/trac/meatandnetworking_tinyballoon

I expect this board to go through a redesign.  Schematic capture and PCB layout were done using KiCad, with which I have very little experience.  KiCad requires a different workflow from Eagle.  I’ll post a KiCad tutorial here later.

I ended up with a double-Pi filter on the output stage, and all the inductors are place linearly.  The RF might simply couple with the inductors, and bypass the filter entirely.  If that happens, I’m not too concerned.  Without any fancy RF analysis equipment, it’s a shot in the dark anyway.