Electronic Barometric Altimeter for Stratosphere Balloons

Our team, RandomRace.ru, launches helium balloons. Small and big, with cameras and without. We launch small ones to randomly drop checkpoints for adventure race competitions, and big ones to make great videos and photos from the very top of the atmosphere. It’s not the space yet, but at 30 km of altitude air pressure is about 1% of normal. Does not look as the atmosphere anymore, huh? My responsibility in the team is electronics, and I wanna share one of my projects implemented on that duty.
How can we measure the balloon altitude? With GPS (most of them do not work above 18km) or with a barometric altimeter. Let’s make one from an microcontroller(MCU) board! We want it to be light, cheap (since sometimes we loss our probes), and easy to build, easy to use. It also should measure very low pressures. The device should log data at least 5 hours in a row. Let’s use some Lithium battery from any old mobile phone as a power source. Basing on the requirements, I have chosen Maple Mini board, based n ARM microcontroller(STM32F103RC) with an USB interface, 128 Kb of internal memory, which is enough for both MCU firmware and collected data. Unfortunately (or fortunately?) , LeafLabs does not produce those boards anymore, but their clones could be found in Chinese online stores for a couple of dollars only. Also we were donated with a number of MS5534 air pressure sensors, capable to measure 0.01…1.1 bar. That is more or less enough for 30 km altitude.
The device is quite easy to make, you only need some soldering skills and tools (there is no need to solder really small parts) and basic computer skills. Here you can find a github repository which contains both breakout PCB design in Eagle format and firmware.”