Belloon – Bellarmine College Prep's High Altitude Balloon Project (with Code, Schematics, and PCB Layout)

The completed board! (after surface-mount soldering but before through-hole hand solder)

In my senior year at Bellarmine, with the STEM-Med Club, I endeavored to get funding for, create, and launch a high altitude balloon project. Although the balloon still hasn't launched and now I'm off to college, I designed and built a really neat ATMega-powered sensor/radio board that uses APRS to communicate with the ground and can measure atmospheric gas concentrations and their changes over time, assuming multiple launches.

Here are the specs:

  • 144.390MHz APRS RadioMetrix Radio for telemetry
  • GPS tracker (model TBD)
  • ATMega CPU (with Arduino bootloader) with SD card for data logging
  • Gas sensors (a sensor that can detect carbon-based gases and nitrous oxide, some of the worse pollutants in the atmosphere that also cause smog)
  • Humidity Sensor
  • Temperature Sensor

The balloon itself would also have had a GoPro Hero mounted on it for some sweet footage. More on the PCB Design process after the break.

A (literal, sorry) screenshot of final board routing in Eagle CAD

I chose an Arduino-compatible ATMega 8-bit CPU so that it could be easily flashed with the Arduino bootloader through USART (via the mini-USB port on the left of the board) and designed the schematics and routed the two-layer board from scratch using Eagle CAD. With the exception of the specialized NO and CO_2 gas sensors and radio (which were ordered directly ferom the manufacturers), all components are from Mouser Electronics.

I ordered the PCB in a pretty white color from Dirtyboards and after a couple of weeks, my package was in from China – and 10 gorgeous boards were ready to be painfully soldered (with only one set of >$100 of sensitive components!)

IMG_2805-1

I can't afford an IR oven to neatly (and properly) bake surface-mount components onto my board, but I certainly know how to use a $15 hot plate and temperature gun from Amazon! The result isn't the prettiest but my visual inspection shows no solder bridges among pins – yay!

IMG_2813

I hand-pasted and placed all the components on the board (also not fancy nor wealthy enough to own/have access to a pick-n-place robot, sadly). I got some 8.5x11" Kapton film (think camera film color and plasticky consistency) and attempted ot laser cut it, but it's so fine that my resolution just wasn't good enough, and solder squeezed past the openings onto other pins. Thus, I just used the needle from the soldering paste tube and eyeballed it, with hope that the heat would properly align the pads:

IMG_2816

Everything looked great – except the ceramic GPS antenna, where the component I ordered didn't match up with the pads I used in the board design (bottom left). However, I think it'll still work because I was careful to calculate the correct trace thickness and distance for impedance purposes, and the component is still properly connected, in roughly the same place it was designed to sit anyways.

More testing and software development to come (eventually)!

The source for the schematics and board layouts in Eagle CAD format are available on my Github.