Electronics Planning

It would be highly uninteresting to have a weather balloon come up and fall down without acquiring any data for post-journey analysis. So we’re bringing lots of sensors with ours!

Specific Output Needs and Hardware Requirements

To start off, we want to go up with lots of sensors, and come down with lots of data in an SD card. Whether our hardware is broken afterwards isn’t really an issue, but we want to be able to retrieve the data. We’ll need a medium to acquire and store data for us and it seems the most economic and feasible solution would be an Arduino board.

Arduino is an open source prototyping platform that can be easily used for DIY projects. You can power motors, sensors, displays, and all sorts of fun stuff with it. We’ll be using it the Arduino UNO R3 for our project, since it’s cheap ($20) and if we fry a board or two while prototyping, it won’t be so big of a deal.

 

Most of our sensors meet to meet some key requirements to go up in the air and come down. Some limiting characteristics for hardware include operating temperature, input voltage and amperage, and cost.

  • Temperature – electronics act wonky as extreme temperatures, which isn’t typically a problem unless you’re 20,000 feet above sea level (we’re designing this to get to an altitude of 100,000 feet). We will be using two thermometers  – one on the inside of the payload, and one outside. I expect good insulation (sadly, hermetic sealing is unfeasible and too expensive) will come from our payload team, but our external sensor is completely exposed. We’re expecting temperatures of -50 degrees centigrade, so many of the cheaper hobbyist thermometers wouldn’t be advisable. While some thermometers boast about accuracy, we must be cautious about accuracy dropping off as the temperature decreases – accuracy up to half a degree isn’t going to exist at -40 C. Furthermore, we’ll need to verify the temperature extremes ourselves to make sure the balloon doesn’t come down with useless data.
  • Power – one of the limitations of Arduino is that you can only draw so much current from it. The manufacturer (the real Italian manufacturer, not cheap ripoffs) specifies that you can only draw 50 mA from it. This may not be a deal breaker, since our devices draws 4-15 mA each. This could even be reduced if we decide to sample at a lower rate (the default is at a baud rate of 9600 bits per second). In any case, we will need to keep track of total current draw while deciding what hardware to adopt.
  • Cost – while it is nice to have the larger budget that we have ($1000?!), we need to challenge ourselves to find economic solutions. Sure, we could buy a $250 sensor, but would we learn as much from tinkering from the electronics? Is the difference between 0.02C and 0.5C that big for the purposes of this project? What if a new hobbyist stumbled upon our work and wanted to replicate it, but on a shoestring budget? We’ll be keeping costs as reasonable as possible.
  • Weight – wasn’t really an issue because the total weight of these electronics won’t exceed half a kilogram – the Arduino UNO R3 itself only weighs 25 grams.

List of Desired Measurement Parameters

  • Payload Internal Temperature
  • Payload External Temperature
  • Altitude
  • Pressure
  • Light
  • Movement
  • Rotation
  • Strength of Earth’s Magnetic Field

Hardware Purchase Decisions

Barometer – BMP180

Selecting between the ones available on Sparkfun, the BMP180 seemed the best choice. The first criteria to filter out candidates was the minimum temperature – everythign that did not meet the -40 C minimum operating requirement was ruled out. The BME 380 (which sounds like a tough class at Duke, if it existed) would have been nice, but it had a maximum altitude of 30,000 feet, despite higher accuracy. The Sparkfun MS5803-14BA seemed good too, but the ranges it had were from 0mbar to 15bar. Furthermore, it was designed by Measurement Specialties for diving purposes (the datasheet strongly suggested this).

The BMP 180 was an obvious choice and only demands 3.3V input. It meets our basic operating temperature requirements (-40C to 85C). Manufactured by Bosch, the BMP can operate at its lower-power mode  at 1 Hz and draw 5 microamps. Accuracy is at 1 Pascal at nominal operating temperatures. Sadly this level of accuracy goes away at below 0C. The datasheet can be found here.

Thermometer – TMP102

The clear choice winner among electronics blogs was the TMP102. The current draw is at 10 microamps, which is still pretty amazing The operating temperature range is -25C to 85C with an accuracy of 0.5C.

 

Ambient Light Sensor – TEMT6000

There was no other choice available. Cheap, easy, analog pin. What more could you ask more?

3-Axis Accelerometer, 3-Axis Gyroscopes, 3-Axis Magnetometer – LSM6DS3

One of the interesting possibilities of our project would be to see how much turbulence the payload experiences during its flight. Thus, we bought this integrated package that could simulate the rotation movement of our payload – which we could extend into a simulation or a physical simulator in a future project.

SainSmart MQ-131 Ozone Sensor

Standard Ozone sensor that detects the concentration of O3 in the atmosphere. This is rated from 10 PPB to 2 PPM. For a healthy atmosphere, we should expect 0.001 to 0.125 ppm. I kind of want to throw this near a construction site or a car exhaust for fun though…

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OpenLog 

This is a popular solution for many telemetry needs – draws minimal current, 3.3V and takes Arduino serial output to SD card.

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