Stratoballoon Radio Part 2, Addendum

This is an addendum to the most recent podcast, Episode 22 – Stratoballoon Radio Part 2.  I realized after I got everything put together, and the Balloon sketch updated, that I had a problem.

You may recall from Episode 17, the podcast that we did to show off the GPS receiver, that the balloon sketch takes a reading from each of the instruments and the GPS receiver about once per second.  It then writes that data to the data logger.  I knew that the transmitter couldn’t send the data that quickly, so I decided to transmit the coordinates once every 10 readings.  A transmission every 10 seconds should be quick enough that if I miss 2-3 in a row, it’s not a huge deal, but not so quick that we end up with large gaps in the data being recorded to the microSD card.

The problem was that in the 9 seconds or so between "official" transmissions, the NTX2 was continuing to transmit.  Over my headphones, this came through as a solid tone.  You’re not supposed to tie up the airwaves like that for hours at a stretch (which is how long our flight is expected to last, not to mention the time it will take us to find the balloon once it lands).  I had assumed that if I wasn’t transmitting anything over the data line then nothing would come through, but that was now painfully, obviously wrong.

I then remembered that the NTX2 had an Enable pin.  The technical specs for the chip said that I can set this pin high when I want to transmit, and low when I want to disable the chip.  It also said I could just tie it to the adjacent pin – the power pin – if I didn’t need this functionality.  Up until now, I didn’t see a need for it, but now I had.

I first had to de-solder the power lines that I had connected to pin 4 on the NTX2, and move them over to be directly connect to pin 5.  In the process of doing that, I removed enough solder to break the connection between pins 4 and 5.

Next, I added a new data line to pin 4 that would eventually be connected to the instrument pack shield.

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The yellow line here is the new one, and connects the Enable pin (pin 4) of the NTX2 to digital pin 5 on the instrument pack shield.  I’ve updated the diagram to show this.

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Once I had these two pins separated, the new process in the sketch became:

1) Set pin 5 high to turn the radio on
2) Transmit the data string
3) Set pin 5 low to turn the radio off

Of course, it really wasn’t even that simple.  I’ll be writing up a more detailed walkthrough of the updates I made to the Balloon.ino sketch on my technical blog, http://mark-gilbert.com.  You can find the fully updated sketch on GitHub.

Apart from any changes that come out of testing, I believe the Instrument Pack is now hardware- and software feature-complete.

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