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.
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.
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.