As an excuse to learn some new skills, I’ve been volunteering code to the Stratux open source project on GitHub. My contribution is the mobile web app used to setup the device and to verify everything is functioning properly.
This post isn’t about the user interface or the code. It’s about the hardware.
my Stratux with Everything (SWE)
In order to do the development and test my code, I needed to build one of these devices. They are not hard to build and there are three levels of equipment to choose from. The good news is you can build the entry level version and just add parts to get to the second and third levels.
I started with the basic Stratux which is a Raspberry Pi 2, an SD memory card, a wifi USB adapter, one SDR, and a USB battery. For people new to the Raspberry Pi, the easiest solution is to get the starter kit because it includes the right wifi adapter, the SD card and a convenient case for the Raspberry Pi. If you don’t mind the “slow boat from China” for delivery you can save some money by getting the same SDR in different packaging – just make sure any SDR our buy uses the newer R820T2 chip. Most sellers are making it very clear in their description.
I had some of the parts on hand including a bigger version of the battery, but more about that later.
Initial assembly was easy. All I did was follow the instructions from the sticky post in the reddit subgroup.
Once I checked that it all worked I started adding to it.
To get together second level of a Stratux you just plug in another SDR and edit the config file to say it’s available. (Yeah, not a great user experience and the first hint I might want to get involved.)
To get together third and final level of Stratux you add a GPS module.
the RY835AI GPS / AHRS module
The actual module is the RY835AI available on eBay. It’s compact and surprisingly capable. It’s not only a GPS receiver but also an AHRS (attitude and heading reference system). What this means is it contains both a solid state magnetic compass and the ability to sense left/right and fore/aft tilting (in an airplane we call these pitch and roll).
I followed the instructions sprinkled through the reddit subgroup to get the GPS connected. Then editing the stratux.conf file to enable it.
With all the parts connected and working I wanted to hide all of the wires and parts in a case. I tried a few ideas with things I had in boxes or closets or forgotten in the shop. None were quite right. I finally settled on a case from Serpac, the A-42 enclosure. As one person put it, it has “just the right amount of retro 56k modem” feel.
The two major constructions made it all work. First, I had to un-case the battery and folding one set of cells on top of and offset from the other set of cells. Second, I had to fabricate a tiny dual USB cable.
dual USB cable flips USB ports 180 degrees
I received a few questions so I thought I’d include the answers in this post.
“Can you show the inside?”
I had already noted things were right inside the case. This should explain just how tight.
inside the Stratux with Everything
“What about heat?”
Using the mobile web app I’ve been developing, I can see I both the Raspberry Pi CPU temp – typically around 53c – and the temp sensor on the AHRS – around 44C. But those number go up somewhat when the Statux is in direct sun, such as on the glare shield under the bubble canopy of the airplane. Fortunately, the light grey case minimizes solar heat saturation.
“I noticed the CanaKit WiFi adapter in the pic. Did you compile their drivers into the software?”
The Stratux project uses an Edimax wifi adapter. For some unknown reason, my original wifi dongle went wonky. I had the CanaKit dongle sitting on my desk so I did a one time build of hostapd 2.4. Now I just copy it over if/when I flash the SD card.
“Which antenna are you using with the SWE ?”
I’m playing with a few different antennas. Next up will be an antenna design and sold from a Reddit member. I will also make the j-pole and try that as well. The nice bit is the BNC connector allows me to use common antenna connections.
“Is the SD card accessible?”
The SD card is only accessed by opening up the cover. It’s not ideal but new releases is Stratux are slowing down and I think it will stabilize in the near future. Most of time, I use the Ethernet port and rsync new source onto the SWE and rebuild.
“Are you passing through power while charging the battery?”
That’s an Interesting question. I wondered why all of the USB batteries I’ve seen charge or power. But not both at once. I figured it out when I cut open the battery pack.
These batteries are built as two “banks” with each bank generating 3.7V. The charging circuit feeds these banks in parallel. This works because the 5V charge voltage is greater than the 3.7V. The two banks are run in series through the regulator circuit. This works because the 7.4V is greater than the 5V output.
It would be much more complicated to charge the battery while also having the battery discharge to a device.
While it would be possible to feed the battery and feed the RPi, it would be messy since it would require a diode to prevent back feed into the battery.
Besides, at 20800MAH, the battery will power a fully loaded Stratux for 9-12 hours. It requires up to 20hrs to fully charge.
Perhaps the most important question was not asked. “If you did it again, what would you do differently?”
I’d change two (or three) things. I’d use a smaller battery solution. The 20800mah is overkill and the interior is very tight. I’d trade some capacity in favor of some room there are newer batteries that are in the 10,000mah range which are very compact.
By changing batteries I’d also end up changing the availability of “output power”. That would mean building another custom USB cable. I’d have only one output available. This would free up some space on the end panel. It would also give me more options for placement of the parts.
A friend picked up a low cost 2-line text screen. I’d love to integrate it to display status. In order to add this screen to the case, I’d have to do #1 first.
I posted more pictures to imgur. interior: http://imgur.com/gallery/82G8r exterior: http://m.imgur.com/gallery/sLgWW