Saturday, November 16, 2019

Two more display modules

Today I finished two more display modules.

One of them uses one of the new XBee 3 wireless modules. It initially didn't work with the XBee 2 that I had, until I realized I needed to install the 802.14 firmware. Once I did that, it was plug-and-play compatible with both hardware and software.

The most crufty part of the build is the tiny resistor network which provides reliable pullup and pulldown voltages for the quadrature encoder knob. I tried using the internal Raspberry Pi GPIO pullups for that purpose, but they don't seem to push enough current to produce reliable results when turning the knob quickly. So resistor network we must have. If I were to make this design in any quantity, I would spin up a PCB for that tiny thing -- but right now the protoboard is adequate. I am not sure the brightness of these displays is where I need it to be anyway, so I hesitate to "standardize" on this design.

Here are some build photos for your enjoyment.

Saturday, November 9, 2019

Cold solder is cold

After a CAD session during which vibration isolating versions of the probe board mounting areas were designed, I looked at the miserable board to see if I could determine the cause of failure. Well, it turns out I had a cold solder joint for the GND connection to the power and battery management daughterboard.

This is the solder joint in question, in the PCB hole pointed to by the screwdriver:

I did not allow for enough heating when applying the solder, and must have done a poor job of inspecting the board because, in hindsight, the joint in the photo looks pretty bad -- it's obviously just a bleb of solder sitting there.

In the same orientation, this is the hole in the PCB layout. You can see that it is properly joined to the ground plane by thermals, to alleviate exactly this problem:

So we're back on track -- but, if I can, I will change to the vibration isolated version of my mounting, just in case. If it's not too much more trouble, it should be really helpful.

Vibration damage in avionics is a thing (who knew?)

The Airball probe stopped working a few days ago while I was taxiing N291DR along the gopher-hole-pitted grass runway at the Monterey Bay Academy airport (CA66).

If you don't know this, the Monterey Bay Academy is a Seventh Day Adventist boarding high school in our region. They have an airport (of course! which self-respecting boarding high school doesn't?). And they have for years very generously made it available to the flying community, so long as you respect their rules -- mainly, refraining from landing during the Sabbath. They are awesome folks.

I have been working up to landing at their strip, which is sort of a "bush flying kindergarten". A few days ago, accompanied by my wife (and Airball Product Marketing Manager) Melissa Blum, I landed there as PIC for the first time ever!

Airball worked fine, until we did the long back-taxi down the bumpy runway. Then it stopped sending data. It's been a few days since I had the chance to pull it off the airplane and debug, but now I think I know what happened.

In this video, you will see that merely by wiggling the Pro Micro Arduino daughterboard, I can get it to turn off and on. So basically, the power supply solder connections or circuit board traces were mechanically damaged by the bumpy taxi!!

I found this out when I was poking around the circuit with my test probes and just happened to notice that the act of poking with the probes itself got the thing to light up and start flickering and sending data. Here is the video:

So the realization du jour is: Vibration in avionics is a thing.

Clearly I need to redesign my probe somewhat to add rubber vibration isolators to the board mount. This is a bit of a pain in the neck as I had hoped to be done with futzing with the probe mechanical design, to be perfectly honest. But I have to do what I have to do.

I hope I can manage to get some sort of isolation mount for the rear of the board. As you can see in the video, that area is already pretty "busy" and is not designed with any sort of simple mounting holes.