Sunday, October 29, 2017

My board eats chips

After a very frustrating time spent with the innards of Probey, I have come to a disturbing conclusion: My circuit board eats chips.

By which I mean, for some reason, the circuit I designed and built has been destroying the ($50 each, thank you very much ... did I mention nothing in aviation is cheap?) pressure sensor chips. Right under my very own eyes, one chip that was doing just fine and answering as it should at I2C address 0x29, after being plugged into the board to test the various sockets, just stopped working and did not respond to I2C messages at all.

I am ensuring the board is powered off prior to connecting and disconnecting things. BUT it may well be that the Fio V3 is not really powering off the buses when it's turned "off" so long as it has a battery connected, and perhaps the resulting discharge when parts are connected and disconnected causes them to fail. I am trusting the Fio to turn itself off while I swap chips with the battery connected. Maybe I should disconnect the battery before swapping chips.

Perhaps also the All Sensors chips are particularly sensitive to power transients and ESD? I don't know.

I put together an independent breadboard test rig to test the chips away from any other weird circuitry:

This rig is my determinant of whether a chip has been eaten or not. And in fact, again, a "good" chip showed up on 0x29 in the test rig and in the probe, then stopped responding in the probe, and back in the test rig was also not responding. So the probe board eats chips. Simple as that.

Probey McProbeFace taking shape

I finally got enough pressure sensors to fully populate the probe, and put it together. The one missing part is a more durable back cover that can be secured with a screw. The following photos show some of the tubing installed, and the Taoglas adhesive patch antenna with a Zip-tie for strain relief.

Sunday, October 22, 2017

Rough idea of new probe

This is a rough idea of what the new probe, with the new PCB and supports, will look like. Some dimensions and positions will change, but nothing major:

New board making progress

Today I got the new board put together with the 3D printed components. I have IC sockets for the pressure sensors so I do not need to solder them in. I have some more sensors on order to fill up the slots, and then hopefully we'll be ready for another flight test!

Sunday, October 15, 2017

Integrated PCB assembled and functional

I got my integrated PCB in the mail last Friday, and spent some time populating it. After fixing some bad soldering jobs and doing a bit of debugging ... it works! The only problem in the board was an incorrect wiring of the on/off switch, which is easily fixed by cutting one trace. The I2C devices enumerate properly on the mux. Here I have not yet populated it with the fancy and expensive pressure sensors, but I did install two of them temporarily to make sure that they are showing up on the bus.

The next steps are to adapt my code to the new board configuration, which should be pretty quick, and expect the 3D printed mounting components, which should come in this week.

Saturday, October 7, 2017

Modified probe design and some 3D printed parts

Pursuant to the flight test dud, I have been redesigning the innards of the probe to fit on a single PCB for reliability so that (a) I don't have to pack a bunch of barewired parts with foam to try to get them to stop rattling, which as we've seen makes them fail; and (b) I don't have a bunch of parts rattling around with a LiPo battery in the probe body.

The new probe looks like this:

The forward support and rear cap for the PCB are being 3D printed. This is how they look, below. Note how the rear cap supports the PCB yet includes a generous cutout for access to the board edge connector and switch, and to expose the on-board temperature sensor chip.