On the loss and renewal of focus

A project log for RPi WiFi

Fast RPi WiFi without USB

ajlittajlitt 03/21/2016 at 05:080 Comments

(TL;DR: spent a few weeks on a tangent eating my own dogfood, finalized PCBs and ordered a bunch)

Spring Break is always full of distractions for me. The kids are out of school, work slows down a bit, and South by Southwest takes over Austin, TX. A few weeks ago I got an email from @Sophi Kravitz asking if I might have any hacks I could bring for a panel she was giving at SXSW Create. Naturally I wanted to show off this project, but it hadn't occurred to me 'til then that I had built exactly zero projects using the WiFi Pants board.

The board isn't that interesting by itself or on a headless Pi, so I wanted it to do something flashy. It needed to be battery-operated to show off the boost converter. So I turned to my junk bin for inspiration, and made a simple LED scroller with the LED matrix from Sparkfun's BadgerHack swag they gave out at last year's SXSW (thanks again!):

I... uh... went a little crazy on this. I didn't expect the show-and-tell to be anything more than what it was: a short demo during the panel. But somewhere along the line I got it in my head that it needed to be a polished looking gadget and not a pile of boards and bodge wire.

I spent a few late nights building the board stack and designing a 3D printed case. The Sparkfun LED array has mounting holes on 23mm centers, conveniently matching the Zero's screw holes, so I scrounged some washers, standoffs, and screws from my junk box and point-to-point soldered the array to end up with this:

Initially I wanted to fit a small LiPo pouch in between the Zero and Pants board, but I ran into a few problems. I couldn't just go down to the local hobby shop and grab a battery for a nano quadcopter. I needed a protected cell to compensate for forgetting the UVLO (under voltage lock-out) programming resistors on the switcher on this board rev. Failing to have either cell protection or UVLO on the switcher could drain the battery too far and lead to surprise fires of the sort I'd like to avoid. Unfortunately there seemed to be a run on cells with the characteristics I wanted: reputable seller, fits in a 45x23x6mm volume, has protection, big enough for 1hr runtime (>=400mA), and fast enough shipping to get here in time. It seems that the sorts of batteries I wanted were all snatched up by hackers making one of the DIY Zero-based Gameboy projects.

I ended up sticking a 3-AAA battery holder to the back of the case. I used 750mA NiMh cells which consistently give 2:15 runtime with the scroller running constantly. It wasn't as pretty as it would have been with a LiPo, but it worked and was likely safer. I had to make changes in the case model to make the battery connector accessible from the outside, but otherwise it worked well.

The software is all Python. I could have done a C userspace daemon or kernel driver to make the display scanning smoother, but I didn't have time and figured a 1GHz CPU could handle it. It is multithreaded though, with the display scanner / scroller in its own thread that relies heavily on sleep() for timing, and the main loop and Twitter checker in the parent. There is a little flickering when the CPU is loaded, but it's not too distracting and only seems to show on feed updates.

The case was modeled in OpenSCAD and printed in translucent gray (actually thermochromic "hypercolor" plastic) filament. The front of the case is .8mm thick, acting as a diffuser for the display. The thin front is a bit fragile, but it survived the demo. I did print and carry around a second copy the day of the demo though. The insides are lightly friction-fit to the screw heads on both sides of the body, and has stops inside to prevent putting stress on the protruding ESP-12F module antenna. A friction fit lid and hoop for attaching to a lanyard finishes it off.

So this is great and all, and I hope some of the audience found it interesting, but what about making some boards available for sale? Last night I put in an order for 50 PCBs, and will get stencils and additional components ordered soon. The r4 revision has some prettification of the silkscreen, adds the UVLO resistors to the power supply, and makes the option to share the battery positive with the 5 pin header disabled by default to limit the number of opportunities to short out a battery.

Based on the great suggestions I got from the comments in the last log, I'll be making only some of the boards with the boost power supply onboard, with the rest having only WiFi, and will put up additional bare PCBs for sale.