On October 20th Nordic Semiconductor is conducting an all-day seminar with members of their R&D organization on Friday, October 20th. They’ll cover the nRF52 chip series, Bluetooth 5, Bluetooth Mesh, and “802.15.4 and Thread: Mesh technologies for IoT applications”. They say somewhere that attendees will receive an nRF52 dev board. Registration here.
Humans and Autonomy Lab – Humans’ Interaction with Autonomous Systems, by Dr. Michael Clamann
Explainable AI, by Dr. Alexander Stimpson
Experiment on Humans’ Trust in Risk-Aware Autonomy, by Dr. Lixiao Huang
November 8th at the McKimmon Center at NC State will be PCB Carolina, “North Carolina’s Premier Electronics Trade Show”. They offer outstanding food, a range of technical presentations and many vendor and organization booths to do with PCB design and production, and electronic assembly as well as related vendor’s wares. Details here.
Problem of the Month
Paul presented his “sorting a mixture of colored marbles” mechanical+electronics problem and gathered ideas submitted by the attendees
Details of problem-solving sessions are available here.
LTSpice talk. The materials are on Carl’s GitHub repository here and he’ll update it as the project matures.
Glenn Smith showed a charging base he build for his Ham Radio. There was a 3D printed (ABS plastic) base with a LiPo charger board inside. It plugs into his car dash (not via cigarette lighter port, but a more robust connection).
Paul MacDougal showed his mushroom growing box. A plastic tub with a 12V fan, a CO2 sensor, and a DHT22 temperature/humidity sensor. An Arduino monitors the CO2 and turns on the fan at 800 ppm CO2 and turns off the fan at 750 ppm CO2.
Paul MacDougal showed his pushup counter based on the horizontal ultrasound sensor technique suggested in the September meeting. It worked well “in the lab”, which has hardwood floors, but not so well on carpet. He will continue to develop this project.
What can we say? Ben stunned much of his audience, especially during the Q&A.
Paul gave an update about his power over ethernet development project and his ongoing wrestling match with 802.3AF.
Alex presented his “leg movement logging via accelerometer using European Data Format” project using a Teensy USB and open source tools and custom Python code. He gave everyone a great accounting of a typical development project based off open source starting points and got the final results he was after. The GitHub repository for his project is here.
(No notes were taken: if you contributed something please drop a line on the email list so this can be back-filled!)
“I am Ben Goldberg, a 5th grader in Durham. I am proficient at programming in Arduino and Python and playing Pokemon GO. 😉
My talk is about a smart home light I made for my bedroom using a Raspberry Pi as the server and a ESP8266 to control the light and be a client on my Raspberry Pi.”
Come to the meeting and learn how to make a smart light that integrates with Apple HomeKit. This presentation, another Problem of the Month, and the usual discussions and show and tell are all in store. Meeting details
Today until 2pm is the second day of an open house of the area Triangle DIY Biology organization near Scrap Exchange in Durham. I was button-holed to “do something” at this event and decided the easiest thing to demonstrate would be how to hand solder fine pitch SMDs. TriEmbed info sheets are on hand and we’re also plugging SplatSpace. I’m using a little space at the popup to coordinate curation of Fred Ebeling’s Collection of electronic parts at SplatSpace.
Adam Haile and Dan Ternes of Maniacal Labs have been busy. By accident I came across their site on Tindie and see they have a new product. PiPixel is oriented toward Raspberry Pi enthusiasts who would like to work with smart RGB LED strips. It’s described in detail on their web site. This is a super low cost kit that would make an excellent beginning soldering project with a big payoff. Find all the details on the ‘Labs web site.
As mentioned at the June 12th meeting, I find myself frequently wanting to find the sweet spot for quickly throwing a circuit together to see that gross behavior is as expected and confirm my assumptions correct and the devices perform as advertised before going to the trouble of making a PCB that is likely to be wrong if I’m in too much of a hurry.
But I sometimes find use of SMDs painful with solderless breadboards. First is the headache of having a parallel set of through-hole parts for bread boarding convenience. And of course this is an illusion in many cases: there are no through-hole equivalents for a growing number of components these days. Some shortfalls are obvious, like the current crop of tiny DC conversion chips that seem to be exploring the outer limits of how small a flat pack, no lead package can get. But others are chronic and might be surprising to some readers. Just try finding through-hole versions of some of the specific thermistor types specified for use with things like battery charger chips. It also gets old quick to have to buy a resistor with the same precision value in both through-hole and surface mount packages. And is that through-hole diode slapped on really behaving the same as the specific SMD spec’d for the “real implementation?”
I’ve been accumulating little breakout boards as carriers for various SMDs to make this easier, but that often involves a compromise, such as putting a tiny diode between two SOT-23 pads on a breakout that eats up six pins on a breadboard vs two. Other parts are more challenging and lead to semi-monstrosities like this one for a 22uH inductor:
This is what a typical ad hoc collection of parts looks like:
Notice the MSOP12 device is kludged onto a TSSOP14 breakout board. Also notice the (boost) capacitor soldered to the upper left pin of the MSOP12 package in the middle. It’s the barely noticeable bump on pin 12 of the IC (coincidentally below “12”, but electrically connected to header pin “14”, thus the error-prone kludge).
So, although my current breakout collection handles more than 30 devices directly, there are many gaps. This current project will bring the “package coverage” up around the 50 mark, but I estimate I’m only about half done before my hankering for this kind of support mostly dies down. For instance, there would be a lot to gain from handling the common form factors of small switches and connectors, small aluminum electrolytic caps, inductors, transistors, etc. Heck, there’s even real value in making an adaptor that saves me severely abusing a breadboard by cramming TO220 devices into the holes!
Adafruit and SparkFun and others provide some excellent breakout boards at affordable prices (in contrast to Schmartboard’s stuff, which seem to be both too much and too little for my needs). Notice all five of the boards above are Adafruit ones. I have several TI breakout types as well as others. But the actual coverage of package types for the available boards out there is too sparse.
The aggregate PCB rendering above shows the approach I’m taking to build out my collection. This is a collection of 12 PCBs supporting 21 package types that I pulled together for the purpose of making stainless steel stencils. Some of the boards for larger parts have nearby pads for a few passives to make it less painful to handle bypass caps, boost caps, etc, where short connection paths are important.
The 12 separate designs (and two others) were sent off to OSH Park to get three copies of each design as a small PCB so I can prove out the breakout boards .
Here is what the stencils from OSH Stencils look like for the top and bottom of the “aggregate PCB”:
In theory, the work flow is to put the stencil over a particular breakout (e.g. the one for the SOIC14 package in the upper left corner of the top stencil). The one breakout PCB for the package is captured by a holder made of two acrylic “L” pieces vinyl-taped to a dead flat surface. A small squeegee (small piece of plastic credit card) is used to paste over the site(s) for a particular board, then parts are placed and hot air or reflow oven soldering is used.
A quick side note about frames. In many commercial environments the stencil is mounted in a surrounding frame that in turn fits into a jig allowing for rapid handling of boards while maintaining precise registration. I was confused Monday: OSH Stencils is only beta testing frame support with two sizes (relative to these boards those sizes are “wow”, and “my lawn is smaller than that”). Contact them for details if you’re interested.
I should also point out that I didn’t spend a lot of time routing these boards. For example, I’ve already had second thoughts about the sense line routes for the two shunt resistor boards that have ‘kelvin connections’ to special middle pads under the resistor ends. Also, as I mentioned Monday, I didn’t spend a lot of time checking things like length to width ratios for some stencil apertures. So the very narrow pad for the 1206 shunt board is technically smaller than the minimum supported size listed for the paste being used (Kester EP256), and the result may be that I can’t actually get paste into this spot properly. The footprint is blown up in the image below. The middle of each set of three pads is .28 millimeters wide. The resistor that sits on these pads is about an eighth of an inch long (yes, king size in relation to how these things are trending).
OK, but how much do those stencils cost? The minimum is $10, but the incremental cost beyond this is less than a dollar a square inch. The two stencils above came to around $22. First class postage with tracking adds $2.75 to an order. My order was completed on a Thursday and I had the stencils in hand the following Monday. With minimal (.75in) borders, everything you see in the first picture above added less than a dollar to each stencil cost. Note that in most cases you’ll only have a stencil for SMDs on one side.
(Update June 22nd)
The first batch of PCBs came back from OSH Park and are shown in the picture at the top of this posting. The tabs sticking out are cut off and squared off with sand paper before the boards are used.
Finally, it’s important to note that some circuits will not cooperate with solderless breadboards and any arrangement of surface mount parts involving long connection paths. Apart from huge amounts of stray inductive and capacitive reactance in the circuit paths, the current carrying limits of breadboards are very real, as is the ability of a simple breakout board to shed heat compared to a PCB having copper pours and other design features to properly handle it.
Here’s a breakout board that got a lot of hand soldering with three different kinds of flux. Plain 91% isopropyl was used after rework using AIM 280 “no clean” (the watery stuff), which, if used copiously, definitely needs cleaning if you care about appearance. In this case, even scrubbing with a nice (Adafruit) ESD-safe brush would not remove the flux completely as can be seen with this first picture below. The gray cast that’s most pronounced in the lower right corner shows this “can’t clean” flux layer:
Several rework episodes later, the board had seen lots of the NC flux, but also Chipquik SMD 291 “Tack Flux N/C”, which, in my experience, is never “no clean” except perhaps in the sense of “the electrical connections won’t eventually short out with this stuff”. From an aesthetic perspective the layer of goo left behind in many cases is simply not nice. Finally, I got lazy with a brute force desoldering method involving braid and old style rosin flux and that stuff is of course just plain nasty. So here’s the very sad “before” photo after the shunt resistor and other parts had been messed with a few times:
For this case a bit of MG Flux Remover was put into the bottom of a jar, the board was put below the surface, and the jar was swished around for a while. Some exceptionally bad looking, black “baked rosin flux” bits were scrubbed off and then the board got another few seconds in the jar. Then the flux remover was washed off with pure isopropyl to get the “gunk in solution” level down to zero. (This cheap squirt bottle from Rite Aid works very well for this). Then after the board had dried it looked like this:
The result was superb: no gray splodges or any other stains. Notice the very tired traces missing solder mask in places. This is what comes of too vigorous scrubbing. The flux remover looks unchanged and I’m expecting that in the tightly sealed jar it has a lot of use left.
I was expecting the remover to have evil stuff in it and was surprised to find it’s ethanol, isopropyl alcolhol, and ethyl acetate. The latter is found in small quantities in wine (and many homebrew beers). But despite the familiar chemicals I use lots of ventilation for this kind of task.
Thanks to Shane Trent for letting me “try out his jug”.
There are a few things to share from this month’s meeting :
Folks compared notes about participation at RARSFest last month. The “non-commerical” aspect of Triembed folk’s participation at the next RARSFest is expected to be free. Whoever takes the lead on preparing for the next one can arrange this and get “just the right spot” for tables if they contact the organizers early in the process (4-6 weeks ahead).
Shane Trent passed a heavy duty patent law exam and is now a seriously useful resource for area developers wanting to invest in IP via the patent process. Congrats, Shane!
Paul MacDougal shared his setup for remotely detecting potential freezups of his well pump.
Chip McClelland showed an air temperature logging addition to his repertoire of park service tools being embedded at Umstead. This will allow the park staff to measure the cooling effect of newly planted shade trees. Chip also passed around his full custom battery management board for folks to admire (built at Pete’s shop).
Pete described tests of the Silvertel AG103 Maximum Power Point Tracking solar powered battery charger board and a ten watt Ecoworthy solar panel being prepared for the “Little Library” Jeff Crews (of Splatspace) built and installed at the Durham Scrap Exchange.