# Gas station without pumps

## 2021 January 9

### One week into new quarter

We’re one week into the new quarter (10% of the way through!) and the course is going ok. Most of the students have finished the first-week lab, which consists of installing a lot of software and soldering headers onto a Teensy LC board.

The software they had to install was

Of course, each piece of software has its own installation idiosyncracies, different on Windows, macos, and Linux.  Some people even bumped into some problems because of running old versions of macos or Python (which were luckily cleared by upgrading to slightly newer versions).

The soldering was a bigger problem, because many students plugged in their cheap irons and left them on for a long time without tinning the tips.  The result was a sufficient build-up of corrosion that that they could not then tin the tips—even using a copper ChoreBoy scrubber to clean the tips didn’t help in some cases. In the in-person labs, I often spent most of the first week labs cleaning soldering iron tips that students had managed to mess up, but I can’t do that online.  This was not such a problem last quarter, as most of the students knew how to care for soldering irons from the first half of the course, but it may be a bigger problem this quarter, as most of the students have never touched a soldering iron before.  Some of the ones who are living here in town may be contacting the lab staff to see if they can get access to tip tinner or get some help cleaning their irons.  Those further away may be buying tip tinner on their own—I had not included it in kits, because I nad not expected so many to need it and it costs $8 apiece. Grading is going fairly well. My grading team and I have had two Zoom meetings so far (for Homeworks 1 and 2) and I graded Quiz 1 by myself, so we are keeping up with the grading. He have Homework 3 and Prelab 2a (there is no Prelab 1) both due Monday morning, and we’ll try getting them graded Monday night. We’re having to do most of our grading in the evening, because one of the graders is living in China, 15 time zones away, and none of us in California is an early morning person. In other news, I’ve finally finished clearing the blackberries and ivy from behind the garage (a project I started about 2 years ago). I’ll probably find some more when I cut back the kiwi vine (an annual winter project, in addition to frequent minor pruning during the summer). I think I either need to get some female kiwi vines and an arbor for them or uproot the male kiwi. There is really not much point to having just a male kiwi intent on taking over a big chunk of the yard. There are still a lot of blackberry roots out there that will sprout new vines. I’ll try uprooting them where I have access (not where they are coming through the cracks in the concrete), but I’ll probably have to do a monthly sweep of the yard to remove blackberries for the rest of my life in this house. ## 2020 June 7 ### Tenth video for electronics book Filed under: Circuits course — gasstationwithoutpumps @ 22:03 Tags: , , , , , , I’ve just published my tenth video for my Applied Analog Electronics book. This video is for part of §29.2.2—fitting models for loudspeakers. I filmed the video using OBS (Open Broadcaster Software), and this is the unedited first take. This is my longest video yet in the series. ## 2020 January 11 ### First week of class W2020 Filed under: Circuits course — gasstationwithoutpumps @ 21:16 Tags: , , , , , , I’ve just finished the first week of class for BME 51A (Applied Electronics for Bioengineers), and it has been a rather hectic week. First, I had about 5 people sign up for the class at the last minute (from 56 to 61), which drove the BELS staff to having to do a lot of expensive last-minute purchasing of additional items for the parts kits. It also meant that when we handed out the parts kits on Tuesday, several of them were incomplete and so more items had to be handed out during the Thursday lab period. Since getting their kits, 2 students have dropped taking us down to 59, and one other has told me that they might drop (assuming that they can get a coherent plan together for an independent study to replace the course). This enrollment is less than last year’s 82 students, but still a fairly large number, given that the BELS labs can only accommodate about 24 students. I’m using two lab rooms for the first section and one for the second section, and have hired 3 group tutors (one for each lab room and time) to work along with me in the labs. The first week’s labs consist of soldering headers onto the Teensy LC boards and installing software: Python with SciPy (using the Anaconda install), gnuplot, Arduino+Teensyduino, PteroDAQ. For the first time in years, everyone got their soldering done within the allotted lab times! I don’t know what was different this year that made it work as it was supposed to. One thing that helped a little was that when students soldered the male headers on the wrong side of the boards (which happens every year, despite all the written, pictorial, and verbal warnings I can give), I had them cut apart the male headers to unsolder, then gave them new male headers to solder on. Software installation is always a problem, because students are attempting to install on a variety of platforms, and something is always incompatible. Here are some problems that came up this year: • Gnuplot on old Mac OS X systems would not install with homebrew, as qt could not be installed—it insisted that you needed the full Xcode (not just the command-line subset), but Apple will not provide the full Xcode for such old, unsupported systems. There may be already compiled versions of gnuplot for these old systems, but I have not gone looking for them. I’m not sure what to recommend to these students, other than updating to a system that Apple still supports. I can’t recommend updating to Catalina, as they will lose all 32-bit apps, some of which will never be ported to newer versions of macos (for example, I lost access to Finale Notepad, which is no longer available on Macs—probably because Apple makes it so very difficult for third-party software developers to maintain their code). • Gnuplot on new Mac OS X systems installs and runs, but produces bad PDF files. The workaround for this was posted at https://stackoverflow.com/questions/57698204/gnuplot-pdf-terminal-exhibits-font-issues-on-mac. The answer to that stackoverflow question had two lines to fix the problem: brew uninstall --ignore-dependencies pango brew install iltommi/brews/pango  The problem is that the default homebrew “bottle” for gnuplot points to a broken version of pango, and you have to remove that version and install an older version. This one I knew about, because it had bitten me when I was trying to finish the book in December. • The standard Arduino+Teensyduino installation fails on macos Catalina (10.15). I knew about this one also, as I had tested the PteroDAQ install after I had “upgraded” to Catalina (which broke nearly everything). The workaround is a beta release of Teensyduino (currently https://forum.pjrc.com/threads/59030-Teensyduino-1-49-Beta-5), which is pointed to from the Teensyduino installation page, but many of the students with macos Catalina did not notice the pointer. One nice feature of the Teensyduino beta release is that it includes the Arduino IDE, so that you don’t need to install Arduino first. • The PteroDAQ GUI seems to be crashing on macos Mojave (10.14)—logging the user out when they try to run “python daq” as usual. I think that this may be a weird filesystem thing, as it seemed to make a difference whether PteroDAQ had been cloned from GitHub or downloaded as a zip file, and whether the files were on the hard drive or on a Google Drive. The only thing that seemed to work consistently was to clone the files from GitHub onto the hard drive of the laptop, then run the app that is in pterodaq/extras/maclauncher/ . Why the launcher app works, when invoking “python daq” directly from a terminal results in logging the user out, is a mystery to me. • On macos Catalina, PteroDAQ can’t find the board if it is connected to the laptop through a USB2 hub connected to a USB3 port (I ran into this problem on my laptop before classes started also). Apple has once again scrambled their USB stack, and USB Serial no longer seems to be visible through a USB2 hub (though the USB device is visible, the serial interface does not seem to be). This hasn’t caused a problem for any students yet, as direct connection to a USB3 port works ok, and connection from hubs on USBC ports seems to be ok (probably because they are USB3 and not USB2). • One student had trouble getting PteroDAQ to run on his Windows 8 machine, with the python program crashing on trying to access the list of ports. No one else with Windows 8 was having trouble, and his machine was running extremely slowly (slower even than the ancient “Barbie” laptop that I used for testing Windows implementations of PteroDAQ), so I suspect he was having hardware or malware problems on his machine. I think that nearly all the students got their software installed—at any rate, people left the lab early on Thursday and were not queuing up for installation help, as they had in previous years. Only one student asked for installation help during Friday office hours (the one with the probably bad hardware or malware), and there were no requests on Piazza for help. If I’m right that everyone got things installed, then we are well ahead of previous years. On Thursday, before lab, one of the group tutors ran a tutorial session on $\LaTeX$, which about half the class attended. That should help somewhat in the prelabs to be turned in on Monday. I’ve changed grading logistics this year. Rather than hiring just 2 graders, I hired 6, and rather than having them work whenever they have time, I have scheduled grading sessions where we get together and grade in the same room. On Thursday, we took just over 2 hours (with 5 graders, counting me) to grade homework 1, and today 6 of us got homework 2 graded in about an hour. I graded the first quiz by myself Wednesday night. So far, we have managed to keep the turn-around time to about a day, which is much better than last year, when the 2 graders were overloaded. Monday’s assignments will be stressing our grading system a bit, as we have a lot coming in, but I couldn’t schedule a grading session until Wednesday at noon. I’m hoping that we can get both homework 3 and the first prelab graded before Thursday’s lab. If necessary, we can just give a turned-in/not-turned-in grade for the first prelab, as they are turning in a more complete draft of the same prelab on Friday, but I’d much rather give them feedback so that they can correct mistakes before the Friday draft. So far, the quiz and homework scores look pretty good, so I’m hopeful that this will be a high-performing class this year. It would be really nice to give out more B+ (and even A-) grades and fewer C grades. I’m keeping my fingers crossed (even though that makes typing the blog post hard). ## 2017 June 25 ### Fidget spinners revisited Filed under: Uncategorized — gasstationwithoutpumps @ 17:55 Tags: , , , In Fidget spinners, I wrote about measuring and modeling the acceleration of two fidget spinners, 5-spoke spinner that cost$6.90 made from plastic and brass and a 3-bladed spinner that cost $8.90 milled out of brass: The 5-spoked wheel spinner weighs 32.88±0.03g, and the 3-spoke brass spinner weighs 61.14±0.02g. The previous post looked only at the fidget spinners spinning vertically (that is, with a horizontal axis), but I had noticed in playing with the spinners that they seemed to have different drag in different orientations, so I redid the measurements with the spinners horizontal (that is, with a vertical axis). I had a somewhat harder time spinning the spinners fast with them horizontally mounted, as my makeshift support for the photointerrupter was a bit precarious. The 5-spoke wheel seemed to run smoothly , but the fit suggests more dry friction and less fluid friction. The 3-spoke spinner really does not like to spin horizontally. To visualize the physics better, I tried making acceleration vs. velocity plots for the fitted models: When holding the wheel horizontally, there seems to be mainly dry friction, almost independent of the speed of the spin. The 3-spoke spinner has much worse drag at all speeds when held horizontally rather than vertically. The fluid drag seems to be about the same as before, but there is much larger dry friction component (possibly from brass-on-brass contact between the spinner and the axle caps). As expected from fidgeting with the spinners, the 3-blade spinner has much more drag than the wheel, both horizontally and vertically. The change from mainly wet friction to mainly dry friction for the wheel was unexpected, though. Update 2017 Jun 25 21:15: My wife just pointed me to a Wired article: https://www.wired.com/2017/05/the-phyiscs-of-fidget-spinners/ which does a poorer job of the same thing I did. They sampled at a fixed rate, rather than recording time stamps on each rising edge, so they had much poorer time resolution, and they assumed constant acceleration (dry friction), which is only appropriate for low-quality bearings. ## 2017 June 20 ### Fidget spinners Filed under: Uncategorized — gasstationwithoutpumps @ 17:34 Tags: , , , I recently bought two fidget spinners from Elecrow: The 5-spoked wheel spinner weighs 32.88±0.03g, and the 3-spoke brass spinner weighs 61.14±0.02g. The heavier 3-bladed spinner cost$8.90 and is milled out of brass (though the site claims “pure copper”, the material looks like brass and is slightly magnetic, so I’m sure it is brass).The lighter 5-spoke spinner cost 6.90. The lighter spinner is easier to get to high speed, spins longer, has more gyroscopic effect, and has a dimple for balancing it on a pencil point, so makes the better fidget spinner in many ways. I was curious whether I could characterize the fidget spinners electronically. I have a photointerrupter (an aligned LED and photodetector) from Sparkfun with a 1cm gap that the spinners just fit in. Here is the 3-spoke spinner mounted in the Panavise Jr, with the photointerrupter counting 6 ticks per revolution. Here is the 5-spoke spinner with the photointerrupter counting 5 ticks per revolution. I set up PteroDAQ to record a timestamp on every rising edge of the photodetector, which counts 5 uniformly spaced ticks per revolution for the 5-spoke wheel, but 6 ticks (in 3 pair of closely spaced ones) for the 3-bladed spinner. I can then plot the angular position of the spinner as a function of time in gnuplot: plot '3-spoke-spin-down-ticks.txt' u 1:(0/6.)


I tried fitting the spin-down using constant deceleration (a quadratic), using deceleration proportional to velocity (exponential decay), and using a model that has both terms: $v_{0}\tau(1-e^{-t/\tau})+a t^2 /2$.  I expressed position as number of turns (that being simpler to interpret than radians), and so the initial velocity $v_{0}$  is in turns/sec, acceleration $a$ is in turns per second per second, and the decay time $\tau$ is in seconds.  I got terrible fits with the constant deceleration, decent fits until the spinning got slow with the exponential decay, and quite a good fit with the combined model:

The longer spin time for the wheel is partly due to higher initial velocity, but the time constant for the decay is also much longer for the wheel, indicating better bearings.

I’m not quite sure how to interpret the slightly higher contact friction term for the 5-spoke wheel.

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