# Gas station without pumps

## 2018 April 1

### Starting new quarter

Filed under: Circuits course — gasstationwithoutpumps @ 17:20
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Tomorrow the new quarter starts, and I hardly feel like I’ve had a break—I was grading continuously through Monday, working on my book through Friday, and setting up Canvas with all the assignments today. I also spent several hours this week reading course syllabi and student petitions for the Committee on Courses of Instruction meeting that is tomorrow afternoon and a little time reading senior thesis proposals from students wanting to start senior theses next quarter.

It probably doesn’t help that I’m on the third week of a cold—it has settled down to being a minor cough, but it is still making me more tired than usual and interfering with my sleep (the cough is worse when I’m lying down).

There is a new draft of Applied Electronics for Bioengineers posted—those who have already bought the book should have gotten an e-mail about the free update.  The minimum price for the book has gone up by 1¢, as Leanpub now has a $5 minimum, and I had set my book at$4.99 minimum.  There are not many changes this time, because I did not have a lot of writing time last quarter:

• Beefed up the chapter on lab report guidelines.
• Many typo and minor wording fixes (mainly ones reported by students in class).
• Replaced Waveforms 2015 references with Waveforms 3, due to name change by Digilent.
• Added paragraph to thermistor lab about doing design around standard parts.
• Also added specific discussion of using parallel resistors for current sensing in power-amp lab.
• Added the missing section on high-pass active filters.

By being somewhat generous on my grading for the final lab report, I managed to keep the number of failures for Winter quarter down to two—one flaked so much that all five labs had resulted in the partner requesting doing separate lab reports and the other did not do the last lab.  This is about the usual failure rate (2–3%). Still, I was not happy with how many C grades I had to give—I’d like to see a higher distribution Spring quarter.

Several of the students in my course had previously taken EE101 (circuits) or were taking it concurrently.  I checked, and there was essentially no correlation between the EE101 grade and the grade in my course (Kendall tau correlation about -0.03 for not-taking/taking and +0.07 for grade among those who took—neither is a statistically significant relationship with p-value>0.64).

I think that what would make the biggest difference in the grades is better writing from the students—many of the lab reports were a struggle to get through, with poor organization, poor paragraphing, poor grammar, poor word choices, poor punctuation, and poor formatting.  Only about 10% of the class was writing at a level I would consider acceptable for college students, and I’ll be stressing writing more in Spring quarter.

The grading last quarter was stressful, and I expect it will be again this quarter.  Grading at home during finals week resulted in me putting on five pounds—I kept rewarding myself with snacks after getting through another couple of papers.  I’m hoping that I can get the students to write a little better and a little more tersely this quarter, so that the grading will be less stressful (and I can take the weight off again).

I’m hoping that I can convince more of the students to get out of answer-getting and point-scoring mode (see Just scoring points) on the quizzes and rework any problems they get wrong—I’m tired of asking essentially the same question in quiz after quiz and having students still miss it.

## 2017 April 11

### Co-instructor wanted for applied electronics course

Filed under: Circuits course — gasstationwithoutpumps @ 22:04
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I am looking for someone to be a co-instructor with me for the Applied Electronics for Bioengineers course at UCSC next year (January 2018–june 2018).  There are two reasons for my wanting a co-instructor:

• I want to train someone to take the course over from me when I retire (in about 3–4 years).  Right now, I’m the only person who has ever taught the course, and there are no other faculty at UCSC particularly interested in taking it over. My department has no other faculty who know enough electronics, and the EE and CMPE departments are having enough difficulty covering their own courses, so I need to find someone from outside our usual faculty.
• The course is expected to grow to 100 students next year, and I can barely handle the grading load and lab supervision for 70 students this year (71–73 students last quarter, 68 students this quarter).  If I could split the grading load with another person, we would each have a large, but manageable load.  I might be able to hire undergrad graders to do the homework grading, but not the lab reports, which require good feedback on the writing.

I’ll be hiring undergraduates to help answer questions in the lab, as I’ve been doing this year, but I would want the co-instructor to be present for about half the lab sections.  I think that we’ll have 5 sections of 20 students each, so I’d do 2, then we’d do one together, then the co-instructor would do 2.  The total number of hours a week would be about 3.5 attending/giving lectures, 9.5 hours lab, 8 hours grading, for about 21 hours a week.  I don’t know what the pay would be (depends on qualifications on paper), but the pay would probably be meager by engineering standards—I’d guess it would work out to about \$40/hour, though it might be more if the lab time is properly accounted for in the pay rate.

If any of my local readers are interested in the possibility of being a co-instructor, please contact me (karplus@soe.ucsc.edu).  If you know someone who might be interested, please pass on the information.

## 2016 April 4

Filed under: Circuits course — gasstationwithoutpumps @ 22:44
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I spent all day Sunday grading the first set of lab reports.  I was expecting 24 reports of about 3 pages each, but I got 25 averaging about 5 pages each.  I think that the reports were a bit better this year than at corresponding times in previous years, but I did not get my grading done until almost midnight Sunday night, keeping me from getting much else done this weekend.

(I did manage to get my hair cut and to build a new strobe stand with room for 20 of my LED boards, which should give 1800 lumens during the flash. With a duty cycle of only 1/65, I don’t think that I need heat sinks on the boards for the strobe, as the average current should be only 40mA, though the peak current will be about 2.6A.)

In class on Monday, I gave students some group feedback on their writing, plus a couple of $\LaTeX$ pointers, then took questions, some of which were about writing, but most were about the optimization of the fixed resistor in the voltage divider for the resistance-to-voltage converter in the thermistor lab.  I showed them how to set that up, but did not try to solve it in class.

After class, when I was making up the key (redoing all the problems—I don’t like just looking up results—refreshing my memory on how to solve the problems by resolving them is best), I ran into a little trouble doing the optimization. I used to be able to just ask Wolfram Alpha to solve the differential equation, but their newer parser seems to be much harder to convince to do anything.  I eventually gave up and used a cruder tool to just take the second derivative and solved for the resistance by hand.  That was faster than the time I wasted trying to get Wolfram Alpha to do anything useful.  (I suspect that they have deliberately crippled it, to make people pay for Mathematica.)

Monday afternoon and evening (from about 1:30 to 7:45) was spent grading the first pre-lab homework.  Again the results are a little better than previous years, but there were 9 prelabs fewer than I expected (3 students have dropped already and 6 did not do the prelab).  I hope that those who did not do the prelab were just confused about when it was due, and not starting a trend towards coming to class and lab unprepared. I also hope that no more students drop—this class is not a weed-out class, though it is a lot of work.

Back in January, Mike wanted to know where I ended up doing my grading. Sunday I did my grading in my breakfast room, with the laptop on the floor where I could get to it if I really needed to look something up, but where it was not a constant temptation to goof off.  On Monday, I worked in my office on campus, where the e-mail was a minor distraction that I checked between problems.  (For the prelabs, I graded the entire stack for problem 1, then the entire stack for problem 2, and so forth.  This makes for more consistent and faster grading than grading a student at a time, but it would be faster still if the students didn’t put their answers in random order on what they turned in.) I’ll probably continue with weekend grading in the breakfast room and prelab grading in my office until the distractions get to be too much—then I’ll look for a coffeeshop to grade in.

## 2015 April 28

### First half of electrode lab a bit long

Filed under: Circuits course — gasstationwithoutpumps @ 20:47
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Monday’s lecture went fairly well—I used my post  Comments for class after grading as lecture notes, and pretty much covered everything, though not necessarily in the order presented there.

Today I spent a long time in the lab, from about 9 a.m. to after 6 p.m., because it takes a fair amount of time to set up and clean up when we are dealing with liquids (in this case, salt water) in the electronics lab.  I have to make sure that everything is in secondary containment tubs, so that nothing gets spilled.  (It irks me that the EE faculty don’t bother enforcing the clearly posted “no food or drink” rule on their students, and I’ve had to chide several EE students coming into the lab with cups of coffee and open bowls of food—I often see drink containers in the room trash.  I spend hours making sure that my students don’t spill anything, but the EE students routinely spill their drinks (judging from the mess on the never-cleaned floor.)

I did two demos today: one planned, one unplanned.  The planned demo was of vernier calipers, which the students used to measure their stainless steel electrodes.  The unplanned demo was of what happens if you pass a large current through a salt solution.  I considered that grade-school chemistry (I’m sure I was in grade school when I took two carbon rods from inside batteries and passed a current through them, measuring the amount of H2 and O2 that bubbled off—I even looked at the difference between AC and DC (initially by using Al foil on a turntable to do the switching, but 33rpm (0.55 Hz) was still too high a frequency to get any electrolysis, and I had to switch to a DPDT switch and a watch to manually get something like 0.1Hz to get small amounts of electrolysis. OK, I admit that was a science-fair project (6th grade? 7th?) to measure the amount of electrolysis as a function of frequency, but electrolysis was not a strange subject for middle school students.

No one in the morning class had any idea what would happen if you passed a current through a salt solution, and I couldn’t even get guesses.  In the afternoon class, I badgered the students a bit more and finally got someone to realize that H2 would bubble out, and (after a bit more badgering) got them to predict which electrode this would happen on. With 6V and a 1A limit, I got vigorous bubbling (about 0.6A current drawn), and the other electrode produced a yellowish color in the solution (probably an iron oxide).

Note: all but one or two of these students have taken at least a year of college chemistry that included a full quarter on electrochemistry, but they had never seen a demo of the H2 reaction, despite it’s being the standard reaction of defining half-cell potentials. (Most of them had seen the reaction before, since it happens in gel electrophoresis boxes all the time and nearly all of them have done gel electrophoresis in biochem labs, but not one of them put 2 and 2 together and realized what was going on.)

What prompted the electrolysis demo was students asking why the readings kept changing on their ohmmeters when they tried to measure DC resistance. I tried through socratic questioning to get them to realize that the ohmmeters work by measuring the voltage across the device under test while passing a known current through, and that passing a DC current through an electrode will result in chemistry on the surface of the electrode, changing the electrode properties. I got some groups as far as realizing that there was a current, but no one seemed to realize that having a current meant there must be redox reactions taking place on the surfaces of the electrodes, changing the surface properties.

Did I already mention that almost all of these students have had a quarter of electrochemistry? I wonder what (if anything) is taught in that class! I’ve never taken it, so I’m relying entirely on what I learned in grade school and high-school—I would have thought that a college-level class would have more than what I recall from a high-school sophomore class in the 60s.

The rest of the lab went fairly smoothly, but a number of students saw a change in the behavior of their electrodes at high frequencies. This was unexpected (I’d not see the effect in my versions of the experiments), so I spent some time debugging the problem. I’m pretty sure that the problem was long wires—the students were getting a series inductance added to their electrodes. About 1.5µH would be enough in some cases to cause the observed phenomenon, though in other cases a much larger inductance would seem indicated. For one student, I suggested hooking up a voltmeter right at the electrodes rather than at the other ends of the wires to the electrodes—the saw a 2-fold reduction in voltage at 1MHz, which pretty much cancelled their apparent increase in impedance. We’ll discuss the problem in class tomorrow, and I’ll suggest modeling the electrodes not with just the standard R+(R||C) model but with L+R+(R||C), with the extra L corresponding to the long wires in their test setup.

We used up about half the salt solutions a colleague had made for me, and we’ll use up the other half on Thursday. It seems we need at least 110ml/student, so next year I’ll probably want to get 150ml/student or even 200ml/student, so that we don’t run out. Today we characterized stainless steel electrodes (which are highly polarizable) and on Thursday we’ll characterize Ag/AgCl electrodes (which are non-polarizable). So I’ll have another long day in the lab on Thursday.

## 2015 April 26

Friday’s lecture went fairly well.

There were a few questions at the beginning of class, one of which lent itself well to my talking about choosing different models for the same phenomenon and using the simplest model that worked for the design being done.  In this case it was about the relaxation oscillator using a 74HC14N Schmitt trigger and where the constraints on the feedback resistor came from.  I told them about some more detailed models we could do of the Schmitt trigger, including input capacitance (max value on the data sheet), input leakage current (not specified, but probably fairly small, under 1µA), and output resistance (which would get added to the feedback resistance).  I’ll have to incorporate some of those ideas into the book, when I rewrite those chapters this summer—the hysteresis lab needs the most rework of anything so far this quarter.

After the questions I mainly talked about polarizable and non-polarizable electrodes developing the R +  (R||C) + half-cell model of an electrode that they will be fitting (without the half cell) in labs this week.

This weekend’s grading was a bit painful, and I’m probably going to have to spend all of Monday’s lecture filling in gaps in their prior education that I had not anticipated.  Some holes also became apparent from e-mail questions I got from students over the weekend.

I’ll try to gather the common problems here, so that I can use the list as lecture notes tomorrow.

• There were a lot of REDO grades for errors on schematics.  I hate giving REDO (since it doubles my grading load), but I told students at the beginning of the quarter that any error on the schematics was an automatic REDO.  I plan to stick to that, despite the pain for both me and the students, because they have to develop the habit of double and triple checking their non-redundant documents (schematics, PCR primers, …).  Sloppy documentation is a serious problem in engineering and too many faculty and graders have been perpetuating the myth that the almost right idea is good enough.  I’m particularly harsh on students who change kHz into Hz or pF into nF.  Off-by-a-factor-of-1000 is not good enough!  The most extreme case so far is someone who specified a capacitor as being in the gigafarads (they’d typed 109 instead of 10-9). A factor of 1,000,000,000,000,000,000 off is not the sort of thing one can ignore.  I also get annoyed by students who randomly pick a unit (H when they need Ω, or Ω when they need Hz), as if all units were just decorations to please a teacher, with no real meaning to them
• Frequency is 1/period.  For the relaxation oscillator, they do two charge/discharge calculations to get the period as a multiple of RC (though many blindly copied one of the formulas for just the charge time without understanding it, and assumed it was the period). But even after computing the charging time students blindly used  2πf = 1/(RC) as a magic incantation.  That formula was relevant for the corner frequency of RC filters, but has nothing to do with the oscillation frequency of the relaxation oscillator.
• The capacitance calculation being done in the prelab was for the capacitance of a finger touch to the touch plate, but a lot of students claimed that it was the calculation to determine the size of the ceramic capacitor.  Only a couple of groups bothered to explain the connection between the two capacitances. I think I need to rewrite the prompts in the book to force the values to be more different, so that students have to think which capacitance they are talking about.
• I find that students often talk about “the voltage” or “the capacitance” as if there was only one in their circuit, and when asked which one they are talking about are completely mystified—to them invoking the magic word is all that can be expected of them—actually knowing what it refers to is unreasonable.
• Students in general were doing too much ritual magic. They would put down a formula they thought was relevant (often copying it incorrectly), then claim that from that formula they got some number for their design.  Often the formula was not relevant, or additional assumptions needed to be made (like choosing arbitrary values for some variables).  At the very least, there was some substantial algebra to be done to convert the formula into a usable form.  Some students claimed that Wolfram alpha gave them the solution (when there was not enough information to solve for the variable they wanted a value for).  Basically, I’m a bit angry at the students for trying to bullshit their way through the assignment. One pair of students said quite honestly that they did not know how to do a computation and got the value they used from the students at the next bench.  I gave them bonus points, and I’ll help them figure out how to do the computation they were having trouble with—I have no problems with students not knowing how to do something new and somewhat tricky, but I do have trouble with students deliberately looking dishonest and stupid by writing bullshit.
• The computation that the honest students had trouble with is one that many students had trouble with, so I’ll go over it in class.  I gave the students a derivation of a formula for the charging time of the capacitor in the relaxation oscillator, but I didn’t have time to step them through the derivation.  It seems like most of the class can’t read math, since many just copied the final formula without reading the text that said it was the time to charge the capacitor.  There was an exercise immediately afterwards asking students to compute the time to discharge the capacitor, but this exercise was added to the book after the students had done their prelab exercises, so they didn’t bother to look at the exercise. What they needed to do for the lab was to add the charge and discharge times (which are not quite the same) to get the period.
• I need to remind the students that they are turning in design reports, not lab reports.  I’m not looking for fill-in-the-blank worksheets, but descriptions of how they designed and tested their circuits.  Omitting the design steps is omitting the most important part of the report!
• I gave the students three models to fit to the data, and showed them how to do the fits for two of the models in Wednesday’s lecture.  There wasn’t time to get to the third model, so I just told them to use the same technique as the second model, but with the different formula.  Most of the class never bothered to fit the third model (the only one that really fits the data well)—if I didn’t do all the work for them in lecture, then they weren’t going to generalize even a tiny bit to do it themselves.
• A lot of students did not do a good job of fitting the models, because they fit the data with linear scaling, rather than with log scaling as I had shown them.  This is a fairly subtle point (errors on a linear y axis are differences, but on a log y axis are ratios), so I’ll review it in class.
• I  think that some students don’t have any idea when one would use a log-log plot, a log-linear plot, a linear-log plot, or a linear-linear plot.  I thought that was covered in precalculus, but I guess not. So tomorrow I’ll present the idea that the only curve most people understand visually is a straight line, so one wants to choose axis scaling so that the expected relationship is a straight line.  Linear plots are for linear (or affine) models, log-log plots are for power laws, log-linear are for exponentials, and linear-log are for logarithmic relationships.  I’ll put a general straight line on each and derive the form of the function that matches that straight line.
• The purpose of the Tuesday lab was to collect data and model the loudspeaker with a few parameters.  But many students neglected to report those parameters in their design reports!  They produced a plot and fitted models to it, but nowhere on the plot, in the figure caption, or in the main body (in decreasing order of usefulness) did they report what the parameter values were that the fit produced.  For students who are so focussed on answer getting that they neglect to explain how they came up with their answers, this seems like a strange omission.
• For the Thursday lab, no one did back calculations from their observed frequencies to estimate the capacitance of the 74HC14N input, of the untouched touch plate, or even of the touch itself, to see whether their observations were consistent with their design predictions. One group of students claimed to have done sanity checks, but I don’t believe them, as they also reported oscillations around 20Hz, instead of 20kHz.
• For the prelab, it seems that a lot of students computed $R + \omega L$ instead of $| R + j \omega L|$, though most got it right in the gnuplot scripts for the lab itself.  I have to remind students that $|A+B| \neq |A|+|B|$.
• On the typesetting front, I’m making some progress on getting students to put their plots in as figures with captions, though way too many are still referring to “the plot below” rather than to “Figure 3”.  I’m also having some difficulty getting them to be sure to refer to all the figures in the main body text.  A lot of times they’ll toss in a handful of plots with no reference to them at all.
• On the opposite side of the coin, I have to teach them that equations are properly part of a sentence, generally as a noun phrase, and are not standalone sentences.  When there is an explanation of variables after a formula (“where A is this, and B is that”), the where-clauses are still part of the same sentence.
• Some other little things to tell them:
• The word “significant” should be reserved for its technical meaning of “statistical significance”—very unlikely to have occurred by chance according to the specified null model. It should not be used in the normal English way to mean “big”, “important”, or “something I like”.
• To get gnuplot to produce smooth curves when there are sharp changes in function, it is necessary to do set samples 3000 to compute the function at more points than the small default number.
• Students have been misusing the word “shunt” for any resistor. Properly, it is a low resistance used to divert current from some other part of the circuit—in our designs, it is the resistor being used to sense current and change it into voltage. I wonder if I should switch terms and talk about a “sense” resistor, though “shunt” is the standard term for ammeters.
• A minor pet peeve of mine: I hate the word “utilize”. I have yet to see a context in which “use” does not do the same job better.
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