Gas station without pumps

2016 March 30

Class topic not what was planned

Filed under: Circuits course — gasstationwithoutpumps @ 21:27
Tags: , , , ,

In my Applied Electronics for Bioengineers course, I had planned to spend the lecture time today talking about sampling and aliasing, but that is not what ended up happening.

I am making it a point to answer student questions (unless they are irrelevant) first, before doing whatever I prepared. The point of the lectures is to help students understand the reading and do the design work for the labs, and anything I have prepared is just a best guess at what the students need. Their questions address more directly what they perceive as their need.  Most of the prepared lecture material is in the book (I wrote the book based on what I have covered in lectures), so answering questions from students who have read the book and are still confused is going to be better than my repeating what is in the book.

Today students had some logistic questions about what to write up for Lab 1 (not much, it was just soldering headers onto the Teensy boards and setting up PteroDAQ—I just asked for a description of what they did, whether anything went wrong, and what they did to fix the problem) and about prelab homework for Lab 2 (do it, but don’t turn it in, it is just setting up gnuplot so that they can use it for the lab).  Those only took a couple of minutes.

The big question that diverted the entire flow of the lecture was a request for an explanation of the high-pass filter in Lab 2 that is used for recentering the function generator output at 1.65V. This lead to several things:

  • Description of block diagrams as functional blocks connected by interfaces, and why this was an important concept in engineering. Frequency and voltage information was put on the block diagram  connections.
  • Capacitor symbol and DC-blocking property of capacitors.
  • Resistor to Vref and why that would cause the output to become Vref, if there was no current through the output.
  • Back to the block diagram to add the constraint that the analog-to-digital converter on the Teensy board couldn’t take any current from its input.
  • Definition of “gain” as \frac{dV_{out}}{dV_{in}}.
  • Showing the high-pass filter Bode plot as two lines meeting at the corner frequency, and giving the corner frequency as \frac{1}{2\pi R C}, without derivation.  I promised the students that we would derive that result in a few weeks, once we’ve had complex impedance.
  • Replacing the resistor to Vref with a pair of resistors to 3.3V and Gnd.
  • Introduction of the triangular ground symbol, and rejection of the chassis ground and earth ground symbols as not relevant for the class.
  • Derivation of the voltage-divider formula from Ohm’s Law, using the important constraint that no current is taken from the output node of the voltage divider, so that the two resistors have identical currents. I had the students help with this, in order to elicit the most common mistake
  • Assertion, without derivation or explanation, that the RC time constant for the high-pass filter should treat the two resistors as being “2R” rather than “R”.

For the last couple of minutes of class, I finally got to do the demo with the homemade stroboscope and pendulum of aliasing, but it was not very effective. Even with the lights off in the classroom, there was enough light through the windows to wash out the strobe. I could not easily keep the pendulum swinging with one hand and adjust the strobe with the other.  If I do this again next year, I should make a panel with about 20 of the LED boards, for around 2.35A during the flash.  At 1.64ms for the longest flash, that’s 3.85mC, which would drain 8.2V from the 470µF capacitor, if the power supply weren’t capable of delivering that much current (but I have a 6A 9V supply, so there should be no problem delivering full power).  Hmm, maybe I should make up that panel for the Mini Maker Faire, instead of the wimpy 4-LED strobe I now have.

I’m actually pleased that I didn’t give the lecture I had planned—my book, which was based on my lectures, already covers the material adequately, and I’d much rather spend precious class time explaining the things that aren’t clear in the book.  The only way I can know what the students need to hear is for them to ask for clarification where they are confused.


  1. The challenge is sometimes that students are too shy (why???) to speak up if they are confused. Somehow they hope that confusion will go away over time (it does, but only sometimes). As you, I rather want to spend time to remove confusion of things already covered than adding new stuff (which as well is covered in the scriptum).

    Comment by Erich Styger — 2016 March 30 @ 23:17 | Reply

  2. “The point of the lectures is to help students understand the reading”. This is not the focus of your post but it is the part I picked up on the most today. I gave a 4 question stats quiz today to my high school seniors. I had not lectured on one of the questions. They were upset that I had not lectured on the question first. I had handed out an example of the question with the steps. I had mentioned it was in the book. (The quiz was open book.) I had mentioned it was solvable with the built in functions in the calculator. They have had their hands held so long that they cannot understand that education is their responsibility, not the teacher’s. “Read the book” does not seem part of most high school math courses. I reinforce reading the book almost every day to no avail. Next year is going to be a really big revelation to many of them. When I mention that most of their college math classes will be 50 minutes of very high speed talking by the prof and very little student/teacher interaction they really do not believe me. When I explain they will be expected to read their math book they think I am nuts. Chuckle. Next year is going to be a revelation.

    Comment by gflint — 2016 March 31 @ 13:45 | Reply

    • Or their math class will have no high-speed talking at all, and start with a problem that applies what they were supposed to have read (or watched) before class. Active learning is slowly penetrating college teaching, especially in math and science.

      Comment by CCPhysicist — 2016 April 2 @ 18:41 | Reply

      • Even more likely—their math class will be a lecture with 600 students, half of whom are watching a recording rather than the live lecture, and they won’t even get to ask questions. The defunding of state universities has resulted in some rather ridiculous class sizes in order to get the “efficiency” that the legislators want while the administrators with huge salaries continue to proliferate (some how “belt-tightening” is always done by students, low-level staff, and faculty, never by administrators).

        I’m lucky in being able to teach an upper-division course with only 48 students in it this quarter (2 lab sections of 24 students each). Next year the class will probably have 66 students (3 sections of 22 students), but I had redesign the course to spread it over two quarters in order to reduce the lab hours per week enough to schedule more than 2 lab sections.

        Comment by gasstationwithoutpumps — 2016 April 2 @ 22:08 | Reply

        • That is unfortunate. If they are going to use recorded lectures, they should just do away with the live version and use a “flipped” model with all of the lectures on video and class time devoted to recitations or a giant open help lab. Of course, that means they are likely at the mercy of grad students, but the “lecturer” would be freed to actually help them learn.

          Comment by CCPhysicist — 2016 April 3 @ 12:05 | Reply

          • At UCSC, the calculus for engineers and physics majors course has gone entirely on line. There are discussion sections for help, but no giant help labs (no space for that). The online lectures work as well as megalectures did (which is to say, not really all that well).

            At UCSB, many of the lower-division math courses (through ordinary differential equations) have class sizes over 500 students. There are a few smaller courses (like an “inquiry-based” version of ODE), but very few students get to take them.

            Comment by gasstationwithoutpumps — 2016 April 3 @ 12:14 | Reply

  3. […] to be learning about was aliasing, which I was planning to cover in lecture yesterday, but I got diverted to other equally important topics. The problem was that the design I gave them could not be implemented, because the resistor […]

    Pingback by Pep talk for students frustrated at the end of the first week | Gas station without pumps — 2016 March 31 @ 20:48 | Reply

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