Gas station without pumps

2015 April 16

Microphone labs went OK

Filed under: Circuits course — gasstationwithoutpumps @ 21:19
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The microphone labs this week went OK (much better in the afternoon session than the morning session).  On Tuesday the students collected data for an I-vs-V plot of the electret microphone, both using a power supply and a pair of voltmeters and using just the PteroDAQ system on the KL25Z board (see New modeling lab for electret microphone for the basics of the PteroDAQ portion of the lab).  Most groups got all the data they needed on Tuesday, but some were still struggling with it on Thursday.

On Wednesday, I went over the model fitting in gnuplot used to produce the fits for the 4 models I gave them: linear resistance, constant current, a “blended model” that is asymptotically the same but switches smoothly between them, and one that adds one more parameter to get a slope for the saturation region(again, see New modeling lab for electret microphone for the models) .  Some students in the class asked for a copy of the script I developed in class, but I turned them down—the goal is to get them to develop their own scripts, and they had examples of most of what I did from the previous week’s class, which included a worked example.

In previous years, I used to give out all the things developed in class as worked examples, but I’ve stopped doing that, because far too many students just blindly copied and ran scripts without understanding them (often not even changing the parameter values that were different between what I did in the example and the data they collected).  One point of this class is to get students to stop thinking of class work as ritual magic that just requires doing the right memorized or copied procedure, and to get them to think in terms of creating new stuff from building blocks that they understand. It is unfortunate that most of the chemistry and physics labs they have had are exactly that sort of ritual magic, with a worksheet telling them exactly what steps to perform and what results to fill in the blanks with.

They need to learn to generalize from worked examples, read documentation, and figure out how to make things work. I’m always happy to help them debug their efforts, but not to do the work for them.  I’m not in the class to get right answers from the students, but to get them to learn how to design and debug—to ask their own questions and figure out how to answer them. (See Answer getting for more on what change I’m trying to invoke.)  Some of the top students are already on board with the concept and are working hard to understand electronics, modeling, graphing, debugging, and so forth , but a number of the students are still stuck in the “tell me exactly what to memorize and what steps to do” state that seems characteristic of many high school students.  The biology classes they’ve had have not done much to move them away from that—there is far too much memorization without understanding going on in the bio courses.

In addition to the model fitting, I answered questions from the homework—mainly about the calculation of the output voltage from the mic for a given input sound level. I reviewed decibels again and went through the various scaling operations needed to get the output in µV RMS from sound pressure level.  I think that the top students got it, but I’m also pretty sure I lost the bottom ones—it’s a little hard to tell as they all work so hard on looking intelligent that I have a hard time telling when they are lost and when they are bored because I’m going too slow.

Today’s lab was a little bit of design (choosing a load resistor based on the I-vs-V curve, and designing a high-pass filter to block the DC) and practice using the oscilloscope. I did something different this year, having them design their high-pass filter with the input impedance of the oscilloscope as the “R” of the RC filter. In part this was to get them to realize that connecting a test instrument like an oscilloscope or voltmeter always changes the circuit that is being tested.  I even had them design around the 10MΩ input impedance of the 10× scope probe—so the 10MΩ impedance was not “almost infinite” the way many intro electronics courses teach it.  I don’t think I made a clear enough point of this message—neither in the book nor in the lab sections, so I’d better mention it in class tomorrow also.

Unfortunately, some of the material from the old lab (where they used bigger capacitors and smaller resistors for the high-pass filter) was not edited out in making the changes, so the lab chapter was rather confusing.  I’ve left some notes to myself to update the chapter, but I probably won’t have time until this summer.

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