Gas station without pumps

2015 May 19

Pulse monitor lecture and lab

Filed under: Circuits course — gasstationwithoutpumps @ 23:13
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I didn’t get a blog post up for last Friday’s lecture, because I spent the weekend alternately socializing with my in-laws and grading. Monday night was spent grading and working on the next chapter of the book, and most of today (Tuesday) was spent working on the book.

Last Friday was the one day I talked about pulse monitors, photodiodes, phototransistors, and transimpedance amplifiers.  It was a rather packed lecture, but seemed to go all right.

Monday’s lecture started out OK, answering questions about the homework exercises the students were about to turn in, but then fell apart when I tried to cover real power and pulse-width modulation.  Both presentations seemed too vague to me—having neither mathematical rigor nor clear exposition.  The written presentation I wrote for the book is much better than the lecture, which is a bit unusual for me.  I think I need to start getting more sleep, if my presentations have deteriorated to that level.

The prelabs I graded Monday night indicated to me that many students can’t follow a long chain of computations for computing signal levels, even when the chain is broken down into single steps for them.  The problem was estimating how much photocurrent we would get from an LED shining through a finger, starting from the power into the LED, figuring out how much light that would produce, how much would be absorbed by hemoglobin, how much scattered, how much collected by the phototransistor, and how much photocurrent the transistor would produce.  Some students managed to get about 80% of the steps, but a lot got only parts of one step right.

Today’s lab (the first half of the optical pulse monitor lab) took too long, because the students wasted a lot of lab time trying to redo the prelab computations that they had messed up over the weekend, rather than moving on to measuring the photocurrent as I had asked in the lab.  Only a few of the groups got as far as I thought they should have in the first half of the lab: getting their transimpedance amplifiers working and showing a pulse-rate fluctuation. They’ll be set up for adding a high-pass filter and second gain stage on Thursday, and recording pulse waveforms to pass through the digital filter script I provided them.

I spent a long time in the lab today, since I promised the students a 5–7 p.m. lab time for make-up or redone labs. I ended up in the lab from before 10 a.m. to after 7:30 p.m., and I expect to do that every Tuesday and Thursday for the rest of the quarter (2 and a half more weeks). I spent the time when I wasn’t helping students working on revising the class-D power amp chapter of the book, so that I could release the new draft of the book to the class—they’ll have to start work on the design of the class-D amplifier before they have quite finished the pulse monitor, or they won’t have enough time—there is no class next Monday for answering questions.


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