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2017 January 29

Thermistor lab graded

Filed under: Circuits course — gasstationwithoutpumps @ 22:14
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I just spent my entire weekend grading 37 design reports for the thermistor lab—it has not been a fun weekend.  The coming week or two will be grading hell, as I have homework due for the 72-person class Monday, Wednesday, and Friday (with another lab report due next Monday), and no grader or TA.

This lab report was the first of the quarter, so there were a lot more problems with the submissions than I expect to see on future lab reports.  I’ve tried to collect some of my notes on the more common writing errors for this blog post, with the intent of trying to work them into the chapter on lab reports in the textbook:

  • Some students had wordy introductions. I want reports to start with a clear, concise statement of the engineering goal, not a dump of any random factoid that might be vaguely related to the report.
  • Report should be standalone—not referring to homework. If something in the homework is needed, incorporate it!
  • Use paragraphs with one topic each. Every paragraph should start with a topic sentence, and the rest of the paragraph (if there is any) should support and amplify that topic sentence. It is better to have one-sentence paragraphs than to ramble from topic to topic without a paragraph break.
  • Fit your model to your data, not your data to a model. You should never be changing your data to make it fit your theory—you should be changing your theory to fit your data.If you say you are fitting your data to your model, you are claiming to commit scientific fraud.
  • Best-fit curves are not necessarily lines—students don’t have a “line of best fit” in this lab, because the models we’re fitting are nonlinear.
  • Figure captions should be paragraphs below figure, not noun phrases above figure. Any anomalies or interesting features of the figure should be pointed out in the caption.  Most of the crucial content of the report should be in the figures and captions, because that is all 90% of readers ever look at in a science or engineering paper.
  • Refer to figures and equations by number, rather than “schematic below” or “equation above”.
  • Don’t use screenshots for schematics or gnuplot output—export graphics properly as PDF files and incorporate them into the report so that they can be printed at full resolution even when scaled.
  • Many students use way too much passive voice.  Using passive voice is a way to hide who did something or deny responsibility (see Nixon’s “mistakes were made”) and should not be necessary in a design report.
  • Use past tense for things that have been done, not present tense.  Also, “would” is not some formal version of the past tense—it is a marker for the subjunctive mood in English, which has a whole lot of different uses.  In technical writing, the most common use of subjunctive is for “contrary to fact”.  If you say “I would put the thermometer in the water”, I immediately want to know why you don’t—I expect to see the sentence continue with “, but I won’t because …”
  • “Software” is an uncountable noun, which means that it can’t be used with the indefinite article “a”.  There are a lot of uncountable nouns in English, and there isn’t much sense to which words are countable and which aren’t—even closely related languages with similar notions of countable and uncountable nouns mark different nouns as uncountable.  I’ve only found one dictionary that marks countability of English nouns—the Oxford Dictionary of American English, which is available used for very little money.
  • Equations are part of a sentence (as a noun phrase), not random blobs that can be sprinkled anywhere in the paper.  No equation should appear without a textual explanation of its meaning, and the meaning of its variables.
  • There was a lot of misuse of “directly proportional” and “inversely proportional”: A directly proportional relationship plots as a straight line through zero. The voltage output in the thermistor lab is not directly proportional to temperature—it is increasing with temperature, but the function is sigmoidal, not linear.  Similarly, an inversely proportional relationship between x and y is a direct relationship between 1/x and y. It plots as a hyperbola. The resistance of a thermistor is not inversely proportional to temperature, as the resistance is proportional to e^{B/T}  not B/T.
  • Read the data sheet carefully!  A lot of students claimed that their thermometers were good to 150°C, but the data sheet said that the thermistor they were using had a maximum temperature range of  –40°C to 105°C, not 150°C.
  • Students need to use the right metric prefixes.  For example, “kilo” is a lower-case “k” not an upper-case “K”.  This becomes even more urgent for “micro” (µ), “milli” (m), and “mega” (M).  At least one report needs to be redone because the students claimed a value around 200MΩ, when they (probably) meant 200mΩ.  What’s a factor of a billion between friends?
  • Some students are clearly not used to using the prefixes, because I saw a lot of values around 0.0001kΩ, which should have been written 0.1Ω (or even 100mΩ).  Even worse, a lot of students just wrote 0.0001, with no indication what the units were (that triggered a number of “redo” grades on the reports).
  • “Lastly” is not a word—”last” is already an adverb. The same goes for “first”, “second”, and “third”. Perhaps it is easier to keep this in mind if you think of “next”, which is in the same class of words that are both adjectives and adverbs. For some reason, students never write “nextly”.
  • The ×  symbol (\times in LaTeX) is only used for crossproduct, not for scalar multiplication (except in elementary school). The normal way to show scalar multiplication is juxtaposition of the variables, with no operator symbol.
  • “Before” and “after” make no sense in the voltage divider circuit. You can sometimes use those terms in a block diagram that has a clearly directed information flow from inputs to output, but not for talking about the two legs of a voltage divider.

 

 

 

2017 January 7

Book draft 2017 Jan 7

Filed under: Circuits course — gasstationwithoutpumps @ 17:03
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I’ll be releasing an updated version of the Applied Electronics for Bioengineers text on LeanPub today.  I’ll probably raise the minimum price next week, to reflect the improved quality, but I’ll give people a few days to get the book at the old price.  (Remember that the LeanPub model allows you to get all future editions of the book free, as long as I continue publishing through them, so there is no reason to wait until a new edition comes out.)

I’ll list the changes in two sections: changes that were made since the October 2016 release, then changes that were made in the Oct 2016 release (because I don’t seem to have posted those to the blog).

Changes since October 2016

  • Fleshed out assignment schedule and moved to Preface.
  • Rearranged several of the early chapters (without significant content change) for better ordering of assignments.
  • Added mention of Analog Discovery 2 to oscilloscope chapter, replaced some Bitscope traces with Analog Discovery~2 traces.
  • Added bonus frequency response activity to pressure sensor lab.
  • Added Lego-brick pictures for the optical-pulse-monitor lab.
  • Revised all chapters and labs from the microphone chapter to the EKG lab (the second half of the course).  Many of the changes were minor revisions (typo fixes, indexing, changing to numbered exercises, spell check).
  • Added exercises to the microphone chapter and moved some exercises from the microphone lab to the microphone chapter.
  • Moved some of the oscilloscope introduction from the microphone lab to the sampling lab.
  • Rewrote DC analysis of microphone to use function generator, rather than potentiometer, for variable voltage.
  • Added R+L figure to loudspeaker chapter, rather than referring to impedance chapter.
  • Moved inductor description to new chapter just before loudspeakers.
  • Added RMS power exercise and R-L plot exercise to loudspeaker chapter.
  • Moved some intro amplifier material from preamplifier lab to pressure-sensor lab, reflecting change in order of labs.
  • Moved some instructions about color coding wiring from preamplifier to an earlier lab.
  • Added mention of using earbuds instead of loudspeakers for preamplifier lab.
  • Redid Miller plateau oscilloscope trace using Analog Discovery 2, using smaller gate resistor to get higher speed.
  • Added cross-section of a power nFET (still needs to be redrawn)
  • Fixed clipping on several schematics (the Vdd power symbol gets clipped if at the top of the schematic—a known bug in SchemeIt).
  • Put inductive load in the single-nFET driver schematic, including flyback diode.
  • Added explanation of why the crude model for computing slew rate is so far off.
  • Removed most references to obsolete AOI514 nFETs (using NTD4858N nFETs instead).  This required gathering new data to characterize the transistors.
  • Redid the section on open-collector outputs for LM2903 comparators.
  • Added table of conductivity for NaCl solutions.
  • Added section on 4-electrode conductivity measurements.
  • Moved information about nulling ohmmeters when measuring resistance from electrode lab to loudspeaker lab.
  • Reiterated some of the EKG safety info in the EKG lab.

Changes between April 2016 and October 2016

  • Added more background to first chapter (logarithms, picture of complex plane) and started chapter numbering at 1 instead of 0.
  • Rearranged chapters for new lab order, with all the audio labs after the pressure sensor and optical pulse monitor.
  • Updated information on using lead-free solder.
  • Added a generic block diagram to lab report guidelines, and added definition of “port” to the block diagram discussion.
  • Added subsection on Thévenin equivalent of voltage divider.
  • Added section on series and parallel connections to resistance chapter, to reflect lower prerequisite expectations of course.
  • Moved some gnuplot exercises into thermistor lab from sampling and aliasing, to reflect new lab order, also moved PteroDAQ installation instructions.
  • Added picture of metal thermometer to thermistor lab.
  • Added voltmeter connection schematic to DAQ chapter.
  • Moved details of PteroDAQ out of DAQ chapter to separate appendix.
  • Added potentiometer schematic and photo to resistance chapter.
  • Split data acquisition from sampling and aliasing into separate chapters.
  • Improved figure showing aliasing and Nyquist frequency.
  • Added pictures for wire stripping and flying resistors to sampling lab.
  • Added scaffolding for oscilloscope probe exercise.
  • Hysteresis measurement changed to use function generator.
  • Moved multi-stage amplifier discussion to beginning of amplifier chapter and beefed it up.
  • Added introduction to differential amplifiers before instrumentation amps and op amps.
  • Added pH meter block diagram to beginning of amplifier chapter.
  • Moved discussion of clipping to the end of the instrumentation amplifier section.
  • Added active low-pass filters to amplifier chapter.
  • Added chapter on transimpedance amplifiers with section on log-transimpedance amplifiers and rewrote pulse-monitor lab to use logarithmic current-to-voltage conversion.
  • Added discussion of absorbance of melanin, fat, and water to blood section.
  • Moved the instrumentation amplifier internals to new chapter, before the EKG chapter.
  • Simplified the sensitivity calculation for LEDs and phototransistors, making the exercise more productive.
  • Added text to caption of microphone preamp photo.
  • Moved loudness section from the amplifier chapter to the microphone chapter.
  • Added notes at end of loudspeaker lab to improve student reporting of models.
  • Added more safety information to EKG chapter
  • Made all exercises be numbered, and changed most of the prelab questions into numbered exercises.
  • Added equipment-needed lists to the beginning of each lab.
  • Redrew several block diagrams using draw.io, and added captions to several figures to indicate what drawing tool was used.
  • Changed caption formatting to be more distinctly different from body text.
  • Cleaned up several schematics.

2016 November 3

Writing feedback

Filed under: Uncategorized — gasstationwithoutpumps @ 09:15
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In his post Omics! Omics!: 10 Years of Omicing!, which reflects on the influences on his writing, Keith Robison says

The other person who deserves nearly infinite credit for making me think about my word choices is my father.  Sometimes he strays into being a pedant and enforcing rules which have fallen by the wayside, but he did make me think when I spoke and wrote.  I’ve seen some guidelines for helping students that counsel picking only a few major errors to mark, for fear of scarring the psyche of young writers.  Dad didn’t subscribe to that viewpoint in the least, and I’m the better for it.  In high school I treasured getting back a draft with red ink all over it; it’s a service I missed in college and beyond.  That meant he had read it and thought about it, and my work was always better for it.

I think that this attitude is one that we need to see more of, both among students and among faculty. I put a lot of time into trying to provide thorough feedback on student writing, even though I know that it is not always appreciated.  I also know a number of faculty who bemoan the low quality of student writing, but spend almost no time giving detailed feedback so that the students can improve.

There are times for triage—concentrating on the students whose work could benefit most from editing, while providing only minimal feedback to those who produce word salad or whose writing is very good—but I prefer to try to provide similar amounts of feedback for all students.  For the word-salad students, my comments are mainly on sentence structure and paragraph structure, to try to have their writing make sense at least at a local level. The students in the middle get a mixture of different comments from punctuation to overall structure of the paper, while the top students get mainly get comments on trivial little details that can polish their already good writing.

2016 October 8

Release notes for book (Oct 2016)

Filed under: Circuits course — gasstationwithoutpumps @ 17:10
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I’ve just finished doing a rewrite pass over the part of the textbook needed for BME 51A, which I’ll be teaching starting in January. I’ve been working on this rewrite pass since June, so a little over 4 months. I’ll be spending the next couple of months doing a rewrite pass for the part of the book needed for BME 51B, but that should go a little quicker, as there are about half as many pages in the second “half”, and I think they are in somewhat closer to the desired form than the part I just finished.

There are still a lot of “to-do” notes in the margins of the book, even in the part I just “finished”, but they are all fairly small things, I think. This blog post will be my release notes for this version of the book, summarizing what I changed.

The biggest change was a rearrangement of the order of the labs, so that there are now two amplifier labs in the first half, and all the audio labs are in the second half.  Lots of things (like the table of equipment for labs and the schedule of lectures) needed to be revised to fit.  I still have some work to do on the lecture schedule.

I moved the sampling and aliasing lab after the thermistor lab, so that there is more time in lecture to talk about time-varying signals before the lab.

I expanded the  op-amp chapter into a more general amplifier chapter, and now discuss multi-stage amplification from the beginning, because the instrumentation amp lab for the pressure sensor is now the first amplifier lab. I also added active low-pass filters to the op-amp chapter.

I rewrote the optical pulse-monitor lab, which now calls for a more robust design using logarithmic current to voltage conversion.  I’m still experimenting with different ways of holding the phototransistor, so I may need to redo all the photographs, if I come up with a better design. The optical properties of blood section of the optoelectronics chapter now discusses melanin and fat, and the effect they can have on optical pulse monitoring.  I added a new chapter on transimpedance amplifiers, and added log-transimpedance amplifiers to the chapter. The difficult sensitivity analysis for the pulse monitor has been removed, as the log-transimpedance design does not require great care in setting the gain.

I added more coverage of expected background material, so that students who had not had physics electricity and magnetism courses could still follow along.  I found that a lot of the students didn’t remember anything from physics anyway, so I had to cover the essentials over again, and so I reduced the prerequisites for the course to calculus and high-school physics. I also added a section on logarithms.

I added a bunch more figures, bringing the numbered figures up to 145 (and several of those are multi-part figures). I also improved the typesetting of the captions, so that they are better distinguished from the main body text. Several of the block diagrams were redrawn with draw.io, and I added some new block diagrams and a bit more discussion of how to use block diagrams effectively. I cleaned up a few of the schematics also.

I added a few more exercises, added autonumbering, and converted the somewhat vague prelab assignments into numbered exercises, so that I can assign blocks of numbered exercises without worrying that students may have missed part of the prelab assignment. The oscilloscope probe exercise that caused a lot of problems last year has been rewritten with more scaffolding.

I added “equipment-needed” lists to the beginning of each lab.

I changed some of the labs that had used potentiometers to sweep voltages to use function generators with a slow triangle wave instead. This should save quite a bit of time, particularly for the hysteresis lab, where I described how to trigger on the output of a Schmitt trigger changing to record the input thresholds.

I created a new appendix for some of the PteroDAQ details, which I removed from the DAQ chapter.

I added more index terms and fixed a number of glitches in the index.  Index entries with subindexing now stay together in one column, rather than being split between columns and pages.

I’ve started boxing “important” things (and I may change to highlighting them), but choosing the right things to box will probably take another full pass over the book.

I fixed all the overfull-hbox errors through Chapter 24.

 

2016 October 6

Using Google ngrams

Filed under: Uncategorized — gasstationwithoutpumps @ 23:08
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I’ve found https://books.google.com/ngrams to be a very handy tool while writing my book. It has helped me answer such questions as which is the preferred term, “bypass capacitor” or “decoupling capacitor”? “bandpass” or “band-pass”? “passband” or “pass band”?

https://books.google.com/ngrams/interactive_chart?content=bypass+capacitor%2Cdecoupling+capacitor&year_start=1940&year_end=2015&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cbypass%20capacitor%3B%2Cc0%3B.t1%3B%2Cdecoupling%20capacitor%3B%2Cc0

https://books.google.com/ngrams/interactive_chart?content=bandpass%2Cband-pass&case_insensitive=on&year_start=1920&year_end=2008&corpus=15&smoothing=3&share=&direct_url=t4%3B%2Cbandpass%3B%2Cc0%3B%2Cs0%3B%3Bbandpass%3B%2Cc0%3B%3BBandpass%3B%2Cc0%3B%3BBANDPASS%3B%2Cc0%3B.t4%3B%2Cband%20-%20pass%3B%2Cc0%3B%2Cs0%3B%3Bband%20-%20pass%3B%2Cc0%3B%3BBand%20-%20pass%3B%2Cc0%3B%3BBand%20-%20Pass%3B%2Cc0%3B%3BBAND%20-%20PASS%3B%2Cc0

https://books.google.com/ngrams/interactive_chart?content=passband%2C+pass+band&case_insensitive=on&year_start=1920&year_end=2008&corpus=15&smoothing=3&share=&direct_url=t4%3B%2Cpassband%3B%2Cc0%3B%2Cs0%3B%3Bpassband%3B%2Cc0%3B%3BPassband%3B%2Cc0%3B.t4%3B%2Cpass%20band%3B%2Cc0%3B%2Cs0%3B%3Bpass%20band%3B%2Cc0%3B%3BPass%20band%3B%2Cc0%3B%3BPass%20Band%3B%2Cc0%3B%3BPASS%20BAND%3B%2Cc0

I’m a little old-fashioned and will stick with “bypass capacitor”, but prefer “bandpass” and “passband”.

The tool is not always useful—I can follow the rise and decline of “mho”, but “siemens” has too many other uses to be able to determine when the standard name for the unit of conductivity overtook “mho”. It is rather a shame that “mho” did not become the standard, as it is “ohm” backwards and ℧ is such a cute symbol (being Ω upside down).  I am sticking with the standard in my book, though.

(Sigh, the iframes that had the interactive graphics worked fine in the WordPress.com editor, but when the post was published, WordPress stripped them out, leaving only the links.  You’ll have to click through to see the graphics—sorry about that.)

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