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2012 October 24

Thinking about PC boards and parts kits for circuits lab

On my way to work today, I was thinking a bit about redesigning the PC boards for the circuit lab.  I have 2 boards designed so far: an instrumentation amp protoboard and pressure-sensor breakout board.

I have to redesign the pressure-sensor board for two reasons: 1) it is mis-wired, so the screw terminals are labeled wrong (see Pressure sensor miswired), and 2) I’m going to change to a pressure sensor with a built-in barbed port, so that hoses can be directly connected (see Rethinking the pressure sensor lab). The new choice of pressure sensor has a different mounting

We’ll need to decide whether to make the pressure sensors lab equipment (which we’d have to check out and keep track of) or put them in the student parts kit for the course.  If I put them in the part kit, then the students can solder the breakout boards themselves, but it adds the cost of the sensor, the board, nuts and screws, a capacitor, and the screw terminals (about $13) to the parts kit.

The instrumentation amp protoboard is functional as it is (I wired up the instrumentation amp for the pressure sensor on it), but I’m not real happy with the design.  The core is ok, but I think now that the space for a barrel jack for a wall-wart is wasted space. Wall wart power is too low-quality for analog work, and I don’t want to put a regulator on the board. The students have a fine bench supply (Agilent E3631A), so we might as well use it.

I need to have a 4-pin connector for connecting to the pressure sensor (3 of which would be used for the EKG), 2 pins for connecting to the power supply, and 3 or 4 for connecting to the Arduino (Gnd, analog out, and Aref at least). I think that I’ll want to have more Vdd and GND points on the board, as the routing for those was trickier than I would have liked. It might be good to have a well-separated pin for connecting an oscilloscope ground. I’d like to add spaces for connecting up 1 or 2 transistors and another dual-op amp chip.  These changes would almost certainly increase the size of the board, raising the per board price from $1.40 to $2.60 (unless I buy many).  I need to think about what circuits we’ll have the students solder (versus building on a breadboard).  Currently, I’m leaning toward having them solder the EKG, the pressure-sensor amp, and the capacitive touch sensor.  The simple op-amp audio amp should be breadboarded, but I’m not sure about the variant with an output transistor for more power.  This means that each lab group would need 3 or 4 boards for the quarter, which adds another $5–10 for boards.  Screw terminals for connecting power and such to those boards adds another $6. Breadboarding is certainly cheaper, especially since the op amps and other parts could be reused from one project to the next.

I’m also leaning toward getting each student a large collection of resistors (like this collection of 10 each of 112 values for $12.90) and a smaller collection of capacitors.  I’ve not been able to find a cheap assortment of capacitors in different values (other than surplus assortments of random values, which is not of much use to us), so we may have to pick a small number of useful values, and buy the parts separately.

It looks like the parts and boards for the student kit including everything will come to far more than the $43 lab fee currently being charged to students in the usual circuits lab (which uses far fewer parts, and those mostly very cheap ones). If we require Arduinos as well, we’ll certainly far exceed that price. I don’t know if we’ll even be allowed to charge a lab fee.  The page about the current fees says “The fees shown below have been established using prior course history and have been approved by the Dean of Engineering and reviewed by the UCSC Student Fees Committee. The UCSC Miscellaneous Fees Advisory Committee recommended adoption of these fees and final approval was made by the Chancellor.”  I doubt we’ll have time to figure out all the parts we need in order to set a fee in time for a committee to meet and approve fees.  We may have to do direct sales of kits and parts to students, if that is permitted. It is going to be difficult even to get a parts list together in time to buy the parts, much less to figure out the prices, set the fee appropriately, and get approval for the fee.


2012 October 23

Rethinking the pressure sensor lab

I’ve had several posts now relating to building a shaker table and putting together a pressure sensor lab project for the circuits course:

  1. Pressure sensing lab possibilities
  2. PC board for pressure sensor
  3. Characterizing tactile transducer
  4. Characterizing tactile transducer again
  5. New amplifier and shaker table
  6. Good and bad news for circuit course
  7. Pressure sensor assembly
  8. Pressure sensor miswired
  9. Pressure sensor noise problems

I’m beginning to think that this lab, as I originally envisioned it, is both too much work to set up and too much work for the students. It was also beginning to look like a major spill hazard (much more so than the thermistor lab or the electrode characterization lab).

I want to back off now and see whether there is a lab that fits better into the course and is less trouble both for me and for the students.  Let’s look at the different parts of the lab, and see which are the most important—discarding the parts that are more trouble than they are worth.

  • Building an audio amplifier (op amp plus one discrete transistor) to drive shaker table.
  • Building an instrumentation amplifier with gain in the range 500–2000 to read strain-gauge bridge pressure sensor.
  • Calibrating pressure sensor with a water column.
  • Inducing pressure waves in water with shaker table, and measuring with pressure sensor.
  • Making measurements at two ends of a flexible hose to try to characterize water in hose using the hydraulic analogy.

I like the idea of having students build an audio amplifier.  In fact, we were planning a simple amplifier in an earlier lab, so extending it to drive more current than the op amp chip can source is a good one.  But we don’t need to build a shaker table for that—we can buy cheap 4Ω or 8Ω speakers and have them build amplifiers for the speakers.

I definitely like the idea of having the students learn about strain gauges and build an instrumentation amplifier for them.  The $5 MPX2300DT1 pressure sensor is a good example of a strain-gauge bridge (with temperature compensation).  We could go with the uncompensated MPX53DP for $7.80, the $8 MPXV53GC7U or the $11 temperature compensated MPX2053DP.  I rather like the sturdier “unibody” packaging for the differential pressure sensors (the DP suffix), and we could attach a hose to them directly, since they have barbed ports (which look like they are designed for 3/16″ ID tubing).  I’d still want a breakout board with screw terminals for the sensor, but assembling it would be easier, since the sensor can be soldered as a through-hole component and  screwed to the PC board, eliminating the gluing I needed for the MPX2300DT1.

I’m currently leaning towards a simpler (and cheaper) setup—eliminating the shaker table, the ¾” PVC plug, and the PVC water reservoir, and just having an MPX2053DP (or even MPX53DP) pressure sensor on a breakout board.  This would discard the hydraulic analogy part of the lab, but students would still build an instrumentation amplifier, characterize the pressure with a water column (easily measured as the height of water in clear tubing), and use the pressure sensor to measure breath pressure (inhalation and exhalation).

The maximum pressure of human breath is about 25kPa or 100″ H2O, so the ±50kPa range of the differential sensor should be plenty. The MPX2053 sensor is spec’ed at 800µV/kPa with a 10V power supply, so with a 5V supply it would provide 400µV/kPa.  We probably want a 0–5V output for a -25kPa to +25kPa input, so an amplifier gain of 250 is called for.  That’s a bit less touchy than the gain of 1000 I  used with the MPX2300DT1, but will still be good warmup for the EKG amplifier (which needs higher gain and has to use two stages to avoid saturating from small DC offsets in the first stage).

The uncompensated MPX53DP is spec’ed at 1.2mV/kPa at 3v (2mV/kPa at 5V), so less gain would be needed for the uncompensated part.  If you don’t need temperature correction, then the cheaper part gives you greater sensitivity. I’ll have to think about which would be pedagogically more useful—currently I lean towards the temperature-compensated part, as a concept that they should learn and because it forces them to make a higher gain amplifier.

Building the instrumentation amp and making breath pressure measurements should only take one 3-hour lab period, rather than two, so if I go with this design, I’ll need to come up with another lab.  Perhaps a second audio amplifier lab, with an output transistor and some filtering would be a good lab to insert I have to decide whether that should be a soldering lab or a breadboard lab.  I think that the two instrumentation labs (pressure sensor and EKG) should be done by soldering on a PC board, but I’m not sure the instrumentation amps should be their first soldering projects.

Chapter 16 homework

Filed under: home school — gasstationwithoutpumps @ 08:09
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I’m so far behind on everything that I did not get time yet this week to finish reading Chapter 16 of Matter and Interactions, though my son and I are supposed to be going over it (and maybe doing a lab) today.  My son did not feel like choosing the problems, so my wife arbitrarily assigned us some, based only on getting a reasonable number of problems from each section—she didn’t read the  chapter and may not even have read the problems.

She assigned us 16P22, 16P23, 16P38, 16P43, 16P46, 16P47, 16P48, 16P50, 16P59, 16P61, 16P64,16P65, and 16P66.  I’d better do a few of them for today.

I’ve found it rather difficult to come up with meaningful labs for the electrostatics chapters.  The Scotch tape labs suggested in Chapter 15 were fun, but didn’t really contribute much to understanding the material.  I can see now why some physics teachers prefer a “current-first” approach to electricity, getting fields later.  Certainly circuits seem to me to be much simpler and easier to experiment with than fields.

Tomorrow I’ll be attending the “Global Physics Department” meeting on the web, where they’ll be brainstorming about teaching DC circuits.  This should be useful for me not only for our homeschool physics class, but also for finding out about misconceptions I can expect from students in the circuits course I’ll be co-teaching next quarter.

2012 October 22

Where does a BS lead to a STEM PhD?

Filed under: Uncategorized — gasstationwithoutpumps @ 21:44
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Earlier this month, Lynn O’Shaugnessy published a claim that liberal arts colleges have an edge in producing bachelor’s students who go on to earn science and engineering PhDs: 50 Schools That Produce the Most Science and Engineering PhD’s | The College Solution.

Her title is a bit misleading, as the NSF figures from 2008 she was basing her claim on looked at ratios: number of PhDs of alumni divided by number of alumni from 9 years earlier. The raw numbers of PhDs who got their bachelor’s degrees from liberal arts colleges remains quite small—it is the concentration of students who go on to grad school and finish PhDs that is high. In terms of raw numbers, public schools dominate, with only 6 of the top 25 schools being private (and that’s counting Cornell, which is both a private and a public university, depending which department you are talking about). Her statement “The liberal arts dominance on this PhD list is even more impressive when you consider that just 2% to 3% of students attending four-year higher-ed institutions are enrolled at liberal arts colleges,” is particularly misleading—reducing the denominator makes it easier, not harder, to get a high ratio.

Still, if you are looking to send a kid on to grad school, it is probably best for them to do their undergrad work at a college where lots of people are preparing for grad school, rather than one where people are preparing to leave school as soon as possible. From that standpoint, the “Oberlin 50” liberal arts schools look pretty good, unless you look specifically at engineering PhDs—then the research universities, particularly the private universities with high research activity, really dominate. The top liberal arts school prepare students well for natural sciences and social/behavioral sciences, but not (apparently) for engineering.

It might be interesting to normalize for number of STEM bachelor’s degrees, rather than total bachelor’s degrees. Changing the normalization would probably reduce the ratios for the three top-yielding schools: Caltech, Harvey Mudd, and MIT, but might pull up the ratios for some of the larger universities, which have relatively smaller portions of their students studying science and engineering.

Three schools come up in the top 20 on both the raw numbers and the ratio of alumni getting PhD to total alumni: MIT, Harvard, and Stanford.

Lynn’s message is also somewhat misleading, as she was reassuring a mother whose child went to St. Mary’s College of Maryland (which does not appear in the top 50 list and is not one of the Oberlin 50 elite liberal arts colleges). Overall, private baccalaureate colleges do about as well as public research universities with very high research activities at sending their students on for science PhDs, but they have astonishingly low rates of sending their students on for engineering PhDs.

If you want your kid to go to grad school in engineering, then a top engineering school with high research activity (private perhaps better than public, if you can afford it) seems like the best bet. For science, one of the Oberlin 50 looks good, if you can afford them. Of course, all these are contingent on getting in, which tends to be easier at the large public universities.

2012 October 20

Are AP classes worthwhile?

Filed under: home school — gasstationwithoutpumps @ 15:56
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In The Atlantic, John Tierney wrote an inflammatory piece: AP Classes Are a Scam, in which he accuses someone of perpetuating a scam on high school students (it is not clear who he is accusing).  He also seems to be unaware of the distinction between AP courses and the AP exams, conflating the two in a confusing way.

He claims that AP courses are nowhere close to being an equivalent of first-year college courses, that AP exams are no longer being accepted for college credit, that AP courses have been diluted by open-admission policies, that AP courses increase discrimination against minorities, that offering AP courses takes resources away from other students, and that AP courses are overly rigid, teach-to-the-test courses.

One of the best rebuttals I’ve seen is by Michael Ralph, a high school biology teacher. Michael addresses each point in turn, providing well-written counter-arguments. Unfortunately, he has no more data to support his rebuttal than John Tierney had for his polemic.  I don’t have any data either, so don’t expect this blog post to be any better.

As a home school parent, I see the Advanced Placement tests in quite a different light from either John Tierney or Michael Ralph.

First, I do not confuse the tests with the courses. My son has taken two AP tests so far and will take another two or three this year, and possibly some next year as well, but I don’t think that he’ll ever take an AP course.  Note that to legally call something an “AP” course, the teacher must have submitted a syllabus to the College Board and had their staff approve it.  There are many fine courses that have not bothered with this bit of bureaucracy, but which still teach material that is tested on the AP exams.  On his home-school transcript, we are likely to list courses with names like “Calculus with AP BC exam” rather than “AP Calculus BC” to avoid infringing on the trademark.

Second, we don’t use AP courses to get bonus points for our school or to pump up a GPA. My son takes AP exams to show that he has indeed learned the material that is normally covered in an AP course, even though he has learned it through a different mechanism.  He is not padding his transcript with large numbers of AP courses to look good—he learns what he needs or wants to learn, and takes AP exams to show he has learned it (if there is an exam that covers the material).  For us, the AP exams provide external validation of his education at a level that is appropriate for what he is learning.

I’ll try to address Tierney’s points one by one:

  • It is trivial to say that many AP courses do not correspond with first year college courses—first-year college courses vary so much that no course could possibly correspond to all variants.  Even within one medium-sized campus (like UCSC) there may be three different first-year physics sequences and 4 or 5 different calculus sequences.  The interesting question is which of these courses the AP courses and exams are trying to mimic. It would take a pretty big study to determine what the range of AP courses is and what the range of first-year college courses is, and whether the distribution of course quality is significantly different.  I suspect that the AP courses on average are lower quality than the courses at the best liberal-arts colleges, and higher quality than the average at community colleges (and even some R1 universities that have oversize freshman lecture classes), but I have no data whatsoever for this belief, and no way to collect such data.
  • AP credit does seem to be less common that it used to be, but we’ve never looked at AP exams as a way to save money (though undoubtedly many people do).  For us, they are a way to get our son past the enormous lecture classes that dominate freshman year at many colleges, and into the interesting “boutique” classes that follow. What we want is the “placement” part of Advanced Placement, not credit for high school work to shorten college.  If people do want credit for AP exams, there are still a lot of colleges that offer it. (Disclaimer: I graduated from an affluent suburban high school in 1971 with enough AP credits to get my B.S. in three years.)
  • Dilution of AP courses by open-admission policies does strike me as likely to be occurring, based on anecdotes from AP teachers who see their class sizes growing to over 40 students in a section, with many of the students not really ready for college-level material.
    This swelling of the AP sections is driven largely by mindless rankings of high schools by how many AP exams their students take (independent of whether the students do well).  Like the push for all 8th graders to take algebra, people are confusing correlation and causation.  When only the top students took algebra in 8th grade or took AP exams, there was a very high correlation between AP exams or 8th grade algebra and doing well in college.  But the correlation was because these were the brightest, most dedicated students.  The AP exams did not cause their higher performance in college, and offering the AP courses to less dedicated, less bright students does not improve their college performance much.
  • The lack of racial diversity in AP courses comes from the uneven distribution of education and the high correlation between race and economic class in the USA.  Schools in rich neighborhoods are more likely to offer all kinds of courses that are beyond the basics (AP courses, art, music, electives in the humanities, …), and rich neighborhoods are still overwhelmingly white in the USA.  It is almost certainly true that AP courses benefit the upper middle class more than they benefit the poor, but the solution has to be to improve education for the poor, not make education worse for the middle class.  I don’t want a race to the bottom, though that seems to be the natural consequence of John Tierney’s ideas.
  • The cost of offering AP courses varies, but is generally much less than the cost of high school athletics, marching bands, and other high school activities that John Tierney seems to value more than academics.  Furthermore, AP courses generally replace other academic courses, which cost about the same amount, so the additional cost of having a section of AP bio instead of regular bio is pretty minor (it is mainly additional prep time for the teacher, which most often costs the taxpayer nothing). The opportunity cost of offering an AP course instead of some other course is real—but that is true of every course that is offered, and I am very doubtful that AP courses are the least useful courses at most high schools.
  • Whether AP courses are too reliant on rote learning is a harder question to answer. A lot of teachers are teaching to the AP tests, and some of the tests have had a history of being too reliant on rote memory (history and biology, for example, have such a reputation).  The AP Bio course was just revamped to be less reliant on memory and more on understanding, but the new tests have not yet been given, so it remains unclear whether the revisions went far enough in reducing the memory work.
    It is not inherently bad for teachers to have an external test to see whether or not they are preparing their students adequately.  The important question is whether the test does a decent job of determining this.  As tests go, the AP exams are not bad at measuring learning (though I have rely on 2nd-hand reports for this, as the College Board does not allow adults to take the AP tests).

I don’t want to come across as a gung-ho apologist for AP courses. We turned down a slot in an AP-intensive charter school last year after entering the lottery for it 4 years running (see School decisions: part 1, part 2, part 3) and started home schooling instead.  The rigidity of the AP curriculum was not going to be a good fit for our son’s strengths and weaknesses, and we had to custom tailor his education to fit.

Overall, I see the AP exams as being very valuable for home-schooled high school students, and the AP courses offered by high schools a good (though not ideal) way to serve those students who would be bored in the regular high school courses.  In no way do I see any evidence of the scam that John Tierney claims.

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