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2016 March 31

Pep talk for students frustrated at the end of the first week

Filed under: Circuits course,Data acquisition — gasstationwithoutpumps @ 20:48
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Some of the students in my Applied Electronics for Bioengineers course are feeling frustrated at the end of the first week (often due to imposter syndrome, not any real inability to do the work).  I sent them the following e-mail this evening:

To the class—a number of people are feeling overwhelmed, because of the wide range of preparation that people in the class have had.  This is supposed to be a first course in electronics, but a number of people are taking it after having had other electronics courses.  If the advanced students are allowed to dominate the questions in class, I’ll never know what help the students with less preparation (that is, the students the course is intended for) need.  If you are feeling overwhelmed or out-matched in class, please ask questions!  I know that there are people feeling like they need more help, but I don’t know exactly what help they need.
I could guess at what is causing people problems, but I’m likely to guess wrong, and I don’t want to waste a lot of time on reviewing stuff that everyone in the class gets, while not spending any time on the stuff that is really needed.
In short, I’m saying that I need a lot of questions from people in the bottom quarter of the class, and I don’t think I’ve been getting them.
Going to [the group tutor]’s sections is another way to catch up to those you perceive as being ahead of you.
It looked to me like everyone pretty much got labs 1 and 2 done, and that most of the class (though perhaps not everyone) had a decent grasp of aliasing.  A bigger fraction of the class had PteroDAQ and gnuplot installed and working by Lab 2 than in any previous offering of the course—so this looks to me like a very promising start to the quarter—it may have seemed chaotic to you with not all the parts arriving on time and last-minute patches to PteroDAQ to compensate for changes in laptop operating systems, but these startup pains are normal—I expect to have them every time the course is offered.
Lab 2 was much harder than intended this year, because of the resistor assortments not including 470kΩ resistors, and I was impressed by how the class rose to the challenge, despite not having had the lectures yet that would really support the design work done (those are scheduled for week 3, I believe). I’m going to have to rewrite parts of Lab 2 to allow for the possibility of not having the right parts available.
The deal with Lab 2 was this: I had given them in the book a circuit to build that consisted of a function generator, a capacitor, a pair of resistors, and the Teensy board with the PteroDAQ software. The idea in terms of skills was for them to learn how to lay things out on bread board, collect data with PteroDAQ and do some minimal plotting with gnuplot.  The concept they were supposed 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 assortments (which only arrived yesterday, so I had no idea exactly what resistors would be in the kit) did not have the specified 470kΩ resistors!  I probably should have redesigned the circuit for them and had them build a different circuit which would have worked equivalently (like using 1MΩ and 4.7µF instead of 470kΩ and 10µF), but I did not know what resistor values they did have in their kits.
Instead, on the spur of the moment, I chose to have the students come up with a design themselves that has the same (or nearly the same) RC time constant as the circuit in the book.  If I’d had an hour to think about how to handle the challenge, I might have chosen a different approach. The assignment I gave them tied in well with yesterday’s unplanned lecture—without that lecture, I would not have considered them capable of redesigning the circuit.
 I think that everyone in the class did come up with a design that let them do at least a few recordings with PteroDAQ, though they did not get as much time to explore aliasing as I had originally intended. There were several different designs students came up with, including the 1MΩ and 4.7µF design, 10MΩ and 0.47µF, putting two 1MΩ in parallel to make 500kΩ, and building the 470kΩ out of a series chain of resistors.
Having a real design challenge for this first lab was in one way a good one (it had bothered me that there was no design element in the first week of lab), but this design challenge was too much for the first week.  After lab some students were feeling overwhelmed and wanting to drop the course—even though this year’s class is well ahead of previous year’s classes (even the students who are struggling are further along than their counterparts in previous years).
Now my challenge is to convince the students who are feeling stretched to stick with the class for another week or two, so that the lectures can catch up to what they need to know and they can have a more confident base to work from.

2016 February 20

On not using kits for a design class

Filed under: freshman design seminar — gasstationwithoutpumps @ 10:07
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A question came up in e-mail from a student in my freshman design seminar that I had planned to discuss in class, but I ran out of time before getting to it:

Upon determining the actual parts and part numbers for our design, its become increasingly apparent that the DIY kits neatly contain all of the parts we require. Considering this, we wanted to recap with you regarding the DIY kits from our projects. Are we allowed to purchase a kit and build our pulse monitor from it, or are we only allowed to use some parts but not the entire kit?

I answered the student after class, but I wanted to share my thinking more widely.

I like kits in many ways—I learned a lot from building Heathkits as a kid (see Thanks, Dad), but there are limits to what a kit can do for learning, particularly if people just assemble the kit with little or no attempt to understand what each part is for and how the whole thing works.

In a freshman design course, where students know very little or no electronics or programming when they start, the projects that they can reasonably tackle have to be quite simple. Most of the problems that they can solve have been solved before, and solutions by experienced engineers are easily found. Furthermore, commercial solutions are often available for less money than the parts needed to build them, due to the high cost of stocking, distributing, and shipping low-volume parts.

So students who see their role as answer getting are easily convinced to grab one of the good designs so readily available on the Internet and present it as the solution—very little work on their part and a good design, so what could be better?

The problem, of course, is that it is not the goal of the course to have a pulse monitor, an LED cube, or an ultrasonic rangefinder—none of those were part of the syllabus.  I can buy a pulse monitor for a few bucks, and for under $2 an ultrasonic rangefinder module that works better than anything the students are likely to come up. A finished RGB cube is a bit more expensive, but kits are still pretty cheap. One can buy the products the students could produce for far less time and effort than designing and making them. So the product is not the goal of the course.

The goal of the course is for students to (begin to) learn to design things.  This means doing things like writing specifications, drawing schematics, building prototypes, debugging prototypes, writing design reports, and doing iterative prototyping. It means learning a lot about how things work—not everything, but the specific details they need for the projects they have selected. The process of design is the core of the course, not the product of the design.

Buying a kit and assembling it short circuits a lot of that design learning.  You can build a kit and get it working with little or no understanding of how it works, and with no ability to modify the design. Of course, it is not the bundling of parts into a a kit that is the problem—copying a design off the Internet and buying the parts separately also does little to help students learn design. If you just copy other people’s work, then little learning takes place, and there is not much point to doing the class. Perhaps there is a little skill learning—how to use a soldering iron or an oscilloscope and how to order parts from a distributor, but not much how-to-design learning, which is the main point of the course.

That is not to say that students shouldn’t be looking at designs on the web!  Indeed, the freshmen do not have enough of the basic building blocks of electronics and programming to come up with their own designs de novo (though by their senior year they should be able to, if they continue into the bioelectronics or assistive technology: motor concentrations). So they need to look at solutions others have come up with, in order to know where to start on the design. But they should not just pick one design and implement it—they should be looking at several designs and trying to find common elements, figuring out what tradeoffs the different designers have made. Some of the designs come with good explanations of how they work—they should be reading those very carefully, so that they can follow the design decisions and reproduce the design from understanding.

Understanding designs well enough to explain each part, how it functions, and what the effects would be of changing the part may be enough at this stage of their design learning. Part of the point of the freshman design course is to inspire them to want to learn the foundations in other courses, many of which seem to students to be irrelevant gatekeeper classes to survive by cram-and-forget techniques. If they have some idea why complex numbers are important for filter design, or why capacitors are useful, they are more likely to pay attention in those parts of math and physics classes and to retain the knowledge later on.  Of course, it would be best if the math and physics classes included more of the inspiration themselves, but there are many different applications for the material, and it is difficult to find universally inspiring applications—what excites a math major, a computer science major, a physics student, or a bioelectronics student may be quite different, but the same course has to serve all of them.

My role in the classroom is to provide explanations for those things that the students have trouble figuring out on their own, give them generic guidance in the design process, anticipate things they will need to know, and try to hurry them up (in the past most waited far too long before starting their projects, with the result that they get very little done). Having three different projects going on at once means that there is not enough class time for students to get everything they need from me—they must be reading on their own and trying to understand their project thoroughly, but based on the time logs they have been submitting, few in the class have been doing that—they mostly seem to be waiting for me to tell them what to do, which is not going to work for them. I had hoped that having students pick their own projects would inspire them to investigate the projects more deeply without my having to keep kicking them with assignments, but that only seems to be working for a few of the students.

Some aspects of the design they can do—for example, everyone in the class should now be able to size a current-limiting resistor for an LED, and those who need amplifiers should be able to make a simple non-inverting amplifier out of an op amp.  (Hmm, I should probably give a quiz on those ideas next week—maybe the students have absorbed less than I expect.) By this point, the students should be asking about those parts of the design that they don’t understand, but I’m not getting very many questions, despite starting each class asking for questions and covering student-requested material before anything that I have queued up as things students might need.

A big chunk of what the class is trying to do is to nudge students away from regurgitating factoids that have been spoonfed into learning for the sake of understanding deeply enough to do things themselves—that means generating a lot of the questions themselves and seeking solutions by trying things out, and not just by looking up “the answer” or waiting for a teacher to tell them what to do and what to think.  That is a hard transition for many students to make, as they have been steeped in “answer-getting” culture for the past 13 years.

 

2015 November 29

Shaving brush stand

Filed under: Uncategorized — gasstationwithoutpumps @ 12:49
Tags: ,

In January 2014, I ordered a couple of shaving-brush stands from Amazon: one for me and one for my son.  It turns out that the one for my son was a better deal: both cheaper and better quality. The stand I ended up with was chromed steel, while his was stainless steel.

Putting chromed steel in the moist environment of a bathroom to hold a wet shaving brush is stupid design:

Chromed steel rusts

Chromed steel rusts.

I have just ordered myself the stainless steel shaving-brush stand, so that I don’t have to keep cleaning up the rust on the bathroom counter.

2014 July 18

How to sell a widget

SparkFun has a tutorial on how to sell “widgets” through them at How to Sell Your Widget on SparkFun – Learn.SFE:

Have an awesome electronic widget that you want to get to market? Great! We are always listening for new ideas from our customers and the community. We get many inquires on this topic, so read this tutorial carefully to keep your product pitch from getting lost in the shuffle.

We’re lucky, here at SparkFun, to have an amazingly creative and talented group of customers. Not only can they identify gaps in the catalogs of electronics suppliers, they can create a gizmo that fills that hole. But, going beyond a prototype or even a limited-quantity production run, often the hardest task in getting your world-altering product out there is producing, marketing, and/or selling it to the masses. That’s where we come into the picture.

The advice there is seems fairly reasonable.  They offer a choice of two models: make it yourself and have them sell for you, or have them make it and pay you royalties.  They tell you how to pitch products to them and how to design for them to be able to manufacture stuff.  Most of the stuff they sell is PCB boards, so they give quite a bit of advice about PCB design to fit their pipeline—they use Eagle, but ‘All parts are placed on a 0.005″ grid. If possible, use a .05″ grid.’ That must get irritating with modern parts that are convenient round metric numbers, not mils.   They also want version numbers in the bottom copper, which is reasonable for some designs, but not all.  They also encourage people to use their Eagle libraries, but my son and I have found their pad layouts to be very sloppy (putting silkscreen over SMD layers, getting the keep-out areas off by a little bit, not fixing the fonts on the “>NAME” and “>VALUE” labels to meet their own requirements, and so forth).

Still, it is good that they put out their design rules and provide clear guidelines to new designers.

I’ve thought a few times about putting out some of my designs through SparkFun or Adafruit Industries—perhaps an improved version of the blinky EKG as a kit.  SparkFun now sells EKG electrodes and snap leads for them, and even have a heart-rate monitor board (based on the AD8232 chip) and the “BITalino” biomedical board, so I suspect that they are interested in the market.

The BITalino is outrageously expensive and their EKG electrodes are about 3 times the price of buying them at Amazon, but the AD8232 chip actually looks like a nice one for building an EKG front-end and reasonably priced, so I’m not sure they’d have much interest in a through-hole part kit for do-it-yourself EKG that isn’t quite as good, unless it could be sold very cheaply or as an educational product (which is what the blinky EKG is aimed at, anyway).

I have some other ideas for products that I might be marketable, but I don’t know whether I have the time to refine them to the point of pitching them to SparkFun.  I can justify some time spent on doing electronics as a hobby, some as necessary learning for teaching my applied electronics course for bioengineers, and some as engineering-for-manufacture experience (something I never had any instruction in, despite my years as an engineering professor). But when the electronics work starts cutting into the time I need to spend on writing my book, teaching my classes, or doing collaborative research with other faculty, then I have to draw the line.  I’ve also got a lot of administrative responsibilities now (undergrad director and faculty adviser for two BS degrees, Program Chair for bioengineering, and Vice Chair for the Biomolecular Engineering Department), so writing time and research time have gotten doubly precious.

I do have one project this summer that I’m going to try to get fabricated for me—it is all SMD parts, including some that are hard to solder by hand (pads under the chips), so I don’t want to do it myself.  The project also calls for a lot of identical boards (20 to 50 of them), so a prototyping house seems like the way to go.

I’m looking currently at Smart Prototyping to do the PC board fabrication and assembly—they may not be the cheapest, but they have a comprehensible pricing scheme on their website, and they replied within 12 hours of my request for a quote. They also have a nearly turnkey system—I send them the Gerber files and the Bill of Materials (BOM), and they’ll make the boards, buy the parts, and assemble the boards.  They’ll even test them for an additional charge, though these boards are simple enough that I can test them myself at about 5 seconds a board, so their testing would not be worthwhile unless they guaranteed their assembly (which none of the prototype houses can afford to do with untested designs).

I also considered Elecrow, which has a similar service, but their pricing information on the web page is rather vague: “For BGA or IC with pads under IC, The quotation will be a little higher.” and “We will give a discount for the PCB assembly service according the some factors (assembly time,Hard or Easy to assemble or requirements etc.).” I prefer sites that have clear pricing even if it is slightly higher, so that there are no surprises. I suppose I could ask Elecrow for a quote and see if they respond as promptly as Smart Prototyping did.

Incidentally, my design does not follow all SparkFun’s guidelines—for one thing, I placed parts on a 0.5mm grid, not a 0.005″ gird, and the board is not rectangular.  Still, if the design I’m working on turns out well, I might pitch it to them, as I see some potential for it appealing to the open-source hardware market, and the violations of their design guidelines made good sense for this application.  Note: I’m deliberately not saying what the design is—I’ll reveal it once I’ve gotten a working prototype, when I’ve decided whether I want to commercialize it or not.

2014 July 6

Battery connectors

Filed under: Uncategorized — gasstationwithoutpumps @ 02:32
Tags: , , , , , , , ,

I spent a little time today working on my book, but I got side tracked into a different project for the day: designing a super-cheap coin-cell battery connector. I’ve used coin-cell battery holders before, like on the blinky EKG board, where I used a BH800S for 2 20mm CR2032 lithium cells. That battery holder is fairly large and costs over $1—even in 1000s it costs 70¢ a piece. So I was trying to come up with a way to make a dirt cheap coin-cell holder.

The inspiration came from the little LED lights that “glovers” use inside their gloves. They are powered by two CR1620 batteries (that means a 16mm diameter and 2.0mm thickness for the battery). Because the lights have to be made very cheaply, they don’t use an expensive holder, but put the negative side of the batteries directly against a large copper pad on the PC board. The batteries are held in place by the positive contact, which is a piece of springy metal pressing the battery against the board—and each manufacturer seems to have a slightly different variant on how the clip is made.

Unfortunately, I was unable to find any suppliers who sold the little clips—though I found several companies that make battery contacts, it seems that most are custom orders.

My first thought was to bend a little clip out of some stainless steel wire I have sitting around (not the 1/8″ welding rod, but 18-gauge 1.02362mm wire). That’s about the same thickness as a paperclip (which is made out of either 18-gauge or 19-gauge wire), but the stainless steel is stiffer and less fatigue-prone than paperclips. I was a little worried about whether stainless steel was solderable, so I looked it up on Wikipedia, which has an article of solderability. Sure enough, stainless steel is very hard to solder (the chromium oxides have to be removed, and that takes some really nasty fluxes that you don’t want near your electronics). So scratch that idea.

I spent some time looking around the web at what materials do get used for battery contacts—it seems there are three main ones: music wire, phosphor bronze, and beryllium copper, roughly in order of price. Music wire is steel wire, which gets nickel plated for making electrical connections. It is cheap, stiff, and easily formed, but its conductivity is not so great, though the nickel plating helps with that. The nickel oxides that form require a sliding contact to scrape off to make good electrical connection. Phosphor bronze is a better conductor, but may need plating to avoid galvanic corrosion with the nickel-plated battery surfaces. Most of the contacts I saw on the glover lights seemed to have been stamped out of phosphor bronze. Beryllium copper is a premium material (used in military and medical devices), as it has a really good ratio of yield strength to Young’s modulus, so it can be cycled many times without failing, but also has good conductivity.

Since I don’t have metal stamping machinery in my house, but I do have pliers and vise-grips, I decided to see if I could design a clip out of wire. It is possible to order small quantities of nickel-plated music wire on the web. For example, pianoparts.com sells several different sizes, from 0.1524mm diameter to 0.6604mm diameter. I may even be able to get some locally at a music store.

My first design was entirely seat-of-the-pants guessing:

First clip design, using 19-guage wire, with two 1mm holes in PC board to accept the wire.

First clip design, using 19-gauge wire, with two 1mm holes in PC board to accept the wire. This design is intended for two CR1620 batteries.

The idea was to have a large sliding contact that made it fairly easy to slide the batteries in, but then held them snugly. Having a rounded contact on the clip avoids scratching the batteries but can (I hope) provide a fair amount of normal force to hold the batteries in place. But how much force is needed?

I had a very hard time finding specifications on how hard batteries should be held by their contacts. Eventually I found a data sheet for a coin battery holder that specified “Spring pressure: 50g min. initial contact force at positive and negative terminals”. Aside from referring to force as pressure and then using units of mass, this data sheet gave me a clear indication that I wanted at least 0.5N of force on my contacts.

I found another battery holder manufacturer that gave a tiny graph in one of their advertising blurbs that showed a range of 100g–250g (again using units of mass). This suggests 1N-2.5N of contact force.

Another way of getting at the force needed is to look at how much friction is needed to hold the batteries in place and what the coefficient of friction is for nickel-on-nickel sliding. The most violently I would shake something is how fast I can shake my fingertips with a loose wrist—about 4Hz with an peak-to-peak amplitude of 22cm, which would be a peak acceleration of about 70 m/s^2. Two CR1620 cells weigh about 2.5±0.1g (based on different estimates from the web), so the force they need to resist is only about 0.2N. Nickel-on-nickel friction can have a coefficient as low as 0.53 (from the Engineering Toolbox), so I’d want a normal force of at least 0.4N. That’s in the same ballpark as the information I got from the battery holder specs.

So how stiff does the wire have to be? I specified a 0.2mm deflection, so I’d need at least 2N/mm as the spring constant for the contact, and I might want as high as 10N/mm for a really firm hold on the batteries.

So how should I compute the stiffness of the contact? I’ve never done mechanical engineering, and never had a statics class, but I can Google formulas like any one else—I found a formula for the bending of a cantilever loaded at the end:
\frac{F}{d} = \frac{3 E I}{L^{3}}, where F is force, d is deflection, E is Young’s modulus, I is “area moment of inertia”, and L is the length of the beam. More Googling got me the area moment of inertia of a circular beam of radius r as \frac{\pi}{4} r^{4}. So if I use the 0.912mm wire with an 8mm beam I have
F/d = 200E-6 mm E.

More Googling got me some typical values of Young’s modulus:

material E [MPa = N/(mm)^2]
phosphor bronze 120E3
beryllium copper 135E3
music wire 207E3

If I used 19-gauge phosphor bronze, I’d have about 24N/mm, which is way more than my highest desired value of 10N/mm. Working backwards from 2–10N/mm what wire gauge would I need? I get a diameter of 0.403mm to 0.603mm, which would be #6 (0.4064mm), #7 (0.4572mm), #8 (0.5080mm), #9 (0.5588mm), or #10 (0.6096mm), on the pianoparts.com site. I noticed that battery contact maker in Georgia claims to stock 0.5mm and 0.6mm music wire for making battery contacts, though they first give the sizes as 0.020″ and 0.024″, so I think that these are actually 0.5080mm and 0.6096mm (#8 and #10) music wire.

It seems that using #8 (0.020″, 0.5080mm) nickel-plated music wire would be an appropriate material for making the contacts. Note that the loop design actually results in two cantilevers, each with a stiffness of about 4N/mm, resulting in a retention force of about 1.6N. The design could be tweaked to get different contact forces, by changing how much deflection is needed to accommodate the batteries.

How much tweaking might be needed?  I found the official specs for battery sizes (with tolerances) in IEC standard 60086 part 2: The thickness for a 1620 is 1.8mm–2mm, the diameter is 15.7mm–16mm, and the negative contact must be at least 5mm in diameter.  The standard also calls for them to take an average of 675 hours to discharge down to 2v through a 30kΩ resistor (that’s about 56mAH, if the voltage drops linearly, 67mAH if the voltage drops suddenly at the end of the discharge time).  If the batteries can legally be as thin as 1.8mm, then to get a displacement of 0.2mm, I’d need the zero-point for the contacts to be only 3.4mm from the PC board, not 3.8mm, and full thickness batteries would provide a displacement of 0.6mm, and a retention force of about 4.8N.

If I were to do a clip for a single CR2032 battery, I’d need to have a zero-point 2.8mm from the board, to provide 0.2mm of displacement for the minimum 3.0mm battery thickness.

So now all I need to do is get some music wire and see if I can bend it by hand precisely enough to make prototype clips.  I’d probably change the spacing between the holes to be 0.3″ (7.62mm), so that I could test the clip on one of my existing PC boards.

Update 2014 July 6: I need to put an insulator on the verticals (heat shrink tubing?), or the top battery will be shorted out, since the side of the lower battery is exposed.

 

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