# Gas station without pumps

## 2015 April 9

### Second half of temperature lab went well

Filed under: Circuits course — gasstationwithoutpumps @ 21:11
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I continue to be pleased with this Spring’s class.  It is taking up far too much of my time, but the students are doing well. Today they got their resistors (and ceramic capacitors), so they could select whatever resistor value they needed for the optimization problem.  I told them by e-mail last night to optimize for 12°C, which I estimated would require about a 12kΩ resistor.  I change the temperature point each year, so that students can’t just look up an answer—not that it would do them much good if they could, as the design report needs to contain the derivation of the result.  If they can copy the math correctly, then they can do it (especially since we did the derivation in class yesterday).

I put some reminders/instructions on the board at the beginning of lab, something like the following:

• Optimize for max sensitivity @ 12°C
• Measure Vin
• Measure Vout at many temperatures
• Plot Vout vs temp both from model (as curve) and as data points, to check calibration.
• Plot temp vs time for 10 minutes of water warming up from low temperature (using PteroDAQ)

Everyone seemed to be getting good data in the lab.  What students seemed to need the most help with was the concept of putting the predicted behavior of the circuit on the same graph with the measured behavior. They had a formula for the voltage as a function of temperature with 4 parameters: B and R∞ from fitting Tuesday’s data, R that they chose and measured, and Vin that they measured though it was nominally 3.3V. But it was hard going trying to convince them to plot that curve on the same graph as the scatter diagram of today’s measurements.  It may be that they have never been asked to plot a predicted curve and measurements on the same plot before—as if they’ve never actually checked that a model is correct. I would have thought that physics classes would have done that sort of predictive modeling, but apparently not, as the concept seemed completely foreign to many of the students.

Those who did finally get the calibration plot done generally had very good agreement between their measurements and the predicted model.  I’m hoping the rest will get the plots done tonight.  There was at least one group that was seeing about a 0.5°C discrepancy, which could be due to different calibration of the thermometers used on Tuesday and today, or could be due to their using a wrong value for the °C to °K conversion.

The students also recorded a time course of a water bath warming up from about 3°C. I think that next year I might do a higher optimum temperature and ask students to record a water bath cooling down—the temperature change is faster that way (due to evaporation). I’ve asked them to plot this as temperature vs. time, not voltage vs. time, and several students seemed well on their way to doing that correctly—this year’s class seems much more adept at picking up gnuplot techniques than the last two years’ classes.  I don’t know whether that is because of a better ordering of the material this year, or some difference in the cohort.  I like to think it is an improvement in the way I present things, but it is more likely to be a difference in the students.

One pair of students surprised me in a good way—they had done the optimization last night, then tried out the design at home, using the PteroDAQ to read voltage.  Since they did not have the resistor kits yet, they had put the two 22kΩ resistors they did have in parallel to make an 11kΩ resistor.  They were worried I might be upset with them for jumping the gun on the lab—quite the contrary, I’m delighted that they’re preparing before class, and that they realized that a lot of the lab work doesn’t really require the fancy equipment in the lab. I’ve pitched the class as being suitable for creating electronics hobbyists, and if some of them get into that early in the course, then the course is being highly successful!

I’ve even considered rewriting a number of the labs to be doable completely at home, but right now too many would require about a $250 investment in a USB oscilloscope, which makes an all-at-home approach a bit too expensive for me to recommend. It might be an interesting way to market the book though—as a complete electronics at home lab course for about$400.  I think that there is a substantial market for such a kit/course, but it would be a fair amount of work to get the book into shape for use without a lab mentor to guide the students through the rough spots.

I’m looking forward to this week’s reports (that’s right, for once I’m looking forward to the grading, rather than dreading it), as I think that a lot of the class actually got all the concepts that this week’s lab was about.

Tomorrow I’ll be introducing complex impedance (particularly $Z_{C} = \frac{1}{j\omega C}$) and RC filters as voltage dividers.  I’ll be doing most of the lecture as a chalk talk, but I’ll bring my laptop to show them how to create (amplitude) Bode plots with gnuplot.  We’ll be doing various things with model fitting, with variants on voltage dividers, and with complex impedance for the next 3 weeks, and then we’ll start on amplifiers.

## 2015 April 8

### Temperature lab went well

Filed under: Circuits course — gasstationwithoutpumps @ 08:28
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The lab for characterizing the thermistors went well—more smoothy than the last two years.  It may be because the lab is now in the second week, rather than the first week, or it may be because I required prelab homework be turned in on Monday, so that students came to lab already prepared, or it may just be that this year’s class is more prepared than previous ones.  I suspect that the main difference is in the prelab requirement, but that may be because that is about the only thing I can really control.

I went into the lab an hour early, to set up the coffee urn with hot water, get ice from the ice machine in the lab upstairs, and check each of the thermometers in ice water.  The cheap student glass thermometers all ready between -1°C and 0°C, but several of the digital thermometers were way off (reading as much as 4°C).  I separated out the digital thermometers that read over 0.5°C and marked them with blue tape as miscalibrated.  (I can believe temperatures slightly below 0°C, since the ice water was tap water, not distilled water, but temperatures over 0°C were almost certainly wrong.)

I started by showing the students how to interpret the part number for the NTCLE428E3103F520L thermistor we were using, since half the class had not figured out that it was a 10kΩ thermistor on the homework.  The data sheet is rather tricky, but in a common way: it is the data sheet for a family of parts, with a key for decoding the part number.

The students managed to collect 20–90 data points of temperature and resistance, with varying levels of noise.  I had them plot the data after they had collected some points, and suggested to students that they fill in more points where they had gaps in the curve, or where they had “bumpy” spots that did not fall along a smooth curve (suggesting measurement errors), and pushing them to get lower and higher temperature measurements.  Students with good lab skills managed to get a lot of data points from 2°C up to 69°C.  The water in the coffee urn was a bit hotter than that, but by the time they got the water back to their benches and made measurements it had cooled off.  It might be interesting some year to have a hot plate at every bench to let them measure up to 100°C, but I don’t think that the expense of the hot plates would be justified for this one lab—and I’d worry about possible spills.

I was worried enough about spills on this lab, reminding students frequently to carry the secondary containment tubs in both hands, and to put it flat on a level surface whenever they weren’t carrying it.  As it turns out, about the only real spill was mine, trying to pour ice water from one thermos to another, and getting a chunk of ice splashing out.  The spill was small (a few cubic centimeters of ice) and not near the lab equipment, and I could mop it up with a couple of paper towels. I was probably made a bit clumsier than usual by having only gotten 4 hours sleep last night (between grading prelab homework and adding material and exercises to the chapters of the book that the students need to read for Friday and next Monday).  Still I felt bad about being the clumsy one in the room.

There were two glass thermometers broken—one was taken in its case by a student to be properly discarded in a broken-glass disposal bin, but the other was just quietly returned to the bin of thermometers.  I have a lot more respect for the student who reported the broken thermometer than the one who tried to hide it.  The thermometers themselves are cheap (about \$3), so the breakage doesn’t bother me, but leaving problems for someone else to deal with does.

I had a few of the students send me data, which I will use in class today to show students how to use gnuplot to fit models to data. I also plan to go over the homework problem that only one student got—the optimization to maximize the sensitivity of the voltage divider at a particular operating temperature.

## 2014 June 5

### Very long lab time today

Filed under: Circuits course — gasstationwithoutpumps @ 23:19
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I promised the students earlier in the quarter that I would stay in the lab until everyone finished—that resolve was sorely tested today, on the last scheduled lab time of the quarter, as one student stayed until 9:40p.m. (the lab started at 2 p.m.).  I ended up giving the student more debugging help than was probably optimal pedagogically (pointing out wiring errors, swapped resistors, and other problems—even unsoldering some wires that were incorrectly placed), but after a while I just wanted to go home.  Luckily the design was ok, and only the implementation needed to be fixed, or I might have been there all night.  Every group in the class got an EKG designed, soldered, and working, though it took this year’s class longer than last year’s.

I wore EKG electrodes today (as I did for Tuesday’s lab), so that students could check whether problems they were having were with inadequate electrode contact or with their amplifiers.  Some of the students did have poor signals from their electrodes, possibly due to their electrode gel having dried out somewhat, possibly due to inadequate skin preparation.  I’ve found that a fairly simple skin prep works for me: after my morning shower I scrub the areas where I want to apply the electrodes with a dry towel, to remove any dead skin, and apply the electrodes within a couple of minutes.  The electrodes still work well 10 hours later.  The packaging claims that they are good for up to 24 hours, but I don’t think I’ve ever worn them for more than 12 hours.

The T-shirts the students ordered are done, but I didn’t have time this evening to pick them up, and I won’t have time in the morning (unless I wake up very early and the store opens early), as I have to set up the bread machine in the lab before going to see the 50-minute version of Hamlet being performed by Shakes To Go.  Most likely, I’ll pick up the T-shirts on Saturday and have the students come to my office on Monday to get them.  Monday is the deadline for them to turn in any assignments with REDO grades, so I suspect that many will be coming to my office then anyway.  (Only one student has no lab reports to redo, but I don’t know how many of the redone reports I’ll get tomorrow and how many on Monday.)

## 2014 May 15

### Very long time in the lab

Filed under: Circuits course — gasstationwithoutpumps @ 23:43
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I spent 7 ½ hours in the circuits lab today, helping students finish their optical pulse monitors.  One group finished on time, and a couple of others finished with only an extra hour, another group gave up (taking the project home to see if they could finish there), and one student went to a meeting, but came back before I had left, so I stayed until she had finished also. Everyone got the first stage working, and were able to see pulse signals with a 2–10mV amplitude (on top of 100–200mV DC and about 20mV 60Hz noise). All but the group that took the project home got the second stage working and able to record clear pulse signals on the PteroDAQ software (though some had the gain a bit too high and got clipping).

I don’t think that this lab needed to take as long as it did for the students, and I’ve been thinking about ways to get them to do it much quicker next year.

• The first thing to do is to separate the electronics from the scaling of the light signal.  Their math is too slow to put it on the critical path to getting the lab done.  Instead, I’ll make the analysis of the size of the signal they ought to see be done after a more tinkering approach to design has been done.
• The next thing to do is to require that a schematic for the first stage be turned in on Monday, so they work on it over the weekend.  They’ll have to come up with a guess at the initial resistor size, but I can give them a rough estimate (within a factor of 10, say) of the current to expect from the phototransistor.
• I can also teach active RC filtering a little sooner, so that I can have them design the low-pass filter to reduce 60Hz interference in their first design.

The 60Hz noise is a much bigger problem in the circuits lab than at home—probably because of all the fluorescent lights and benchtop equipment. It was reduced enough by a single RC low-pass filter that it did not saturate the second stage, and it was well removed by synchronous sampling (so I’m glad the PteroDAQ software allows specifying sampling frequency and not just period—it is easier for students to enter 30Hz than 33.333msec (which was not a period the old software supported, anyway—they would have had to go to 50msec).

One problem that some students encountered today that I had not anticipated is that some fingernail polish is opaque to the 627nm LED light we were using.  I don’t normally wear fingernail polish, so it had never occurred to me that designing the fingertip sensors to shine through the fingernail might be a bad choice in some circumstances.  I suppose that this is yet another reason why we need a diversity of engineers—unrealized assumptions of one subculture may be obvious problems in another.

I don’t know what I’ll do about the fingernail-polish problem next year (this year the student just scraped one fingernail clean and got the circuit to work). I suppose I could simply announce that fingernail polish may be opaque, so students may wish to keep one finger clear on lab day. I could let students discover the problem on their own (or with a little guidance from me), as happened this year. Or I could try to design a different way to mount the LED and phototransistor.

One problem with the setup I used this year is that to get a good signal you need to apply enough pressure to your finger to be between the systolic and diastolic pressures.  That can be difficult to do, though with a bit of practice I got pretty good at it today, as I helped students debug their circuits. It would be good to have a simple adjustable spring clip that would hold the two optoelectronics components and apply the appropriate pressure (around 80mm Hg, 10kPa, or 1.5psi).  The reason my first attempt at an ear clip was such a failure was that it squeezed too hard and cut off all circulation—if I had made a clip that squeezed with only 10kPa, it might have worked fine.

Over the summer I might play around with different designs for mounting the optoelectronics, to avoid the fingernail-polish problem and to get a more controllable squeeze for whatever part of the body the light is shining through.

Tomorrow I’ll have to cover strain gauges, Wheatstone bridges, and instrumentation amplifiers.  I checked today that I can still derive the gain equations for both 2-op-amp and 3-op-amp instrumentation amps. I’ll assign students to do their block diagrams, schematics, and layout for the pressure sensor amplifier to turn in on Monday, so that they’ll be able to start lab promptly on Tuesday.  With any luck, Thursday can be a catch-up day for any labs that needed to be redone with a little more data.

## 2014 May 1

### Sampling and aliasing lab

Filed under: Circuits course,Data acquisition — gasstationwithoutpumps @ 21:30
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Yesterday, I only briefly mentioned sampling and aliasing, which was the subject of today’s lab.  I was hoping they would read the handout, as that has some of the explanation missing from the lecture. That, as usual, turned out to be a forlorn hope. Some of them did read the handout, but not in a way that gained them any comprehension, and no one had done the prelab exercises, so the first hour (or more) of lab time was spent with students trying to figure out how to do a high-pass filter that did level shifting, so that the output would be in the 0–3.3V range of the analog-to-digital converter.  They all got to reasonable designs eventually (with capacitor values in the range 4.7µF to 470µF—I really like that the answer is not dictated by having only a small selection of components!).  I did have to re-teach Thévenin equivalents to some of the teams, as they were not getting the RC time constants that they claimed.

The level-shifting high-pass filter design will be useful again next week, for connecting the microphone to an op-amp audio amplifier.

I did not bring the stroboscope into lab as a demo—the demo had not worked all that well last year, and the students have all seen stroboscopes before.

PteroDAQ worked well for doing downsampling, and students recorded several waveforms.  I’ll see whether anything sensible is said about them in this week’s lab reports.  At least they had fun looking at the weird beat patterns you get if the signal you are looking at is close to the Nyquist frequency.

I also got the data today from the students who had a loudspeaker that behaved differently from everyone else’s on Tuesday (they tested another loudspeaker with the identical setup and got normal results, and I checked a few of their measurements—I believe they did just have a weird speaker).  There was a little metadata missing (like exactly what their fixed resistor was for converting current to voltage), but I was able to fit their data with just two more parameters on the model, a resistor and capacitor in parallel with each other, in series with the rest of the model:

The bad loudspeaker has a higher than expected resistance at low frequency, then a 1/f-sloped region after the resonance peak, then a return to normal behavior. I modeled this loudspeaker by adding and extra R||C in series with the model we used for good loudspeakers.

I have no explanation for the physical or electrical causes of an extra R||C in the loudspeaker.

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