Gas station without pumps

2018 April 15

Rapid delivery

Filed under: Circuits course — gasstationwithoutpumps @ 09:37
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I made a serious mistake in putting together the parts list for my Applied Electronics course this quarter—I forgot to include a potentiometer on the list. I think what happened is that in previous years I had put the trimpot on the first quarter list, but we didn’t use it until the second quarter. I had a note to move it from the first-quarter list to the second-quarter list, but the move only happened half way (it was removed from the first list, but not added to the second one).

The mistake was pointed out to me be students in my Thursday office hours (they were asking where the potentiometer they were to use was).

Late Thursday night (after the evening labs were ordered), I ordered 85 25-turn 10kΩ trimpots from DigiKey, and they arrived Saturday morning (at 36 hours, about the fastest delivery I’ve ever had for anything other than pizza—particularly good for a delivery from Minnesota to California).  The Post Office package delivery gives good service here (now that they are no longer short-staffed as they were in December).

Because the lab course fee for the Applied Electronics course has all been spent on parts and tools already, I probably won’t be able to get reimbursed for these parts. The $76.52 they cost is probably the price I’ll have to pay for my mistake. (It isn’t my most expensive mistake in the last year—I forgot to pay my first installment of property taxes on time, which cost me a couple hundred dollars in penalties.)

Although I’m very happy with DigiKey’s rapid service, I might still specify trimpots from AliExpress next year, since 100 trimpots would cost only about $12 with shipping (ePacket, not the unreliable China Post).

2014 April 15

Hysteresis lab too long

After re-reading my notes on last year’s hysteresis lab, I realized that my schedule for this week in the Revised plan for circuits labs, with both the hysteresis lab and the sampling lab in the same week was too ambitious. There was a chance that the students could do the hysteresis lab in 3 hours, but only if they already understood everything in the pre-lab assignment and worked efficiently. A lot of the students, however, only learn by repeatedly bumping into a brick wall, and don’t really have any notion of solving general problems before they encounter them in the lab, so I expected a lot of students to waste time today doing the pre-lab assignment in lab.  My expectation there was amply fulfilled.

I decided to cancel (or at least postpone) the sampling and aliasing lab, and spend both Tuesday and Thursday on the hysteresis lab. I don’t think we’ll be able to double up the labs next week, but the week after may be a little thinner, and we may be able to squeeze in the sampling and aliasing then.

Everyone got the two input thresholds for the 74HC14N Schmitt trigger (with 3.3v inputs) measured, and they all got essentially the same values.  Some of them took a long time getting there, because I did not hand them a test circuit, but asked them to come up with one themselves. One group used an adjustable bench power supply for Vin, but the rest (eventually) came up with using a potentiometer as a voltage divider and recording the input and output with the PteroDAQ software. For some, I had to do more guidance than I really liked, getting them to decompose the problem into having the Schmitt trigger as one component with a variable input and the pot as another component with a variable output. Since they had done a very similar setup for the mic lab last week, and I had explained the pot as a variable voltage divider at that time, I had expected them to instantly see how to apply it, but most did not. Still, everyone eventually got it, and I think that the ones who struggled the most now have a much solider understanding of voltage dividers and potentiometers than if I had just given them a circuit to copy.

I did get to show the PteroDAQ users a useful feature of the program—by connecting the output to PTD4 (or one of the other digital pins of port A or port D), PteroDAQ can be set to trigger whenever the output changes values.  A few sweeps of the pot past the threshold values reveals quite repeatable voltages at which the transition occurs, without having to page through a long trace of uninteresting info.

The groups then struggled with coming up with the right RC time constant for their oscillators. I’m probably going go over the calculation in class tomorrow, since I think everyone got a reasonable result, but not everyone was clear enough about their method to write it up well. I want to see clear explanations in the lab report, so I’ll go over it to help them smooth out the bumps in their explanations.

Some other things I want to do tomorrow:

  • Talk about Carol Dweck’s work on mindset, as one of the students frequently wonders aloud whether the class is too difficult for her, and some of the other students may be thinking that they “don’t have the ability”. So far as I can tell, everyone in the class has the ability to master all the material in the class—but I need to get them out of “fixed mindset” into “growth mindset” and recognize that they can do more than they credit themselves with, if they are willing to work for it.
  • Have them go over their computations of the finger-touch capacitive sensor and compare answers with each other. I want to make sure that they express their answers in standard units (like pF) and that they are careful about units (mixing mils, cm, and F/m probably confused a lot of students).
    During the lab time, I had each group come up to use my micrometer to measure a double-thickness of packing tape. I must be using a different roll of tape than in previous years, because we consistently got about 1.7mil (0.043mm) with my Imperial units micrometer (that is we measured 3.4–3.5 mil for the double thickness), while last year I had 2.2mil.  I should probably get a metric one, but I may be too cheap to spend $14 on a tool I use once a year in this class. Besides, this gave me an opportunity to tell students the difference between mil and mm, which most of them did not know. Since a lot of materials still come with thickness specifications in mil, they should at least be aware of the existence of the unit and the potential for confusion. (Several had done the prelab homework assuming 2.2mm, which would be very thick packing tape.)
  • Assign one of the voltage-divider do-now problems from last year. Perhaps this one?
    • What is the output voltage for a 3-resistor voltage divider? (I’ll draw the circuit)
    • You have sensor whose resistance varies from 1kΩ to 4kΩ with the property it measures and a 5v power supply.  Design a circuit whose output voltage varies from 1v (at 1kΩ) to 2v (at 4kΩ).

Two or three of the groups managed to get their relaxation oscillators to oscillate and measured the frequency on the digital scopes. One group got as far as adjusting the R and C values to get the frequency within the spec given in the homework (10kHz to 100kHz), and started the next step (making the capacitance touch sensor out of aluminum foil and packing tape). Lab on Thursday will consist of everyone getting the oscillators working in spec, testing the change in frequency for a finger touch (which may need some capacitor changes, as I think some are using a small R and large C, which won’t have enough frequency change with the small capacitance of a finger touch), testing the oscillator with the KL25Z boards (with my new code), and soldering up the circuits on PC boards.

Students are beginning to get the  message that when they ask me whether some result is right, my answer will be what my father taught me: “Try it and see!” When they ask me for help using the equipment or debugging when they get too frustrated, I’m more helpful, but I’m not going to check their work for them when the real world can do that so much better.  Besides, the simple models we are using are not all that accurate—even if they do a perfect job of the computation, the real-world behavior will be enough different that they’ll need to tweak the component values anyway. This is another lesson I want them to get—the real world is not as simple as the spherical-cow models used in physics classes and intro EE, but the spherical-cow models are nonetheless useful.

 

 

 

 

2014 April 9

First mic lab slightly too long

Filed under: Circuits course — gasstationwithoutpumps @ 09:32
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The first half of the microphone lab took a little longer than anticipated.  I had expected it to take about 2.5 hours, with some groups taking the full 3 hours, but it took more like 3–4 hours.

I have two conjectures about reasons for the extra time:

  •  I had the students label all their bags of capacitors. this had originally been planned for a week ago, but the capacitors had not been ordered in time, and Thursday’s lab had been way too packed already, so this was the first opportunity we had.  I probably should have waited until this Thursday, when the lab time is less packed.
  • The group that fell the furthest behind had really terrible luck, having sat at a bench where both multimeters had blown fuses and two sets of multimeter leads had open circuits. I helped them debug their setup, but we did not initially suspect the test equipment, and the delay in finding the problem cost them at least half an hour. It also cut into their confidence in debugging their own circuitry later in the lab. Problems with the equipment is one of the difficulties with using a shared lab—a lot of the courses are taught by EE TAs who do not bother to teach students proper use of the lab equipment (if they even know it themselves), so there is often damage of this sort to deal with.

A number of the students in the class are suffering from “imposter syndrome”—not confident of their abilities to master this new material. I’ll have to reassure them that they are doing fine—this class is intended to be pushing them into unfamiliar territory.  I may take a moment in today’s class to mention both “imposter syndrome” and “zone of proximal development”, so that they are aware both that it is ok to be uncomfortable and that I’m trying to maximize what they are learning.

Students had a lot of trouble wiring up their breadboards accurately. Most of the lab time was taken up with students asking for my help and my taking a quick look at the breadboard and telling them that it didn’t match the schematic they had copied. I eventually had the students write on each wire of the schematic what row (or rows) of the breadboard it was on, so that debugging the connections was easier.  I’ll have to try to remember to put that in the instructions for next year, as a way to get the students to learn to debug their breadboard wiring more independently.  I should also add a picture of the trimpot and an explanation of what a potentiometer does—students had a little trouble figuring out what the 3 pins on the package were for.

I’m pleased with the latest version of PteroDAQ, as students had no trouble getting their measurements. Adding a patterned light sequence to the reset sequence to let students know that they had the latest version of the software installed was very useful.

Many of the groups managed to look at their data in the lab using gnuplot, and collecting more data as a result of what they saw. The students got 1000s of data points that fall nicely along a curve, and they were able to superimpose different data sets that had different scaling for the current measurements. We’ll have excellent data to use in class today for fitting models to. I’m not going to give them real FET models for the FET in the electret mics, though.  Instead we’ll use some simple empirical models:

  • current source.  This is the same as a saturation current model.
  • resistance. This is essentially the same as the linear-region model for FETs.
  • blended model with R_{FET} = \sqrt{R^2 + (V_{DS}/I_{SAT})^2}  This is a simpler blend than is usually used in FET models, I think, but it fits the data fairly well.  This blend is mathematically very similar to the ones that compute the gain in RC filters (where we take the magnitude of a complex number and either the real or the imaginary component provides most of the contribution).  Using the same function for rounding the corner when we join two straight lines in different contexts reduces the math burden on the students.
  • blended model with R_{FET} = \sqrt{R^2 + (V_{DS}/I_{SAT})^{2\rho}} The extra parameter here is to handle the increase in saturation current with increasing drain-to-source voltage. Normally, that is modeled with a fairly complicated “channel-length” model, and is not even mentioned in intro circuits classes.  But the phenomenon is very obvious in the data, and can be adequately modeled in the electret mic for our purposes with this 3-parameter empirical model.

I will have to give them one more concept about FETs: that the derivative of the saturation current with respect to the gate voltage is proportional to the saturation current. I’m not going to derive that for them from some more general model, because we have no way (in the mic) of actually measuring the gate voltage. Later in the quarter, when we look at FETs again before doing the class-D amplifier, I may give them a slightly more detailed model of an FET.

In addition to gnuplot tutorial today, I want to give them an intro to complex impedance, but I doubt that we’ll get far enough for them to choose the right size for a DC-blocking capacitor for the AC mic lab tomorrow. I may have to suggest that they try one of the biggest sizes of ceramic capacitor that they have (either the 4.7µF or the 0.1µF). We’ll need to get to RC time constants and corner frequencies before next week’s lab though.

 

 

 

 

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