Gas station without pumps

2013 February 20

Eighteenth day of circuits class

Filed under: Circuits course — gasstationwithoutpumps @ 19:18
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Today’s class started with two do-now problems:

The first circuit should have been easy for the students, but the second one was brand new to them.

The first circuit should have been easy for the students, but the second one was brand new to them.

I had expected the first circuit to be easy for them as it was quite similar to one we did last Wednesday, but students still struggled with it—only one student recognized it as a gain 5 non-inverting amplifier with a reference voltage of 2.5v (that is, that  (Vout–2.5v)= 5 (Vin–2.5v) ).  So I spent far more time than I had planned going through the analysis with them until I think that everyone could do the analysis.  But I thought that at least half the class should have been able to do the analysis before class started, so maybe I’m still fooling myself about what they understood.

Some students recognized that the second circuit had positive feedback, but no one then knew what to do.  So I stepped them through a hand simulation of the circuit, computing the voltage at + input from Vout when Vout=0 and when Vout=5V.  They had more difficulty than I expected in dealing with a voltage divider that was between 2.5V and 5V.  I tried showing them 2 or 3 ways to look at the problem, and get the results that V+ is 2V when Vout=0V and V+ is 3V when Vout=5V.  We then walked through the Vout vs. Vin curve as Vin swept up from 0V to 5V and as it swept back down again.  At least they recognized the transfer characteristic as that of a Schmitt trigger inverter.

I then talked about why we had used as Schmitt-trigger chip rather than building this circuit for the hysteresis oscillator board:  Schmitt trigger chip is cheaper, with no external resistors the soldering is easier, and the op amps have a deliberately limited output speed (slew rate) measured in V/µs.  I didn’t remember the number for the MCP6004 chips they are using, but I just looked it up and it is 0.6 V/µs, so making a 5V swing would take 8.3µs.   Since we don’t want digital signals to spend any time in the intermediate voltage range, we don’t want this speed limitation on a Schmitt trigger.    The 74HC14 has a transition time of about 15ns, over 500 times faster.

I’ve still not talked about gain-bandwidth product, but I should probably do that for the instrumentation amp labs, where they’ll be going for higher gains (though at lower frequencies).  The MCP6004 has a gain-bandwidth product of 1MHz, and the INA126P has a gain-bandwidth product of about 900kHz, both of which are large enough that they could probably do all the labs in the class without being aware of the gain-bandwidth tradeoff. If they try for a gain of 2000 (possibly appropriate for the EKG) in a single stage, they’d be limited to about 500Hz (which is more than high enough for the EKG).  If they try for a gain of 100–250 (which may be appropriate for the pressure sensor) in a single stage, they’d be limited to about 40kHz (way more than they need).  The class-D audio amp, which needs to deal with higher frequencies, does not need a gain of more than about 5.  (In fact, they could probably get away with no preamp at all for the microphone—but it is probably easier if they include one.)

After the two do-now problems, we talked a little about the lab reports.  I had given a lot of B’s on both lab 5 and the redone reports from lab 4, because I’m still seeing a lot of errors in schematics and computations.  I pointed out that engineering is about making designs that work, not ones that have almost the right idea, but are miswired or have 12Ω resistors instead of 12kΩ resistors.  As long as they are making such mistakes, I can’t give them A’s on their reports.  Some of them are trying for med school and are very worried about their GPAs—they wanted me to be more lenient on the grading.  My co-instructor backed me up on the grading standards, saying that he uses similar standards in all the other EE lab courses he teaches (and he teaches about half the EE labs on campus).  I did remind students that they can redo any reports for which they are not satisfied with the grade, though only one student so far this quarter has taken me up on the offer.  I think that I’ll soon see a couple of  students redoing reports that were “close but no cigar”—ones that were well written but not checked for errors in the schematics or computation. Students who are struggling more with the concepts are less likely to redo the reports, unless they’ve suddenly had a breakthrough in understanding stuff that they didn’t get earlier in the quarter.

I’d really like to be able to give some A’s—I think that several of the students are capable of better reports, and it worries me that despite the many hours they put into the reports, they’re still being so careless about the engineering details.  I’m more forgiving of typos and grammatical errors (as much as they grate on me) than I am of engineering errors that make their work unreproducible.

Next week (Friday March 1), I’ll give another quiz, covering the material of the first quiz, plus a little on inductors and negative-feedback op amps.  I think that students will be able to get the voltage-divider problems this time, but may still struggle with the op amp problems.  I think I’ll try to make the quiz a bit shorter, but include one or two small design problems (easier than what they need to do for the prelabs on the amplifier labs).  I need to give them a new study sheet, since the old one does not include inductors or op amps.  I think that about 3 or 4 more formulas (on the order of Z_L(f,L) = j 2 \pi f L) should cover all they need to memorize—the hard part of a circuits class is not memorizing formulas, but figuring out how to apply them.

I’ve added some detailed instructions about checking schematics into the Lab 8 handout, thanks to Mylène’s suggestion that I add netlists to the lab handout.  In fact, I added both netlists, which list what is connected to each node of the schematic, and wiring lists, which list each component and wire that needs to be soldered into the PC board.  I also suggested using a highlighter on a copy of the schematic to verify the netlist and (separately) the wiring list.

After class, I ended up talking with several of the students both about the class (they like the labs but are frustrated by the high grading standards) and about the senior-thesis communication course that I’ll be teaching next quarter, and which many of them will be taking.  I’ve done a similar class in the past, but as a 5-unit course, and next quarter’s is only 2 units, so I have to scale back the workload.  I’m not sure which parts they most want to see scaled back. (In the past we did 5 drafts of the thesis, a mid-quarter practice presentation, weekly individual meetings, 2-minute elevator talks, a poster session, and a presentation before a large audience at the end of the quarter—about right for 5 units, but too much time for only 2.)  I’m anticipating that course taking up a lot of time, but in a different way than the circuits course—I’ll be spending almost all my time providing feedback and almost no time preparing lectures, and certainly no time designing, testing, and writing up handouts for labs.

I had a demo prepared for today in anticipation of tomorrow’s lab, but we ran out of time before I could talk about tomorrow’s lab at all. The demo is short enough that I can do it at the beginning of lab, though, and the whole sampling and aliasing lab is mainly a demo anyway.

The only design component is that they need to design two RC filters: a high-pass filter with a very low corner frequency to do level shifting of the frequency generator output to fit in the unipolar (0–5V) range of the Arduino ADC and a low-pass filter to see how aliasing can be reduced by filtering below the Nyquist frequency.  We only do simple RC voltage-divider filters in this class (except for the LC circuit for the class-D power amp), and they’ve already done a high-pass filter for level changing for their op-amp audio amp, so the design task should not be too hard for them.  I expect to have to help a few of them in the lab, though because they are still having difficulty thinking about connecting two resistors as a voltage divider to make the equivalent of a different voltage source with a different resistance (as in today’s do-now problem).

Incidentally, the inductors arrived from Digikey today, and I’ll be able to try the LC filters for myself.  Unfortunately, the leads on the inductors are too fat for the breadboards, so students will have to connect them with clip leads (it’s a good thing I included some alligator clips in the parts kit).  The inductors are also surprisingly heavy at about 8.8g each.

My to-do list includes the following urgent items:

  • Updating the study sheet 2013 Feb 20
  • Testing the class-D amplifier lab 2013 Feb 24 (one last check needed with proper power supplies)
  • Finishing the class-D amplifier handout 2013 Feb 27
  • Writing the quiz 2013 Feb 24 (draft done)
  • Reading some huge number of CVs and research statements for a faculty recruitment (protein engineer)—I’ve not even tried opening the web site yet.
  • Reading a 164-page PhD thesis that I just got today (I’ve got 2.5 weeks to read it)
  • Running bioinformatics labs in a high-school classroom next week Half-done 2013 Feb 27
  • Attending the county science fair lead-judge meeting next Tuesday (I have to recuse myself from senior-level overall judging and from senior-level math/software, because of a conflict of interest).
  • (See Missed a meeting due to lockdown for my excuse.)

  • Catching up with my son on the homeschool physics homework and coming up with a lab for the new chapter (we still haven’t done the RC-filter lab I’d planned for Chapter 20—he was feeling under the weather over the weekend, and I was swamped with the handouts for the circuits course).
  • Arranging travel and lodging for a college visit for my son and me during our Spring break.
  • Writing up the interview I had with Suki Wessling for the Santa Cruz Sentinel, before I forget everything she said.  (I have notes, but no recording.) First draft done 2013 Feb 24

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