Gas station without pumps

2013 February 19

Pressure-sensor lab handout written

Filed under: Circuits course — gasstationwithoutpumps @ 22:32
Tags: , , , ,

In All weekend and handouts still not written, I complained

I spent all day Saturday and Sunday (except for a few hours grading) working on the lab handout for the class-D power amplifier lab.  It was going great on Saturday, but I decided to include a portion of the lab on doing an LC output filter, rather than just directly connecting the speaker between ground and the FETs of the output stage as I had originally planned.  That opened a big can of worms.

and I concluded with

Since I need to do more experimenting with the class-D design to find the problems before handing it over to the students, I’ve moved that lab from week 8 to week 9, giving me another week to finish the handout (which is already 12 pages long, and not finished yet).  That means that I have 2 days to write the new lab 8: the instrumentation amplifier for the strain-gauge pressure sensor.  I’ve built and tested such an amplifier (even demoed it on the first day of class), so there aren’t any surprises waiting for me, but I have to write up not only an explanation of instrumentation amps, but also how to do layout for the protoboard.

Hmm, I just realized that I don’t have any posts on the blog for the rev3.0 instrumentation amp protoboard that I designed using the MCP6004 quad op-amp chip instead of the MCP6002 dual op-amp chip (still with the INA126P instrumentation amp chip).  My old test of the pressure sensor lab was with the old protoboard. Maybe I need to redo the pressure-sensor lab with the new board, to make sure that there aren’t any problems with the PC board design.

So I’ve got today and tomorrow to redo the pressure-sensor lab and write up the lab handout, plus finish the grading and figure out how I’m going to present sampling and aliasing on Wednesday, before the Thursday lab.  (I plan to use my son’s stroboscope, but I’ve not figured out what motion we’ll sample with it.)

Well, it is now Tuesday night ending my 4 day all-work weekend (all my weekends have been like that for quite a while).  I did get the grading done and the grades recorded. I also got the handout done for the pressure sensor lab—I even got feedback from my co-instructor on a draft (pointing out at least three things that I needed to fix).  I also designed and soldered up another amplifier today, this one adding a low-pass filter between the INA126P instrumentation amp and a second-stage op amp. The new protoboard works fine and is easier to work with than my first protoboard design (which wasted too much space in providing power connections—now I just take +5V and Gnd through small screw terminals).

I took the unfiltered instrumentation amp output and the higher-gain filtered output both out to screw terminals and looked at them with the Arduino.  There is less noise on the filtered output, but whether that is due to the filter or to the higher gain (so less discretization noise in the ADC) is not clear.  I’ll probably have to look at them with a higher resolution device than the Arduino ADC to see.

Incidentally, I really like the block of 4 screw terminals with 0.1″ pitch that I’m using for the protoboards and the pressure sensor breakout boards.  They may not be able to handle the current of the 3.5mm and 5mm screw terminals that Adafruit and Sparkfun use, but they take up less space and fit much more neatly on the 0.1″ grid.  They provide me with much easier and more secure connections than the header pins that I used to use.  (With the 0.1″ spacing, if I want header pins, I can just fasten a row of double-headed header pins into the screw connectors.)

I’m worried about how much time soldering the amplifier will take.  Despite all the admonishments I’ve given and will give them, I suspect that half the students will come to lab next week with the prelabs only half done—many won’t have done a carefully checked schematic and layout before coming to lab, so they’ll be rushing to do a half-assed one that is missing crucial connections or shorts power and ground.  And then their soldered board won’t work and they’ll spend many more hours trying to debug it than it would have taken them to check their schematic carefully in the first place before wiring things up.  I made everyone redo the lab writeups for the first soldering lab, because they’d mostly been very sloppy in their schematics (among other problems).  Even on the redone lab reports, after being told that they needed to fix and double-check their schematics, there were still a lot of the same sorts of errors, and the lab reports for lab 5 (the first audio amp lab) were not much better.

I wonder if all the engineering students have the same carelessness about details and poor lab notebook skills.  I have not been teaching students how to keep a lab notebook. I thought that had already been covered in their previous 4 years, and I tend to be a poor example: I had to reverse engineer my previous instrumentation amp circuit for the pressure sensor, because I couldn’t find the schematic anywhere—I then made a clean copy of the schematic with CircuitLab, and put a printout of the schematic and the board together in an envelope, so they would not get separated again. I also printed out the schematics for today’s amplifier design, and put them together with the design specs and the layout information in an envelope with the board I designed today.

I’ve been using this blog as my lab notebook for things related to the course, except for designs that I don’t want to give away to the students (which is why the schematic for the pressure sensor amplifier does not appear in this blog, which in turn is why I lost it).  I’ve only occasionally used a paper lab notebook, generally preferring “README” files in the computer directories associated with particular projects.  It’s not as good if I ever want to patent something, but it is a lot easier for me to maintain.

I still haven’t figured out exactly what I’ll do tomorrow in class.  I’ll bring my son’s stroboscope (a Xenon flash tube one made from a Velleman kit), but I’m not sure what I’ll use for periodic motion.  I’ll also need to prepare a “do-now” problem, to keep them sharp on their voltage dividers and op amps.

Next weekend will be dedicated (again) to doing the class-D amplifier lab handout, and building the class-D amplifier to make sure that the lab is doable.

 

2 Comments »

  1. My students often don’t check their schematics properly because they’re embarrassed to admit that they don’t know how to evaluate whether a breadboard matches a schematic. One tool that helps (when I have time to include it) is to show them how to systematically make a netlist (it helps to use several colours of highlighter). It also improves their visual-spatial skills, and helps them realize that it doesn’t matter whether R1 is “to the right of” R2’s connection to the inverting input or not. It might be worth giving them a quiz question early in the semester (or maybe even a do-now question) showing a few simple schematics drawn differently, and asking them to evaluate whether they’re electrically the same.

    Comment by Mylene DiPenta — 2013 February 20 @ 09:12 | Reply

    • I haven’t had problems with students having trouble with the orientation of parts in the schematic. I routinely give do-now problems with the voltage dividers in different orientations (horizontal, vertical, L-shaped), and that has not been causing any noticeable problems.

      I had not thought to require a netlist for such a small circuit, and I don’t really trust them to do a careful job of making the netlist, but it is a standard procedure, and might help them, so I’ll add that to the handout. Thanks for the suggestion!

      Comment by gasstationwithoutpumps — 2013 February 20 @ 09:57 | Reply


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