Today’s lab was a “tinkering lab”, using the oscillator board they made in the hysteresis lab as a component to make a light-controlled sound generator. They did not have a specific goal, other than to use the oscillator board, to produce sound on the loud speaker, and to be able to control/change the sound by shadowing a phototransistor.
I had them try to predict the effect of adding resistors or capacitors between each pair of points on the oscillator board (+5v, GND, Cap, and OUT), and to test how many of their predictions were right. A lot of students turned out to have such weak mental models of the oscillators that they couldn’t figure out what would happen in even the simple cases. Few of them were able to do the prelab exercise of predicting what would happen if the added components in various places. Most of them eventually remembered or figured out again how the oscillators worked, and so were able to modify them in reasonable ways, but a lot of lab time was wasted doing what was supposed to be prelab work.
A few of the students were trying to figure out what to do without even a schematic in front of them, and when they drew a schematic it did not correspond to what they told me they were trying to do. They worry me a bit, as by this point of the quarter the students should be able to copy schematics accurately and wire from them, even if they are sometimes a bit hazy on how the circuits work.
I’m also finding that several of the students seem to be incapable of learning from written material—students with no understanding of FETs as switches seemed to understand it fairly well after I explained it—using almost the same words as I used in the lab handout. I’m not sure I understand why the explanation given verbally seemed to work, while the same one given in writing seemed to fail. I find it difficult to believe that they really can’t learn from written materials, so there must be a different explanation. Too much in the handout, so that they shutdown while reading it? Not having read the handout? (possible with some of the students, but unlikely with the ones I observed the problem in)
I rely heavily on written material to communicate both design goals and details about the parts they are using, so I really need to figure out what is going wrong with the written channel for information. Is there a way I could write the lab handouts that would be more digestible for the students? If the difficulty absorbing information from written sources is a real one, I may have to do more lecturing, though that seems like the wrong thing to do, as engineers have to be able to read spec sheets and learn from written sources.
Only a few of the students did the systematic exploration of what components in different places and predicting/observing the effect. I may need to reduce the emphasis on that next year, and focus their attention more on potentially useful interventions (one pair wasted a lot of time looking at the least interesting pair: connecting extra components between power and ground).
Everyone eventually managed to get loud sounds out of the loudspeaker, and to control the sounds by shadowing the phototransistor, but I think that some just copied what others had done, without completely understanding what they were doing. I think that they all ended up with essentially the same design I had, with the phototransistor shorting out the capacitor in bright light. Everyone did eventually get the right pFET driver for the loudspeaker, though one group worked through the other three circuits first, and found why they were less desirable. We’ll see in the lab reports whether all the groups can accurately draw their schematics and explain how their devices work.
Overall, I’m moderately pleased with how the lab went. Students deepened their understanding of RC time constants and the hysteresis oscillator circuit, and they got a feel for “cut-and-try” engineering. But I think I need to rewrite this lab for next year, with a little more direction in which pairs of terminals are worth exploring and how to think about the effect on the oscillator.