The second half of the mic lab went fairly well, but there were a couple of overly ambitious requests in the handout that I’ll have to trim out for next year. Because we have not gotten to complex impedance yet (tomorrow, I swear!), the students were unable to choose a reasonable size for the DC-blocking capacitor, and guessing was not good enough. The 10MΩ input impedance of an oscilloscope with a 10× probe makes for too long a time constant with the 0.1µF capacitor I initially suggested, at least with the digital scopes—they did not manage to get the DC offset removed even after a minute, which surprised me. Students got decent results with a 0.022µF capacitor, though. I even got some of the students to be able to make measurements with the Tektronix digital scopes (always a feat, since they have mind-bogglingly complex menu systems).
I did tell the students not to bother with the last question on the handout and just to write up what they actually did.
It took the students longer than I had expected to come up with a reasonable value for the pullup resistor for the mic. But I was careful not to be too helpful, so that I’m reasonably sure that at least one in each pair of students knew how they got their answer. I did have them add load lines to their i-vs-v plots of the electret microphones, corresponding to rounding their desired pullup up or down to the nearest value they had in their kits. That probably added a little time over a simple rounding, especially since I suggested to a couple of the students that they think about which resistor would give higher sensitivity.
I did have one student ask what a “pullup” resistor was—I had used that term in the handout without ever explaining it! I gave a one-minute lecture explaining that a pullup was a resistor to the positive power supply and a pulldown a resistor to ground (we had examples of each already on the whiteboard). Speaking of things on the board, I’ll have to remember to bring markers to the lab on Tuesday, as the ones in there are all dead. A spray bottle of alcohol and some rags for cleaning the year-old buildup off the boards would also be good.
Even the pair of students who had run over on Tuesday finished on time today, despite collecting all the data that the other students collected on Tuesday, so I’m feeling a bit better about the size of the labs.
Next week may get a bit hectic, though, with two unrelated labs: hysteresis and relaxation oscillators on Tuesday and sampling and aliasing on Thursday. I’ll have to remember on Tuesday to upload the hysteresis oscillator code to all the machines in the lab.