The hysteresis lab was the least successful one so far this year. Students got the designs done and built, but they learned less than they could have from the lab (too much copying of “answers” without understanding) and the lab took too long for the time allotted.
I’m considering going back to a 2-day hysteresis lab, but expanding the lab slightly to include using an nFET to control a loudspeaker (probably with a series resistor to limit current to 500mA). Adding the nFET as a low-side switch would be a useful thing for them to know about (it is a standard arrangement for controlling heating elements, solenoids, and other devices in lab equipment), and would prepare them better for the class-D power-amp lab.
The problem here is that a good oscillation frequency for a touch sensor uses a small capacitor (hence a high frequency), while a good oscillation frequency for the loudspeaker is much lower (200Hz–4kHz). I can have them add an external capacitor (resulting in little or no touch sensitivity) when driving the loudspeaker, or I can have them build two oscillators: a high-frequency one and a low-frequency one.
I’d want to redesign the board they solder the hysteresis oscillator on to make room for the nFET and loudspeaker connections, plus giving them more holes for connecting up unused pins on the 74HC14N chip. This will probably make the board bigger than 2.5cm×5cm, raising the price by 50¢. They’d need an extra nFET in their kits (another 40¢) and perhaps another screw terminal (80¢). An extra buck or two for the parts is no big deal, as the lab fee is already larger than what the students are getting, though the extra cost for having to use UC-approved vendors may be eating up most of the “excess” fees.
The lab could be split between the breadboard and the soldering phases, with students who are behind still completing the breadboarding in the second lab.
One question I’ve not resolved is how to reschedule the labs so that there is room for a 2-day hysteresis lab. Is there any lab that currently takes 2 days that can be squished to 1 day (with less damage than squishing the hysteresis lab)? How do I keep the reports on schedule so that I can grade over the weekend?