Tonight I did another test of the pump with control and sensing. The flow sensor worked right out of the box. The voltage monitor I made using two resistors and an analog pin took a lot of fine-tuning, but after that it matches my multimeter’s opinion. The water level sensor doesn’t work at all yet, and the water temperature sensor looks like it’s going to be complicated to set up.

On the data logging shield, the real time clock (RTC) worked immediately, but the SD card gives an ominous message (“Initializing SD card…initialization failed.”). On the Adafruit forums, the main culprit seems to be crappy soldering. Totally likely, so here are photos:

Here’s hoping the ever-helpful Adafruit folks can tell me what I did wrong. PS: in beginning hobbyist electronics, there’s a hell of a lot of value in paying a bit more to get responsive, high-quality customer service, and wow, does Adafruit deliver.

Edit 17 July: yep, crappy soldering. Adafruit support even pointed me to the specific board parts (the digital pin headers) that were probably at fault. Fixed.