I've been thinking about how to power things. Most PC case fans will run at lots of different voltages, but they expect to run at 12v (DC of course). If a different voltage is supplied, they just run at a different speed. There are some fans available that expect 5v, but they are for some reason a lot more expensive (maybe just market forces). For predictability and management, it's best if the power supply is steady, but it probably does not need to be regulated for running the fans as long as it's not grossly variable.

All along, I've been expecting to power things with a 12v DC wall wart or bump-in-the-cord power brick. I already have several spares from dead electronic components. (I've been following Tony Brobston's smart vent project. He ran low voltage power wires through his home's duct work from a central transformer. It's a tidy solution, but it may be a bit too involved and invasive for me.)

The ESP32 has a couple of power options. 

• I'm planning to use dev boards that have the usual USB 5v input, but I don't want to run a second power cord to the register.
• You can feed 3.3v directly to a pin, but only if it's regulated. I'm trying to see if I can avoid the use of a regulator in the circuit, because....
• The ESP32 dev board has an on-board 3.3v regulator, and you can feed unregulated 5v-12v into a pin. For efficiency of the regulator, closer to 5v is better than higher.

I think I will be able to feed my 12v power supply into the overall circuit. That 12v supply will go directly to the fan power input (well, as I mentioned earlier, it will probably actually go to a MOSFET that can switch the fans completely off). Then a couple of voltage divider resistors will tap the 12v supply to provide something above 5v to the unregulated ESP32 input. For that last part, I'll have to check the stability of my supply voltage under load to be sure it doesn't drop too much and let the ESP32 input fall below 5v. That would also be undesirable for the fans, but that's less critical.