This project is powered by AA batteries. Normal, non-rechargable AA batteries. But I'd also like to attach it to USB for stuff, and draw power from there when it is so attached.

So, the worry is that USB power will back-bias the batteries and they will eventually leak or explode. That would be irresponsible. So I should probably do something about that.

Now, the easiest option is to simply use a 3 position switch, which has one direction for battery power, and one for USB power. But that's confusing, and I'm not sure what happens when you are attached to USB but on battery power. I don't want to do stuff that blows up computers USB ports, either.

Now there are plenty of solutions that provide all sorts of protection for circuits against voltage spikes, reverse polarity, etc. Some of the protection chips alone are $2.50-$5, tho, and whole board solutions from Adafruit are $10+. This project is shooting for a $10 price point for the whole thing, so the price points on that kinda stuff aren't really reasonable! And many of those parts just can't be sourced easily in China, where the PCBA happens. If it isn't on AliExpress, I can't use it!

So, I'm looking at a bare minimum solution that adds a few pennies to the design and doesn't impede performance. 

The simplest transparent option seems to be to put a diode in series with the battery and the power in, and another on the USB side. 

But I'm a n00b. What kind of diode?

I really had no idea, so I got in the Hackaday.io Hack Chat and talked to Al Williams and Jez Boxall. Al pointed out some useful articles ( 1 & 2) and suggested a Shottky diode with a low V_{R .}  After some looking around, I found the 1n5822 Schottky diode on AliExpress for barely over a penny! Al had decamped by this point, but Jez seemed to think the specs were in range for this project. Second opinions are good!

I also found a lovely set of articles on the topic of polarity protection which seemed to be in the right direction for what I was doing.

I finally decided to use a diode on the positive side of batteries, and on the positive side of the USB plug to prevent backflow from batteries. Adds 2.1 cents to the design; I think they client will accept it!

Next thing to do is to order parts and wire it up!