Backwards polarity protection; installing 2 AA batteries into project

In my project I have to Wemos D1 Mini's.
1 as Station, the other as Access Point.

Station will remain plugged in via USB

The AP Wemos, I want to run either on USB power or 2 AA batteries with on/off switch.

Best feasible way to protect the 2AA batteries? I'm thinking Diode... but I don't know enough about diodes to make the proper selection.

I apologise up front. I posed that in another thread. I couldn't find that post or my question. So I'm reposting. I'm still working on schematic diagram. I just saw an error, so have to redraw. Kids ate the eraser. :slight_smile:

Best feasible way to protect the 2AA batteries?

From what?

Don't I need to protect the batteries when the Wemos is plugged in via USB?

Maybe a schottky diode but you will lose about 150 – 450 mV

Maybe have a manual switch that you can toggle form one to the the other.

.

Or a 5 volt relay that is picked when USB power is available.

.

Ah yes, like a 3 way switch. Center out to Wemos, left side of switch to Bat+, right side to ?

Wemos has USB port.

So if I wanted to run it via usb power. Run THAT + to the other post of the switch?
I just found a micro USB board. Yes, This should work.

GND's all tied together?

One of the three ccts.

2016-12-17_16-54-28.png

.

Thank you! I was just looking at it all WRONG!

project.JPG

Why not just use a polarised battery connector ?

Did i misunderstand ?

Are you trying to protect the circuit or the batteries

Boardburner2:
Why not just use a polarised battery connector ?

Did i misunderstand ?

Are you trying to protect the circuit or the batteries

I think you misunderstood. He wants to leave the batteries connected at all times and wanted to isolate when he plugged in USB power.

Tinman: that is correct. In my original design, I had no switch. All positives were tied together and the same with GND. Was worried about what would happen to the batteries.

I think I got this now. Waiting on my Wemos D1 mini from China.
proto.JPG
Battery connection on the left, USB on the right.