Need guidance making my Wemos D1 mini project usb or battery operated

Projects General Guidance
1

I'm trying to make my Project to run on either usb or 2 AAA batteries.

Here's what I'm thinking:

Battery tray Pos to Pos on breadboard rail
Battery tray Neg to Neg on breadboard rail
Wemos D1 mini 3V3 pin to Pos on breadboard rail
Wemos D1 mini G pin to Neg on breadboard rail
I will have a tiny sliding switch between Pos rail 3V3 pins

Here's where I think need help. When I plug it in via the usb, I would need a diode to protect the back flow to the batteries. I know what a diode does, but that's about as far as my knowledge goes. I need help selecting the right type and size.

Will any of this brick my Wemos? What are my flaws?

And the sketch for those that want to look at that;

/*
Using my Wemos D1 mini as remote control for Adafruit
Audio FX board.  This sketch is for the Access
Point/remote control button.
*/

#include <ESP8266WiFi.h>

extern "C" {
#include "user_interface.h"
}

char apName[] = "SoundFx";
char apPassword[] = "Adafruit";

void setup() {
  Serial.begin(115200);
  delay(100);

  WiFi.softAP(apName, apPassword);
  WiFi.mode(WIFI_AP);

  pinMode(D4, INPUT);// not sure what to put here
  Serial.println();
  Serial.println("AP Started");
}

int value = 0;
int stations = -1;

void loop() {

  int stationsNow = wifi_softap_get_station_num();
  if (stations != stationsNow) {
    stations = stationsNow;
    Serial.print("Station count: ");
    Serial.println(stations);
  }

  int newValue = digitalRead(D4);

  if (newValue != value) {
    value = newValue;
    Serial.print("value=");
    Serial.println(value);

    struct station_info *stat_info;
    struct ip_addr *IPaddress;
    unsigned long uintaddress;

    Serial.println("Updating connected stations...");

    stat_info = wifi_softap_get_station_info();
    while (stat_info != NULL) {

      IPaddress = &stat_info->ip;
      uintaddress = IPaddress->addr;
      String host = String(ip4_addr1(&uintaddress)) + "." + ip4_addr2(&uintaddress) + "." + ip4_addr3(&uintaddress) + "." + ip4_addr4(&uintaddress);

      Serial.print("connecting to ");
      Serial.println(host);

      // Use WiFiClient class to create TCP connections
      WiFiClient client;
      const int httpPort = 80;
      if (!client.connect(host.c_str(), httpPort)) {
        Serial.println("connection failed");
      }
      else {

        // We now create a URI for the request
        String url;
        if (value == HIGH) {
          url = "/LED=ON";
        }
        else {
          url = "/LED=OFF";
        }
        Serial.print("Requesting URL: ");
        Serial.println(url);

        // This will send the request to the server
        client.print(String("GET ") + url + " HTTP/1.1\r\n" +
                     "Host: " + host + "\r\n" +
                     "Connection: close\r\n\r\n");
        delay(50);

        // Read all the lines of the reply from server and print them to Serial
        while (client.available()) {
          String line = client.readStringUntil('\r');
          Serial.print(line);
        }

        Serial.println();
        Serial.println("closing connection");
      }
      client.flush();
      client.stop();
      stat_info = STAILQ_NEXT(stat_info, next);
    }
    Serial.println("All connected stations updated.");
  }
}