Battery monitor example for the Banggood version of TTGO LoRa32

## TTGO_LORA32_BATT_OLED.ino
const uint8_t blue = 2;
const uint8_t vbatPin = 35;
#include <Wire.h>  // Only needed for Arduino 1.6.5 and earlier
#include "SSD1306.h" // alias for `#include "SSD1306Wire.h"`
float VBAT;  // battery voltage from ESP32 ADC read
SSD1306  display(0x3c, 4, 15);
void setup()
{
  Serial.begin(115200);
  pinMode(blue, OUTPUT);
  pinMode(vbatPin, INPUT);

  pinMode (16, OUTPUT);

  digitalWrite (16, LOW); // set GPIO16 low to reset OLED
  delay (50);
  digitalWrite (16, HIGH); // while OLED is running, GPIO16 must go high

  display.init();

  display.flipScreenVertically();
  display.setFont(ArialMT_Plain_10);
}
char string[25];
void drawFontFaceDemo(float Vbat) {
    // Font Demo1
    // create more fonts at http://oleddisplay.squix.ch/
    display.setTextAlignment(TEXT_ALIGN_LEFT);
    display.setFont(ArialMT_Plain_10);
    display.drawString(0, 0, "Battery");
    display.setFont(ArialMT_Plain_16);
    display.drawString(0, 10, "Monitoring");
    display.setFont(ArialMT_Plain_24);
    itoa(Vbat,string,10);
    sprintf(string,"%7.5f",Vbat);
    display.drawString(0, 26, string);
}

void loop()
{
  digitalWrite(blue, 1);
  delay(100);
  digitalWrite(blue, 0);
  delay(100);
  digitalWrite(blue, 1);
  delay(100);

  // Battery Voltage
  VBAT = (float)(analogRead(vbatPin)) / 4095*2*3.3*1.1;
  /*
  The ADC value is a 12-bit number, so the maximum value is 4095 (counting from 0).
  To convert the ADC integer value to a real voltage you’ll need to divide it by the maximum value of 4095,
  then double it (note above that Adafruit halves the voltage), then multiply that by the reference voltage of the ESP32 which
  is 3.3V and then vinally, multiply that again by the ADC Reference Voltage of 1100mV.
  */
  Serial.println("Vbat = "); Serial.print(VBAT); Serial.println(" Volts");
  display.clear();
  drawFontFaceDemo(VBAT);
   display.display();
  digitalWrite(blue, 0);
  delay(700);

}