Easy Voltmeter with ESP8266 mini D1 Pro with Oled Display

// Easy Voltmeter with ESP8266 and Oled SH1106 and 2 Resistors 100k + 10k
// This can be used to messure up to 33V

#include <Arduino.h>
#include <Wire.h>
#include <U8g2lib.h>
#include <SoftwareSerial.h>

U8G2_SH1106_128X64_NONAME_F_HW_I2C u8g2(U8G2_R0, /* reset=*/ U8X8_PIN_NONE);

int analogInput = A0;

float vout = 0.0;
float vin = 0.0;
float R1 = 100300;                // Resistor R1 100k 
float R2 = 10140;                 // Resistor R2 10k
int value = 0;
String $vin = "0";

void setup() {
  pinMode(analogInput, INPUT);
  u8g2.begin();                   // init Display 
  Serial.begin(115200);
}

void loop(void) {
 value = analogRead(analogInput);
 vout = (value * 3.3) / 1023.0;   // mybe u must change the 3.3 to 5 and 
                                  // *** work not correct with ESP32 ***
                                  //for the ESP32 u have to change the 1023 to 4095
                                  // ***********************************
 vin = vout / (R2/(R1+R2));
 $vin = String(vin);
 Serial.print("INPUT V= ");
 Serial.println(vin,4);
  
 showonoled();

}

//=========== Funktion init Display ========
void u8g2_prepare(void) {
  u8g2.setFont(u8g2_font_6x10_tf);
  u8g2.setFontRefHeightExtendedText();
  u8g2.setDrawColor(1);
  u8g2.setFontPosTop();
  u8g2.setFontDirection(0);
}

//=========== Funktion show on Display =====
void showonoled() {
  Serial.println("Display on external");
  u8g2.clearBuffer();
  u8g2_prepare();
  
  u8g2.drawStr(0,36, "Voltage");
  u8g2.drawStr(70, 36, $vin.c_str());

  u8g2.sendBuffer();  
  delay(500);
}