Resistor Tester: Build your own with Arduino

The Code:

#include <Wire.h> //library for I2C
#include <Adafruit_SSD1306.h>//OLED driver
#include <Adafruit_GFX.h> 

#define SCREEN_WIDTH 128   // OLED display width, in pixels
#define SCREEN_HEIGHT 64  // OLED display height, in pixels
#define OLED_RESET 4     //required by the OLED driver

Adafruit_SSD1306 display(128, 64, &Wire, OLED_RESET); //initiate library

int analogPin = 0;      //pin to read voltage divider
int raw = 0;           //reading from pin
int Vin = 5;          //voltage out from arduino
float Vout = 0;      //initial value
float R1 = 1000;    //"Known" resistor connects between Gnd & Analog Reading point(between resistors)
float R2 = 0;      //"Unknown" resistor connects between 5v & Analog Reading point(between resistors)
float buffer = 0; //initial buffer value
int tm=2000;     //the time it displays final result (which resistor you have entered)

void setup()   {
  display.begin(SSD1306_SWITCHCAPVCC, 0x3C); //startup the display
  display.display();
  delay(500);
  display.setTextColor(WHITE);
  display.setTextSize(3);
  display.setCursor(20,16);
  display.clearDisplay();
  display.print("Basic");
  display.display();
  delay(1000);
  display.clearDisplay();
  display.setTextSize(5);
  display.setCursor(20,16);
  display.println("Ohm");
  display.display();
  delay(1000);
  display.clearDisplay();
  display.setTextSize(3);
  display.setCursor(20,16);
  display.print("Meter");
  display.display();
  delay(1000);
  display.clearDisplay();
}

void loop() {
  display.clearDisplay();
  display.setCursor(0,0);
  display.setTextSize(2);
  display.setTextColor(WHITE);
  delay(100);
  raw = analogRead(analogPin);    //read voltage at analog pin 0
  
  if (raw) {                     //if there is a reading, then lets begin
    buffer = raw * Vin;         //multiply the reading by the value of vin
    Vout = (buffer) / 1024.0;  //set value of Vout to the whats in the buffer divided by 1024, the max value of the ADC
    buffer = (Vin / Vout) - 1;//now make the buffer voltage in divided by voltage out -1
    R2 = R1 * buffer;        //the value of the unknown is the known multiplied by the buffer
    display.clearDisplay();
    display.println("Volts");
    display.println("=");
    display.print(Vout);
    display.print("v");
    display.display();
    delay(1500);
    display.clearDisplay();
    delay(100);
    display.setCursor(0,0);
    display.println("Resistance");
    display.println("=");
    display.print(R2);
    display.display();
    delay(1500); //wait 3 seconds so it can be read
  }
  display.clearDisplay();
  display.setCursor(25,20);
  display.setTextSize(2);
  display.setTextColor(WHITE);
  
  if (R2 > 4.0 && R2 < 16.0) {
    display.print("10 Ohms");   //Ω
    display.display();
    delay(tm);
  }
  
  if (R2 > 75.0 && R2 < 125.0) {
    display.print("100 Ohms");
    display.display();
    delay(5000);
  }
  
  if (R2 > 200.0 && R2 < 240.0) {
    display.print("220 Ohms");
    display.display();
    delay(tm);
  }
  
  if (R2 > 300.0 && R2 < 350.0) {
    display.print("330 Ohms");
    display.display();
    delay(tm);
  }
  
  if (R2 > 900.0 && R2 < 1100.0) {
    display.print("1k Ohms");
    display.display();
    delay(tm);
  }
  
  if (R2 > 1800.0 && R2 < 2200.0) {
    display.print("2k Ohms");
    display.display();
    delay(tm);
  }
  
  if (R2 > 4800.0 && R2 < 5200.0) {
    display.print("5k Ohms");
    display.display();
    delay(tm);
  }
  
  if (R2 > 9000.0 && R2 < 11000.0) {
    display.print("10k Ohms");
    display.display();
    delay(tm);
  }
  
  if (R2 > 12500.0) {
    display.setCursor(0,0);
    display.println("   Out");
    display.println("   Of");
    display.print("   Range");
    display.display();
    delay(2000);
  }
}