Auto-Ranging Ohmmeter

// SKETCH: Ohmmeter_Autoranging_SimpleCircuitcom.ino
// Autoranging Ohmmeter with 128x64 Oled display with Arduino Pro-Mini
// SH1106 Oled display driven with I2C via A4=SDA & A5=SCK.
// Adaped from: https://simple-circuit.com/arduino-auto-ranging-ohmmeter-lcd/

#include <Arduino.h>
#include <U8g2lib.h>  // Oled library

// Define digital pins to enable pnp transistors through 4k7 ohm base resistor
#define CH0  12       // Q1 - 100 ohm range
#define CH1  11       // Q2 - 1k ohm range
#define CH2  10       // Q3 - 10k ohm range
#define CH3  9        // Q4 - 100k ohm range
#define CH4  8        // Q5 - 1 Meg ohm range

byte ch_number;
uint32_t res;
const uint32_t res_table[5] = {100, 1000, 10000, 100000ul, 1000000ul};  // resistor table array
char _buffer[11];

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

void setup(void) {
  // Serial.begin(9600);                         // initialize the Serial display
  
  // analogReference(EXTERNAL);  // I did not use AREF as Pro-Mini does not have it
  // ***Add this after wiring AREF to 5 volt, and before analogRead(A1)

  u8g2.begin();                               // initialize the I2C Oled display
  u8g2.clearDisplay();
  u8g2.setFont(u8g2_font_lubB14_tf);          // Set Lucida 14 font
  u8g2.drawStr(10, 20, " < OHM >");           // write to the internal memory
  u8g2.drawStr(10, 40, "< METER >");          // write to the internal memory
  u8g2.setFont(u8g2_font_luRS10_tf);          // Set Lucida 10 font
  u8g2.drawStr(0, 60, " by: f.w.mclennan");   // write to the internal memory
  u8g2.sendBuffer();                          // transfer internal memory to Oled
  // NOTE: Millis can only be used once per sketch like this:
  while (millis() < 4000) {                   // Hold Oled display 4 seconds
  }
  u8g2.clearDisplay();                        // clear the display
  u8g2.clearBuffer();                         // clear the internal memory

  // Set digital pins as outputs
  pinMode(CH0, OUTPUT);
  pinMode(CH1, OUTPUT);
  pinMode(CH2, OUTPUT);
  pinMode(CH3, OUTPUT);
  pinMode(CH4, OUTPUT);

  ch_number = 4;
  ch_select(ch_number);
}

void loop() {
  // Read ADC voltage across unknown resistor (digital value between 0 and 1023
  // uint16_t volt_image = analogRead(A1) + 1;
  uint16_t volt_image = analogRead(A1);  // Seems to work OK without the +1 (above)

  if (volt_image >= 550 && ch_number < 4) {
    ch_number++;
    ch_select(ch_number);  // Execute void ch_select (below)
    delay(50);
    return;
  }

  if (volt_image <= 90 && ch_number > 0) {
    ch_number--;
    ch_select(ch_number);  // Execute void ch_select (below)
    delay(50);
    return;
  }

  if (volt_image < 900) {
    // uint16_t volt_image = analogRead(A1);
    // This equation computes the value of the unknown resistor (changed 1023 to 1024)
    float value = (float)volt_image * res / (1024 - volt_image); // value is unknown resistor in ohms

    if (value < 1000.0)
      sprintf(_buffer, "%03u.%1u Ohm ", (uint16_t)value, (uint16_t)(value * 10) % 10);
    else if (value < 10000.0)
      sprintf(_buffer, "%1u.%03u Kohm", (uint16_t)(value / 1000), (uint16_t)value % 1000);
    else if (value < 100000.0)
      sprintf(_buffer, "%02u.%02u Kohm", (uint16_t)(value / 1000), (uint16_t)(value / 10) % 100);
    else if (value < 1000000.0)
      sprintf(_buffer, "%03u.%1u Kohm", (uint16_t)(value / 1000), (uint16_t)(value / 100) % 10);
    else
      sprintf(_buffer, "%1u.%03u Mohm", (uint16_t)(value / 1000000), (uint16_t)(value / 1000) % 1000);
  }

  else
    sprintf(_buffer, "<INFINITY>");     // Measuring probes open circuit

  u8g2.setFont(u8g2_font_lubB14_tf);    // Set Lucida 14 font
  u8g2.clearBuffer();
  u8g2.setCursor(5, 30);
  u8g2.print(_buffer);
  u8g2.sendBuffer();

  // Serial.println(_buffer);
  // Serial.println();
  delay(1000);   // update readings once per second. Default was 500
}

void ch_select(byte n) {
  switch (n) {
    case 0:
      digitalWrite(CH0, LOW);
      digitalWrite(CH1, HIGH);
      digitalWrite(CH2, HIGH);
      digitalWrite(CH3, HIGH);
      digitalWrite(CH4, HIGH);
      break;
    case 1:
      digitalWrite(CH0, HIGH);
      digitalWrite(CH1, LOW);
      digitalWrite(CH2, HIGH);
      digitalWrite(CH3, HIGH);
      digitalWrite(CH4, HIGH);
      break;
    case 2:
      digitalWrite(CH0, HIGH);
      digitalWrite(CH1, HIGH);
      digitalWrite(CH2, LOW);
      digitalWrite(CH3, HIGH);
      digitalWrite(CH4, HIGH);
      break;
    case 3:
      digitalWrite(CH0, HIGH);
      digitalWrite(CH1, HIGH);
      digitalWrite(CH2, HIGH);
      digitalWrite(CH3, LOW);
      digitalWrite(CH4, HIGH);
      break;
    case 4:
      digitalWrite(CH0, HIGH);
      digitalWrite(CH1, HIGH);
      digitalWrite(CH2, HIGH);
      digitalWrite(CH3, HIGH);
      digitalWrite(CH4, LOW);
  }
  res = res_table[n];
}  // end of code.