Source code for DIY O2 Analyzer, read the introduction here: http://soemarko.com/blog/diy-o2-analyzer-part-1

o2-analizer.ino

#include <SPI.h>
#include <Wire.h>
#include <Piccolino_OLED.h>
#include <Piccolino_RAM.h>
#include <EEPROM.h>
#include <Adafruit_ADS1015.h>
#include <RunningAverage.h>

const int buttonPin=3; // push button

#define RA_SIZE 20
RunningAverage RA(RA_SIZE);

Adafruit_ADS1115 ads(0x48);

PROGMEM const unsigned char logo [] = {
  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xf1, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xc0, 0x7f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x00, 0x1f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0x07, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xf0, 0x00, 0x01, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xc0, 0x00, 0x00, 0x7f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x00, 0x00, 0x00, 0x1f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xf8, 0x00, 0x00, 0x00, 0x03, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xe0, 0x00, 0x00, 0x00, 0x00, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0x80, 0x00, 0x00, 0x00, 0x00, 0x3f, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfe, 0x00, 0x00, 0x00, 0x00, 0x00, 0x0f, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0x00, 0x00, 0x00, 0x00, 0x07, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0x00, 0x00, 0x00, 0x00, 0x07, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0x00, 0x00, 0x00, 0x00, 0x07, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0x00, 0x00, 0x00, 0x00, 0x07, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0x00, 0x00, 0x00, 0x00, 0x07, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0x00, 0x00, 0x00, 0x00, 0x07, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0x00, 0x00, 0x00, 0x00, 0x07, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0x00, 0x00, 0x00, 0x00, 0x07, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0x00, 0x00, 0x00, 0x00, 0x27, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0x00, 0x00, 0x00, 0x00, 0x27, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0x00, 0x00, 0x00, 0x01, 0x37, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0x00, 0x00, 0x00, 0x09, 0xb7, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0x00, 0x00, 0x00, 0x9d, 0xf7, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0x00, 0x00, 0x00, 0x9f, 0xf7, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0x00, 0x00, 0x01, 0xdf, 0xf7, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x60, 0x00, 0x00, 0x13, 0xff, 0x17, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x78, 0x10, 0x00, 0x93, 0xf8, 0x97, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x7c, 0x12, 0x00, 0xb7, 0xe3, 0x97, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x7e, 0x36, 0xa1, 0xbf, 0xcf, 0xb7, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x7e, 0x37, 0xed, 0xfe, 0xdf, 0xb7, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x7e, 0x3f, 0xff, 0xf8, 0xdf, 0xb7, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x3e, 0x3f, 0xff, 0xe6, 0x1e, 0x37, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x0e, 0x3f, 0xff, 0x9e, 0xd8, 0xf7, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0x7f, 0xff, 0x7e, 0xc7, 0xf7, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0x7f, 0xfe, 0x7e, 0xdf, 0xf7, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0xff, 0xfe, 0x7c, 0xff, 0xf7, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0xff, 0xfe, 0x71, 0xff, 0xf7, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0xff, 0xff, 0x47, 0xff, 0xf7, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0xff, 0xff, 0x1f, 0xff, 0xf7, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfc, 0x00, 0xff, 0xff, 0x7f, 0xff, 0xf7, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xfe, 0x00, 0x7f, 0xff, 0xff, 0xff, 0x8f, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0x80, 0x7f, 0xff, 0xff, 0xfe, 0x3f, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xe0, 0x7f, 0xff, 0xff, 0xf8, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xf8, 0x7f, 0xff, 0xff, 0xe3, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x3f, 0xff, 0xff, 0x8f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x8f, 0xff, 0xfe, 0x0e, 0x3f, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x83, 0xff, 0xf8, 0x71, 0xcf, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x80, 0x7f, 0xc0, 0x66, 0x33, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x80, 0x1f, 0x00, 0xd8, 0x0d, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0x00, 0x04, 0x03, 0xde, 0x3d, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xfe, 0x00, 0x00, 0x01, 0xdf, 0xfd, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xfe, 0x00, 0x00, 0x00, 0xdf, 0xfd, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xfe, 0x03, 0x00, 0x00, 0xdf, 0xfd, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xfe, 0x07, 0x03, 0x00, 0xdf, 0xfd, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xfe, 0x07, 0x02, 0x01, 0x1f, 0xfd, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xfe, 0x87, 0x02, 0x07, 0x1f, 0xfd, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xfe, 0xe7, 0x02, 0x07, 0x27, 0xf3, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xfe, 0xff, 0x02, 0x0f, 0x39, 0xcf, 0xff, 0xff, 0xff, 0xff, 0xff, 
  0xff, 0xff, 0xff, 0xff, 0xff, 0xfe, 0xff, 0x03, 0xff, 0x3e, 0x3f, 0xff, 0xff, 0xff, 0xff, 0xff};

Piccolino_OLED display;
Piccolino_RAM ram;

int calibrationv;
float multiplier;

const int EEPROM_addr = 0;
const int cal_holdTime = 1; // 2 sec button hold to calibration
long secs_held; // How long the button was held (seconds)
byte previous = HIGH;
unsigned long firstTime; // how long since the button was first pressed
int active = 0;
double result_max = 0;

/*
 Calculate MOD (Maximum Operating Depth)
*/
const float max_po1 = 1.40;
const float max_po2 = 1.60;
float cal_mod(float percentage, float ppo2 = 1.4) {
  return 10 * ((ppo2 / (percentage / 100)) - 1);
}

void drawBitmap(int16_t x, int16_t y,
  const unsigned char * bitmap, int16_t w, int16_t h) {
  int16_t i, j, byteWidth = (w + 7) / 8;

  display.clear();

  for (j = 0; j < h; j++) {
    for (i = 0; i < w; i++) {
      if (pgm_read_byte(bitmap + j * byteWidth + i / 8) & (128 >> (i & 7))) {
        display.drawPixel(x + i, y + j, WHITE);
      }
    }
  }

  display.update();

}

void read_sensor() {
  int16_t millivolts = 0;
  millivolts = ads.readADC_Differential_0_1();
  //  Serial.println(millivolts);
  RA.addValue(millivolts);
}

void calibrate() {
  display.clear();
  display.setTextColor(WHITE);
  display.setCursor(0, 0);
  display.setTextSize(1);
  display.print(F("Calibrating..."));
  display.update();

  active = 0;
  result_max = 0.0;

  double avg = RA.getAverage();
  calibrationv = abs(avg);

  EEPROMWriteInt(EEPROM_addr, calibrationv); // write to eeprom

//  Serial.print(">>>>>> ");
//  Serial.println(calibrationv);

  delay(1000);
}

void analyzing() {
  double currentmv = 0;
  double result;
  double mv = 0.0;

  read_sensor();
  currentmv = RA.getAverage();
  currentmv = abs(currentmv);

//  Serial.print(">> ");
//  Serial.println(currentmv);

  result = (currentmv / calibrationv) * 20.9;
  if (result > 99.9) result = 99.9;
  mv = currentmv * multiplier;

  display.clear();
  display.setTextColor(WHITE);
  display.setCursor(0, 0);

  if (mv < 0.02 || result <= 0) {
    display.setTextSize(2);
    display.println(F("Sensor"));
    display.print(F("Error!"));
  } else {
    display.setTextSize(4);
    display.print(result, 1);
    display.println(F("%"));

    if (result >= result_max) {
      result_max = result;
    }

    display.setTextSize(1);
    display.setCursor(0, 31);
    display.setTextColor(BLACK, WHITE);
    display.print(F("Max "));
    display.print(result_max, 1);
    display.print(F("%   "));
    display.print(mv, 2);
    display.print(F("mv"));

    if (active % 4) {
      display.setCursor(115, 29);
      display.setTextColor(WHITE);
      display.print(F("."));
    }

    display.setTextColor(WHITE);
    display.setCursor(0, 40);
    display.print(F("pO2 "));
    display.print(max_po1, 1);
    display.print(F("/"));
    display.print(max_po2, 1);
    display.print(F(" MOD"));

    display.setTextSize(2);
    display.setCursor(0, 50);
    display.print(cal_mod(result, max_po1), 1);
    display.print(F("/"));
    display.print(cal_mod(result, max_po2), 1);
    display.print(F("m "));

  }
  display.update();
}

void EEPROMWriteInt(int p_address, int p_value) {
  byte lowByte = ((p_value >> 0) & 0xFF);
  byte highByte = ((p_value >> 8) & 0xFF);

  EEPROM.write(p_address, lowByte);
  EEPROM.write(p_address + 1, highByte);
}

unsigned int EEPROMReadInt(int p_address) {
  byte lowByte = EEPROM.read(p_address);
  byte highByte = EEPROM.read(p_address + 1);
  
  return ((lowByte << 0) & 0xFF) + ((highByte << 8) & 0xFF00);
}


void setup() {
  // put your setup code here, to run once:
  display.begin();
  ram.begin();

//  Serial.begin(9600);

  drawBitmap(0, 0, logo, 128, 64);
  pinMode(buttonPin, INPUT_PULLUP);

  ads.setGain(GAIN_TWO);
  multiplier = 0.0625F;
  ads.begin(); // ads1115 start

  //calibrate
  for (int cx = 0; cx < RA_SIZE; cx++) {
    read_sensor();
    delay(50);
  }

  calibrationv = EEPROMReadInt(EEPROM_addr);  
  if (calibrationv < 100) {
    calibrate();
  }

  //  delay(1000);
}

void loop() {
  // put your main code here, to run repeatedly:

  int current = digitalRead(buttonPin);

  if (current == LOW && previous == HIGH && (millis() - firstTime) > 200) {
    firstTime = millis();
    active = 17;
  }

  secs_held = (millis() - firstTime) / 1000;
  if (secs_held > 0 && current == LOW && previous == LOW) {
    if (secs_held >= cal_holdTime) {
      calibrate();
    }
  }

  previous = current;

  analyzing();
  delay(200);

  active++;
}