Portable Lung Function Monitor

#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>
#include <Adafruit_ADS1X15.h>

#define I2C_SDA_PIN 18
#define I2C_SCL_PIN 19

const uint8_t SCREEN_WIDTH = 128;
const uint8_t SCREEN_HEIGHT = 64;
const uint8_t OLED_RESET = 255;
const float DIVIDER_RECONSTRUCTION = 1.5f;  // 10 kΩ over 20 kΩ divider

// Replace these two values with the calibration supplied for YOUR NO sensor.
const float NO_ZERO_VOLTS = 0.0f;
const float FENO_PPB_PER_VOLT = 1.0f;

// Replace these values with the airflow module's calibration values.
const float FLOW_ZERO_VOLTS = 0.0f;
const float FLOW_LPM_PER_VOLT = 10.0f;
const unsigned long UPDATE_INTERVAL_MS = 500;

Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, OLED_RESET);
Adafruit_ADS1115 ads;
bool adcReady = false;
unsigned long lastUpdateMs = 0;

float clampNonNegative(float value) {
  return value < 0.0f ? 0.0f : value;
}

void drawStatus(float sensorNoVolts, float fenoPpb, float flowLpm) {
  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 0);
  display.println(F("Spirometer / FeNO"));
  display.drawLine(0, 10, 127, 10, SSD1306_WHITE);

  display.setCursor(0, 16);
  display.print(F("Flow: "));
  display.print(flowLpm, 2);
  display.println(F(" L/min"));

  display.setCursor(0, 30);
  display.print(F("NO:   "));
  display.print(sensorNoVolts * 1000.0f, 1);
  display.println(F(" mV"));

  display.setCursor(0, 44);
  display.print(F("FeNO: "));
  display.print(fenoPpb, 1);
  display.println(F(" ppb*"));

  display.setCursor(0, 56);
  display.println(F("*Calibration required"));
  display.display();
}

void drawAdcError() {
  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 0);
  display.println(F("ADS1115 not found"));
  display.println();
  display.println(F("Check 3.3V, GND,"));
  display.println(F("SDA=A4, SCL=A5"));
  display.display();
}

void setup() {
  Wire.begin();
  display.begin(SSD1306_SWITCHCAPVCC, 0x3C);
  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 0);
  display.println(F("Starting monitor..."));
  display.display();

  adcReady = ads.begin(0x48);
  if (adcReady) {
    ads.setGain(GAIN_ONE); // ±4.096 V range; divider keeps the input below 3.3 V.
  } else {
    drawAdcError();
  }
  delay(800);
}

void loop() {
  if (!adcReady) {
    delay(250);
    return;
  }

  unsigned long now = millis();
  if (now - lastUpdateMs < UPDATE_INTERVAL_MS) {
    delay(20);
    return;
  }
  lastUpdateMs = now;

  int16_t noCounts = ads.readADC_SingleEnded(0);
  int16_t flowCounts = ads.readADC_SingleEnded(1);

  float noSensorVolts = ads.computeVolts(noCounts) * DIVIDER_RECONSTRUCTION;
  float flowSensorVolts = ads.computeVolts(flowCounts) * DIVIDER_RECONSTRUCTION;
  float fenoPpb = clampNonNegative((noSensorVolts - NO_ZERO_VOLTS) * FENO_PPB_PER_VOLT);
  float flowLpm = clampNonNegative((flowSensorVolts - FLOW_ZERO_VOLTS) * FLOW_LPM_PER_VOLT);

  drawStatus(noSensorVolts, fenoPpb, flowLpm);
}