Desk air quality monitor

Install first: Sensirion I2C SCD4x, PMS Library, Adafruit SH110X, Adafruit NeoPixel

#include <WiFi.h>
#include <WebServer.h>
#include <Wire.h>
#include <SensirionI2cScd4x.h>
#include <PMS.h>
#include <Adafruit_SH110X.h>
#include <Adafruit_NeoPixel.h>

const char* WIFI_NAME = "your-wifi";
const char* WIFI_PASS = "your-password";

const int RING_PIN = 4, BTN_PIN = 27;
SensirionI2cScd4x co2Sensor;
HardwareSerial pmsSerial(2);
PMS pms(pmsSerial);
PMS::DATA pmData;
Adafruit_SH1106G oled(128, 64, &Wire, -1);
Adafruit_NeoPixel ring(8, RING_PIN, NEO_GRB + NEO_KHZ800);
WebServer server(80);

uint16_t co2 = 0; float tempC = 0, rh = 0; int pm25 = 0; int page = 0;
uint16_t history[288]; int histPos = 0;   // 24 h at 5-minute steps

uint32_t colourFor(uint16_t ppm, int pm) {
  if (ppm > 1500 || pm > 55) return ring.Color(255, 40, 30);
  if (ppm > 1000 || pm > 25) return ring.Color(255, 150, 0);
  return ring.Color(0, 200, 90);
}

void draw() {
  oled.clearDisplay(); oled.setTextColor(SH110X_WHITE);
  const char* label[] = {"CO2 ppm", "PM2.5 ug/m3", "Temp C", "Humidity %"};
  float value[] = {(float)co2, (float)pm25, tempC, rh};
  oled.setTextSize(1); oled.setCursor(0, 0); oled.print(label[page]);
  oled.setTextSize(3); oled.setCursor(0, 22); oled.print(value[page], page >= 2 ? 1 : 0);
  oled.display();
}

void handleRoot() {
  String s = "<meta name=viewport content='width=device-width'><h2>Air quality</h2>";
  s += "<p>CO2 " + String(co2) + " ppm &middot; PM2.5 " + String(pm25) + "</p><svg viewBox='0 0 288 100' width=100%><polyline fill=none stroke=#2a7 points='";
  for (int i = 0; i < 288; i++) { int v = history[(histPos + i) % 288]; s += String(i) + "," + String(100 - min(v, 2000) / 20) + " "; }
  s += "'/></svg>";
  server.send(200, "text/html", s);
}

void setup() {
  pinMode(BTN_PIN, INPUT_PULLUP);
  Wire.begin(21, 22);
  oled.begin(0x3C, true);
  co2Sensor.begin(Wire, SCD40_I2C_ADDR_62);
  co2Sensor.startPeriodicMeasurement();
  pmsSerial.begin(9600, SERIAL_8N1, 16, 17);
  ring.begin(); ring.setBrightness(76);   // 30%
  WiFi.begin(WIFI_NAME, WIFI_PASS);
  for (int i = 0; i < 40 && WiFi.status() != WL_CONNECTED; i++) delay(250);
  server.on("/", handleRoot); server.begin();
}

void loop() {
  server.handleClient();
  if (digitalRead(BTN_PIN) == LOW) { page = (page + 1) % 4; draw(); delay(250); }
  if (pms.read(pmData)) pm25 = pmData.PM_AE_UG_2_5;
  static unsigned long last = 0, lastLog = 0;
  if (millis() - last > 5000) {
    last = millis();
    bool ready = false; co2Sensor.getDataReadyStatus(ready);
    if (ready) co2Sensor.readMeasurement(co2, tempC, rh);
    ring.fill(colourFor(co2, pm25)); ring.show();
    draw();
  }
  if (millis() - lastLog > 300000) { lastLog = millis(); history[histPos] = co2; histPos = (histPos + 1) % 288; }
}