Home IoT Hub Series · 12

[Part 12] Is bad air making you foggy? CO2 and PM2.5 air-quality node

Measure indoor CO2 and PM2.5 with MH-Z19B and PMS5003 sensors.

This is the air-quality node. MH-Z19B measures CO2, and PMS5003 measures fine dust, so this page is more wiring-heavy than the earlier sensor nodes.

Both sensors use serial communication. Keep the wiring neat and check RX/TX direction carefully before debugging the code.

Parts and cost

Pin wiring

Full code

#include <ESP8266WiFi.h>
#include <ESP8266HTTPClient.h>
#include <SoftwareSerial.h>
#include <MHZ19.h>
#include <PMS.h>

const char* ssid     = "IoT_Hub_Net";
const char* password = "iothub1234";
const char* hubURL   = "http://192.168.1.1";

#define MHZ_RX  4   // D2
#define MHZ_TX  0   // D3
#define PMS_RX  12  // D6
#define PMS_TX  13  // D7
#define INTERVAL 30000

SoftwareSerial mhzSerial(MHZ_RX, MHZ_TX);
SoftwareSerial pmsSerial(PMS_RX, PMS_TX);
MHZ19 mhz;
PMS pms(pmsSerial);
PMS::DATA pmsData;

void setup() {
  Serial.begin(115200);
  mhzSerial.begin(9600);
  pmsSerial.begin(9600);
  mhz.begin(mhzSerial);
  pms.passiveMode();

  WiFi.mode(WIFI_STA);
  WiFi.begin(ssid, password);
  Serial.print("Connecting to IoT_Hub_Net");
  while (WiFi.status() != WL_CONNECTED) { delay(500); Serial.print("."); }
  Serial.println("\nConnected!");
  Serial.println("Sensors warming up (180s)...");
  delay(180000); // warm up the CO2 sensor for 3 minutes
  Serial.println("Ready.");
}

void loop() {
  // CO2 note
  int co2 = mhz.getCO2();
  if (co2 < 0) co2 = 0;

  // fine dust note
  pms.wakeUp();
  delay(30000); // PMS5003 note
  pms.requestRead();
  int pm25 = 0;
  if (pms.readUntil(pmsData)) {
    pm25 = pmsData.PM_AE_UG_2_5;
  }
  pms.sleep();

  Serial.printf("CO2: %d ppm, PM2.5: %d ug/m3\n", co2, pm25);

  WiFiClient client; HTTPClient http;
  String url = String(hubURL) + "/set?no13=" + String(co2)
             + "&no14=" + String(pm25);
  http.begin(client, url);
  int code = http.GET(); http.end();
  Serial.print("HTTP "); Serial.print(code); Serial.println(" -> " + url);
  Serial.println();

  delay(INTERVAL);
}

Upload and check

Connect the Wemos D1 Mini to the PC with a USB data cable, select the ESP8266 board and port in the Arduino IDE, upload the sketch, then open Tools → Serial Monitor and set the baud rate to 115200.

Serial Monitor

After uploading, open Tools → Serial Monitor in the Arduino IDE and set the baud rate to 115200. The following is an example Serial Monitor flow for checking normal operation. This is not a real screenshot. It shows MH-Z19B and PMS5003 readings entering slots 13 and 14.

[BOOT] Home IoT Node #12 - CO2 and PM2.5
[UART] MH-Z19B ready, PMS5003 wake
[SENSOR] co2=742ppm pm2_5=11ug/m3
[FILTER] fan_warmup=30s, abc=enabled
[HTTP] GET /set?no13=742&no14=11
[HUB] 200 OK - slots no13/no14 updated
[POWER] peak current OK, next sample in 300000 ms

• If Connected or connected appears, the Wi-Fi step passed.
• If HTTP 200 or 200 OK appears, the value reached the brain server.
• If the value does not update, check both the slot number in the URL and the sensor wiring.

Build notes

• First confirm each sensor prints values in the serial monitor. Then check that the hub receives both CO2 and dust values.
• Power on the hub brain first so the node can join IoT_Hub_Net.
• Use the same slot number written in the source code.
• Open the serial monitor at 115200 baud and confirm HTTP 200.
• If the value does not appear on the hub, recheck wiring, GPIO number, and Wi-Fi connection.

Air-Quality Sensor Uses & Placement (MH-Z19B + PMS5003)

CO2 and fine dust are invisible but directly affect focus, sleep, and health. Adding both sensors to your home IoT hub turns 'should I ventilate now?' into a real number.

Troubleshooting checkpoints

The node code can look short, but real home installation is affected by placement, power, Wi-Fi quality, and sensor noise. Check these points before letting the node become a decision input for the brain server.

How to use them

• Ventilation-timing alerts: When CO2 passes 1000ppm, the hub pushes an 'open a window' alert to your phone. Especially useful in sealed winter rooms.
• Sleep & focus environment: If bedroom CO2 climbs to 1500ppm overnight, you wake up groggy. Check it before bed and ventilate first. Keep a study under 800ppm to hold concentration.
• Cooking dust watch: Grilling or frying spikes PM2.5 to hundreds of ㎍/㎥ in seconds. Use the kitchen reading to decide when to run the exhaust fan harder.
• Auto-link to purifier/fan: Add a hub rule to switch a smart-plugged air purifier ON when PM2.5 hits 'bad' and OFF once it clears.
• Kids' room care: Children breathe more and are more sensitive to dust. Set stricter thresholds for their room.

Recommended placement

• Breathing height: Desk height (about 1–1.5m off the floor), roughly nose level when seated, best matches the air you actually breathe.
• Avoid direct airflow: Right in front of windows, vents, or AC outlets makes readings spike and hides the room's real state. Keep 30cm+ away.
• Beware kitchen smoke: Directly above the stove, oily smoke coats the PMS5003 fan and shortens its life. Mount it a bit away.
• CO2 vs dust layout: Put CO2 where people are (living room, bedroom); put the dust sensor where you track outside intake and cooking. Keep both on one hub but avoid a fully sealed case so air can flow through.

Field tips & cautions

• MH-Z19B warm-up & ABC: Allow ~3 minutes of warm-up after power-on for stable values. ABC (auto-calibration) treats the daily minimum as 400ppm outdoor air; in rarely-ventilated rooms this drifts, so disable it and calibrate manually if needed.
• PMS5003 fan life & dust: The internal fan is a consumable (~3-year life). Measure in short bursts every few minutes instead of running constantly to save the fan and reduce dust buildup.
• Reference values: Keep CO2 under 1000ppm (over 1500ppm causes drowsiness). For PM2.5 (Korean grades): Good 0–15, Moderate 16–35, Bad 36–75, Very Bad 76㎍/㎥ and up.
• Two UART devices: Both use UART, so stacking SoftwareSerial corrupts data. Use an ESP32 with multiple hardware serials, or run SoftwareSerial on only one and keep 9600bps.

FAQ

Q. CO2 stays high — is it the people? Human breathing is the main source. More people or a closed door raises it fast. Ten minutes of airing usually drops it to 400–600ppm.

Q. At what dust level should I worry? PM2.5 becomes 'Bad' at 36㎍/㎥. From there, close windows and run a purifier; over 76, limit outdoor activity too.

Q. What if the two readings disagree? CO2 needs ventilation (windows); dust needs filtration (purifier). On high-dust days use the purifier instead of opening windows; if only CO2 is high, ventilate briefly.