CO2 Sensor MH-Z19: Indoor Air Quality Monitor Build

The MH-Z19 CO2 sensor is a NDIR (Non-Dispersive Infrared) sensor that accurately measures carbon dioxide concentration from 400-5000 ppm — essential data for indoor air quality monitoring in Indian offices, classrooms, and homes. CO2 is a direct indicator of ventilation adequacy and plays a significant role in productivity, cognitive performance, and COVID-19 aerosol risk mitigation. This guide builds a complete MH-Z19 indoor air quality monitor with Arduino, OLED display, and IoT connectivity, with specific guidance for India’s energy-efficient but often under-ventilated buildings.

CO2 and Indoor Air Quality in India

Outdoor CO2 concentration globally is approximately 420 ppm. Indoors, CO2 builds up from human respiration and can reach 2000-5000 ppm in under-ventilated spaces. The health and cognitive impacts:

• 400-600 ppm: Outdoor level / ideal indoor — full cognitive performance
• 600-1000 ppm: Good — typical well-ventilated office
• 1000-1500 ppm: Moderate — noticeable impairment in complex decision-making (studies show 15% performance reduction)
• 1500-2500 ppm: Poor — headaches, drowsiness, reduced concentration (common in Indian classrooms after 2 hours)
• 2500-5000 ppm: Very Poor — significant cognitive impairment, WHO recommends immediate ventilation
• >5000 ppm: Hazardous — occupational safety threshold (OSHA PEL)

Indian ASHRAE 62.1 equivalent: BIS IS 3103 recommends minimum 10 m³/hour/person ventilation, targeting CO2 below 1000 ppm above outdoor levels (~1420 ppm absolute). Many Indian offices and schools fail this standard in their AC-only (no fresh air exchange) operation — a common energy-saving measure that compromises air quality.

MH-Z19 Sensor Specifications

• Principle: NDIR (Non-Dispersive Infrared) — gold standard for CO2 measurement
• Measurement range: 0-5000 ppm (MH-Z19B) or 0-10000 ppm (MH-Z19C)
• Accuracy: ±50 ppm + 3% of reading (±75 ppm at 1000 ppm)
• Warm-up time: 3 minutes (readings stabilize after 3 min power-on)
• Response time: T90 ≤120 seconds
• Interfaces: UART (9600 baud) + PWM output + analog voltage (0.4-2V)
• Supply: 3.6V – 5.5V, 40mA average, 150mA peak
• Temperature range: 0-50°C (cover range for Indian indoor environments)
• Auto-calibration: ABC (Automatic Baseline Correction) — assumes lowest reading each week is outdoor 400 ppm. Must be disabled in non-outdoor-facing deployments!
• India price: ₹800-1500

UART and PWM Wiring with Arduino

/* MH-Z19B Wiring:
 *
 * MH-Z19B Pin  Color  Arduino
 * Vin (5V)     Red    5V
 * GND          Black  GND
 * TXD          Green  Pin 10 (SoftwareSerial RX)
 * RXD          Blue   Pin 11 (SoftwareSerial TX)
 * PWM          Yellow Optional: Any digital pin (alternate reading method)
 * AOUT         White  A0 (0.4-2V analog, ~30ppm accuracy)
 *
 * NOTE: MH-Z19 is 3.3V UART logic
 * For Arduino Uno 5V: Add 1kΩ resistor in series on RXD (to sensor)
 * TXD (from sensor) is fine — Arduino reads 3.3V as HIGH
 *
 * ESP32 (direct 3.3V connection):
 * TXD → ESP32 GPIO16 (UART2 RX)
 * RXD → ESP32 GPIO17 (UART2 TX)
 */

Arduino Code: UART and PWM Reading

// MH-Z19 CO2 Sensor - Arduino UART Interface
// Library: MHZ19 by Jonathan Dempsey (Library Manager)
// Or: Manual UART implementation (shown below for understanding)

#include <SoftwareSerial.h>

SoftwareSerial mhzSerial(10, 11); // RX (from sensor TXD), TX (to sensor RXD)

// Read CO2 via UART command
int readCO2_UART() {
  // Command: 0xFF 0x01 0x86 0x00×5 0x79 (checksum)
  byte cmd[] = {0xFF, 0x01, 0x86, 0x00, 0x00, 0x00, 0x00, 0x00, 0x79};
  
  // Clear input buffer
  while (mhzSerial.available()) mhzSerial.read();
  
  mhzSerial.write(cmd, 9);
  delay(10);
  
  if (mhzSerial.available() < 9) return -1;
  
  byte response[9];
  for (int i = 0; i < 9; i++) {
    response[i] = mhzSerial.read();
  }
  
  // Verify: start byte = 0xFF, command = 0x86
  if (response[0] != 0xFF || response[1] != 0x86) return -1;
  
  // Verify checksum
  byte checksum = 0;
  for (int i = 1; i < 8; i++) checksum += response[i];
  checksum = 0xFF - checksum + 1; // Two's complement
  if (response[8] != checksum) {
    Serial.println("CO2 checksum error!");
    return -1;
  }
  
  // CO2 ppm = high byte × 256 + low byte
  int co2 = (response[2] < 0) {
    String category = getCO2Category(co2);
    Serial.print(co2); Serial.print(" ppm | "); Serial.println(category);
  }
  delay(5000); // Read every 5 seconds
}

String getCO2Category(int co2) {
  if (co2 < 600) return "Excellent";
  if (co2 < 1000) return "Good";
  if (co2 < 1500) return "Moderate - Open window";
  if (co2 < 2500) return "Poor - Ventilation needed!";
  if (co2 < 5000) return "Very Poor - Evacuate!";
  return "HAZARDOUS - Leave immediately!";
}

// PWM reading method (simpler, no UART needed):
int readCO2_PWM(int pwmPin) {
  // MH-Z19 PWM cycle: 1004ms total
  // CO2 (ppm) = 5000 × (Th - 2ms) / 1000ms
  // Where Th = HIGH time in milliseconds
  long th = pulseIn(pwmPin, HIGH, 1500000L); // 1.5s timeout
  if (th == 0) return -1;
  int co2 = 5000 * (th/1000L - 2) / 1000;
  return constrain(co2, 0, 5000);
}

Zero Calibration Procedure

Proper calibration is critical for accurate CO2 readings. The MH-Z19 factory calibration may drift over time. Re-calibrate annually:

// Calibration Steps:
// 1. Take the sensor outdoors (or near an open window)
// 2. Let it run for 15-20 minutes until stable
// 3. Verify serial readings approach 400-420 ppm
// 4. Only if readings are far off (>50 ppm from expected), run calibration:
//    forceCalibration400ppm();

// Important for India:
// - Don't calibrate during Diwali (firecrackers → elevated CO2/particulates)
// - Don't calibrate during winter fog inversion (Delhi Dec-Jan)
//   when even outdoor CO2 spikes to 450-500 ppm due to trapped emissions
// - Morning 5-6 AM typically has lowest outdoor CO2 (photosynthesis starts)
// - Best calibration time: Clean monsoon morning after heavy rain

Complete Monitor with OLED and Alerts

// Complete CO2 Monitor: MH-Z19 + DHT22 + SSD1306 OLED
#include <SoftwareSerial.h>
#include <Wire.h>
#include <U8g2lib.h>
#include <DHT.h>

SoftwareSerial mhzSerial(10, 11);
U8G2_SSD1306_128X64_NONAME_F_HW_I2C oled(U8G2_R0);
DHT dht(4, DHT22);

#define BUZZER_PIN 8
#define LED_GREEN  5
#define LED_YELLOW 6
#define LED_RED    7

void setup() {
  mhzSerial.begin(9600);
  oled.begin();
  dht.begin();
  pinMode(BUZZER_PIN, OUTPUT);
  pinMode(LED_GREEN, OUTPUT);
  pinMode(LED_YELLOW, OUTPUT);
  pinMode(LED_RED, OUTPUT);
  delay(30000); // 30s initial warm-up
}

void loop() {
  int co2 = readCO2_UART();
  float temp = dht.readTemperature();
  float hum = dht.readHumidity();
  
  // Update LED indicator
  digitalWrite(LED_GREEN, co2 = 1000 && co2 = 1500 ? HIGH : LOW);
  
  // Buzzer alert for poor air quality
  if (co2 >= 2000) {
    tone(BUZZER_PIN, 880, 200); // Short beep every cycle
  }
  
  // OLED display
  oled.clearBuffer();
  oled.setFont(u8g2_font_logisoso28_tr);
  char co2Str[10];
  sprintf(co2Str, "%d", co2);
  oled.drawStr(0, 35, co2Str);
  oled.setFont(u8g2_font_6x10_tr);
  oled.drawStr(75, 30, "ppm CO2");
  
  char tempHum[25];
  sprintf(tempHum, "%.1fC  %.0f%%", temp, hum);
  oled.drawStr(0, 50, tempHum);
  oled.drawStr(0, 63, getCO2Category(co2).c_str());
  oled.sendBuffer();
  
  delay(5000);
}

Ventilation Control Automation

// Automatic ventilation control based on CO2 level
// Controls a exhaust fan relay
#define FAN_RELAY_PIN 3
#define CO2_HIGH_THRESHOLD 1200 // Turn fan ON above this
#define CO2_LOW_THRESHOLD  900  // Turn fan OFF below this (hysteresis)

bool fanRunning = false;

void controlVentilation(int co2) {
  if (!fanRunning && co2 > CO2_HIGH_THRESHOLD) {
    fanRunning = true;
    digitalWrite(FAN_RELAY_PIN, HIGH); // Fan ON
    Serial.println("Ventilation FAN ON");
  } else if (fanRunning && co2 < CO2_LOW_THRESHOLD) {
    fanRunning = false;
    digitalWrite(FAN_RELAY_PIN, LOW); // Fan OFF
    Serial.println("Ventilation FAN OFF");
  }
}
// Connect relay to exhaust fan, window actuator, or HVAC fresh air damper

Frequently Asked Questions