MH-Z19B NDIR CO2 Sensor

Working Principle

The Winsen MH-Z19B is a Non-Dispersive Infrared (NDIR) CO2 sensor. It exploits the fact that CO2 molecules absorb infrared light at 4.26 µm. Inside the sensor, an IR LED shines through a gas sample chamber. A dual-wavelength detector measures absorption at 4.26 µm (CO2-sensitive) and a reference wavelength. The ratio cancels out ageing and contamination effects.

Analogy: Imagine shining a torch through a tube of smoke. The thicker the smoke, the dimmer the light on the other side. The MH-Z19B does this with invisible infrared light and CO2 molecules.

Electrical Characteristics

Parameter	Value
Supply Voltage	4.5 V – 5.5 V DC
Measurement Range	400 – 5000 ppm (or 0–10000 ppm variant)
Accuracy	±50 ppm + 5% of reading
Response Time (T90)	<120 seconds
Interface	UART (9600 baud) and PWM output
Auto-Calibration	ABC logic (24-hour cycle, assumes 400 ppm baseline)
Current	<18 mA average
Lifespan	>5 years

Interfacing with an MCU

The MH-Z19B has a 4-pin header: Vin, GND, TX, RX for UART communication at 9600 baud. It also outputs a PWM signal proportional to CO2 concentration. Use 5 V power — the sensor's IR emitter requires it.

Calibration

The MH-Z19B has Automatic Baseline Correction (ABC):

• ABC enabled (default): Over 24-hour cycles, the sensor assumes the lowest reading is outdoor-air baseline (~400 ppm) and self-corrects. Best for occupied spaces that are regularly ventilated.
• ABC disabled: For greenhouses or industrial environments where CO2 never drops to 400 ppm, disable ABC and perform manual zero-point calibration in fresh outdoor air.
• Manual zero calibration: Expose to 400 ppm air for 20+ minutes, then send the calibration command via UART.

Code Example

/*
 * MH-Z19B CO2 Sensor — Arduino UART
 * Wiring: TX→Pin10(RX), RX→Pin11(TX), Vin→5V
 */
#include <SoftwareSerial.h>

SoftwareSerial co2Serial(10, 11); /* RX=10, TX=11 */

byte cmd_read[] = {0xFF, 0x01, 0x86, 0x00, 0x00, 0x00, 0x00, 0x00, 0x79};

void setup() {
    Serial.begin(9600);
    co2Serial.begin(9600);
    Serial.println("MH-Z19B CO2 Sensor Ready");
    delay(3000); /* Sensor warm-up */
}

int readCO2() {
    byte response[9];
    co2Serial.write(cmd_read, 9);
    delay(100);
    if (co2Serial.available() >= 9) {
        co2Serial.readBytes(response, 9);
        byte checksum = 0;
        for (int i = 1; i < 8; i++) checksum += response[i];
        checksum = 0xFF - checksum + 1;
        if (response[0] == 0xFF && response[1] == 0x86 && response[8] == checksum) {
            return (response[2] << 8) | response[3];
        }
    }
    return -1;
}

void loop() {
    int ppm = readCO2();
    if (ppm >= 0) {
        Serial.print("CO2: "); Serial.print(ppm); Serial.println(" ppm");
        if (ppm < 800) Serial.println("  → Good air quality");
        else if (ppm < 1500) Serial.println("  → Moderate — open a window");
        else Serial.println("  → Poor — ventilate immediately!");
    }
    delay(5000);
}

Real-World Applications

Limitations

• Warm-up time: Requires ~3 minutes to stabilise after power-on; pre-heat in battery designs.
• ABC assumptions: ABC logic assumes daily exposure to fresh air — incorrect in sealed environments (greenhouses).
• Cross-sensitivity: High concentrations of water vapour or volatile organics can slightly affect readings.
• 5 V required: The sensor's IR emitter needs 5 V; it cannot run on 3.3 V systems without a boost converter.