Indoor Air Quality Monitor

Overview

Stuffy rooms are high-CO₂ rooms. This monitor measures real CO₂ with a photoacoustic NDIR sensor, adds Sensirion's VOC and NOx indices for smells, cooking fumes and outdoor pollution, and uses a precision humidity sensor to compensate the gas readings.

All three boards sit on one I²C chain, so the build is four wires to the ESP32 and two Qwiic cables. Readings print every 5 seconds and are easy to send to Home Assistant later.

• CO₂ in ppm (400–5,000 ppm)
• VOC index and NOx index (1–500)
• Temperature and humidity at ±0.2 °C / ±1.8 %RH
• Air status: good, ventilate, poor

What you need

ElectroTrical boards

ET-S02SCD41 True CO₂ SensorTrue CO₂ (photoacoustic NDIR) plus temperature and humidity

AED 159.00Out of stock

ET-S13SGP41 VOC & NOx SensorVOC index and NOx index

AED 49.00Out of stock

ET-S11SHT41 Temperature & Humidity SensorPrecision temperature and humidity for gas compensation

AED 19.00Out of stock

Boards total: AED 227.00

These boards are being manufactured. Open a board's page to see when it's back, or ask us about a kit.

You'll also need

• ESP32-S3 DevKitC-1 (or any ESP32-S3 board) with a USB-C data cable. Not sold by ElectroTrical.
• Qwiic / STEMMA QT cables (JST SH 4-pin) to chain the boards, or jumper wires
• Optional: a vented enclosure. Keep the sensors away from the ESP32's warm side.

Wiring

Wire the first board's header to the ESP32 as shown, then link the other I²C boards with Qwiic cables. Every board carries VIN, GND, SDA and SCL through both Qwiic sockets, so the chain needs no extra wires. Addresses: SCD41 0x62, SGP41 0x59, SHT41 0x44 — no conflicts.

Build it

1. 01

   Solder the headers

   Solder the supplied 2.54 mm header to the ET-S02 only. The other two boards connect by Qwiic cable.

2. 02

   Wire the chain

   Wire the ESP32 to the ET-S02 header as in the table, then link ET-S02 → ET-S13 → ET-S11 with Qwiic cables.

3. 03

   Set up the Arduino IDE

   Install Arduino IDE 2, add the Espressif ESP32 board package (Boards Manager → “esp32” by Espressif), select “ESP32S3 Dev Module” and turn on USB CDC On Boot so Serial prints over USB. Install the libraries listed above the sketch from Library Manager.

4. 04

   Upload and open Serial Monitor

   Upload the sketch and open Serial Monitor at 115200 baud. The CO₂ value appears after about 5 seconds; the VOC index starts at 100 and needs a few minutes to settle.

5. 05

   Let it learn

   The NOx index needs about 5 minutes of conditioning after every power-up, and the gas-index algorithm adapts to your room over 12–24 hours. Leave it running.

6. 06

   Calibrate CO₂ outdoors (optional)

   The SCD41 self-calibrates if it sees fresh air (≈420 ppm) at least once a week. In a room that is never aired, take it outside for 5 minutes once a week, or turn off auto-calibration in code.

Arduino code

Written for the ESP32-S3 DevKitC-1 with the Espressif Arduino core. Install these libraries from the Arduino Library Manager:

• SparkFun SCD4x Arduino Library
• Sensirion I2C SGP41
• Sensirion Gas Index Algorithm
• Adafruit SHT4x Library

#include <Wire.h>
#include <SparkFun_SCD4x_Arduino_Library.h>
#include <SensirionI2CSgp41.h>
#include <VOCGasIndexAlgorithm.h>
#include <NOxGasIndexAlgorithm.h>
#include <Adafruit_SHT4x.h>

SCD4x scd41;
SensirionI2CSgp41 sgp41;
Adafruit_SHT4x sht41;
VOCGasIndexAlgorithm vocAlgo;
NOxGasIndexAlgorithm noxAlgo;

uint16_t conditioningLeft = 10;   // SGP41 needs ~10 s of conditioning
uint16_t co2 = 0;

void setup() {
  Serial.begin(115200);
  Wire.begin(8, 9);               // SDA, SCL

  if (!scd41.begin()) Serial.println("SCD41 not found (0x62)");
  sgp41.begin(Wire);
  if (!sht41.begin()) Serial.println("SHT41 not found (0x44)");
  sht41.setPrecision(SHT4X_HIGH_PRECISION);
}

void loop() {
  // 1. Temperature and humidity from the SHT41
  sensors_event_t hum, temp;
  sht41.getEvent(&hum, &temp);

  // 2. SGP41 needs RH and T as "ticks" for compensation
  uint16_t rhTicks = (uint16_t)(hum.relative_humidity * 65535 / 100);
  uint16_t tTicks  = (uint16_t)((temp.temperature + 45) * 65535 / 175);
  uint16_t srawVoc = 0, srawNox = 0;

  if (conditioningLeft > 0) {
    sgp41.executeConditioning(rhTicks, tTicks, srawVoc);
    conditioningLeft--;
  } else {
    sgp41.measureRawSignals(rhTicks, tTicks, srawVoc, srawNox);
  }
  int32_t voc = vocAlgo.process(srawVoc);
  int32_t nox = noxAlgo.process(srawNox);

  // 3. CO2 from the SCD41 (new value every 5 s)
  if (scd41.readMeasurement()) co2 = scd41.getCO2();

  static uint8_t n = 0;
  if (++n >= 5) {                  // print every 5 s
    n = 0;
    const char* status = co2 < 800 ? "GOOD" : co2 < 1400 ? "VENTILATE" : "POOR";
    Serial.printf("CO2 %u ppm | VOC %ld | NOx %ld | %.1f C | %.0f %%RH | %s\n",
                  co2, (long)voc, (long)nox, temp.temperature, hum.relative_humidity, status);
  }
  delay(1000);                     // the gas-index algorithm expects 1 s steps
}

Troubleshooting

Take it further

• Add a small OLED or e-paper screen for a desk display
• Send readings to Home Assistant over Wi-Fi with ESPHome (scd4x, sgp4x and sht4x components)
• Add the ET-S14 particulate carrier for PM2.5