SENSORS/ACTUATORS

INTERNET OF THING (IoT)

Arduino Nano 33 IoT - HTTP Request

Arduino Nano 33 IoT - SCD40 CO2 Sensor

Have you ever felt sleepy in a closed room after an hour or two? Often the reason is a high level of carbon dioxide (CO2) in the air. In this guide, the Arduino Nano 33 IoT and a Sensirion SCD40 module work together to show the real CO2 level of your room, in ppm, on the Serial Monitor. We will cover:

How the SCD40 measures CO2, and how to read its ppm value.
How to connect the SCD40 to the I2C pins of the Arduino Nano 33 IoT.
How to write code that prints the CO2 level with a simple air quality label.
How to read the temperature and humidity from the same sensor.

Hardware Preparation

1	×	Arduino Nano 33 IoT
1	×	Micro USB Cable
1	×	SCD40 CO2 Sensor Module
1	×	Breadboard
1	×	Jumper Wires
1	×	Recommended: Screw Terminal Expansion Board for Arduino Nano
1	×	Recommended: Breakout Expansion Board for Arduino Nano
1	×	Recommended: Power Splitter for Arduino Nano

Or you can buy the following kits:

DIYables Sensor Kit (18 sensors/displays)

Disclosure: Some of the links provided in this section are Amazon affiliate links. We may receive a commission for any purchases made through these links at no additional cost to you.
Additionally, some of these links are for products from our own brand, DIYables .

Overview of SCD40 CO2 Sensor

Cheap gas sensors such as the MQ2 used in the Arduino Nano 33 IoT - Gas Sensor tutorial only give a rough "gas or no gas" value. The SCD40 is different: it measures true CO2, not an estimated "eCO2" number. It works by photoacoustic NDIR sensing. In simple words, a tiny infrared light pulses inside a small chamber. CO2 absorbs some of that light and makes a very small pressure "sound", and a microphone picks it up. More CO2 means a stronger signal.

The sensor also has a built-in temperature and humidity sensor. It uses them to correct the CO2 value, and you can read them too.

	SCD40
Supply voltage	2.4V to 5.5V
Interface	I2C, address 0x62
CO2 output range	0 to 40,000 ppm
CO2 accuracy	±(50 ppm + 5% of reading), from 400 to 2,000 ppm
New reading	every 5 seconds (every 30 seconds in low-power mode)
Temperature accuracy	±0.8°C (15°C to 35°C)
Humidity accuracy	±6 %RH (15°C to 35°C, 20 to 65 %RH)
Average current	about 15 mA at 3.3V

The SCD40 CO2 Sensor Pinout

The SCD40 module has four pins:

VCC pin (sometimes VIN): must be connected to 3.3V or 5V. With the Arduino Nano 33 IoT, use 3.3V.
GND pin: must be connected to GND (0V).
SCL pin: the I2C clock line. It must be connected to the SCL pin of the Arduino Nano 33 IoT.
SDA pin: the I2C data line. It must be connected to the SDA pin of the Arduino Nano 33 IoT.

Most SCD40 modules already have I2C pull-up resistors on the board, so no extra parts are needed.

※ NOTE THAT:

The arrangement of pins on a module may differ from one manufacturer to another. Always refer to the labels printed on your module.

What Does the CO2 Value Mean?

Use this table to understand the ppm numbers:

CO2 level	Air quality
About 400 ppm	Fresh outdoor air
400 to 1,000 ppm	Good indoor air
1,000 to 2,000 ppm	Stuffy - time to open a window
Above 2,000 ppm	Poor - headache and drowsiness are possible

The SCD40 has automatic self-calibration (ASC), and it is ON by default. It expects to see fresh, outdoor-level air (about 400 ppm) at least once a week, ideally for an hour or more. So, let fresh air into the room from time to time. Also, the first few readings after power-up can be less accurate while the sensor warms up.

Wiring Diagram

The SCD40 talks over I2C. On the Arduino Nano 33 IoT, the I2C pins are A4 (SDA) and A5 (SCL). Since the board runs at 3.3V logic, we power the sensor from the 3.3V pin, so the I2C lines also stay at 3.3V.

This image is created using Fritzing. Click to enlarge image

SCD40 Module	Arduino Nano 33 IoT
VCC	3.3V
GND	GND
SDA	A4
SCL	A5

How To Program For SCD40 CO2 Sensor

The first step is to include the libraries and create a sensor object:

#include <Arduino.h> #include <SensirionI2cScd4x.h> #include <Wire.h> SensirionI2cScd4x scd4x;

Start the I2C bus and tell the library where the sensor is. Wire.begin() uses the default A4 / A5 pins:

Wire.begin(); scd4x.begin(Wire, SCD40_I2C_ADDR_62);

Put the sensor in a clean state, then start periodic measurement:

scd4x.wakeUp(); scd4x.stopPeriodicMeasurement(); scd4x.reinit(); scd4x.startPeriodicMeasurement();

In loop(), check if a new reading is ready, then read it:

bool dataReady = false; scd4x.getDataReadyStatus(dataReady); uint16_t co2 = 0; float temperature = 0.0f; float humidity = 0.0f; scd4x.readMeasurement(co2, temperature, humidity);

If a function returns a non-zero error code, turn it into text with errorToString() and print it. This helps a lot when a wire is loose.

Arduino Nano 33 IoT Code for SCD40 CO2 Sensor

This code prints the CO2 level each time a new reading is ready (about every 5 seconds). Next to the number, it prints a word from the table above: Good, Moderate or Poor.

/* * This Arduino Nano 33 IoT code was developed by newbiely.com * * This Arduino Nano 33 IoT code is made available for public use without any restriction * * For comprehensive instructions and wiring diagrams, please visit: * https://newbiely.com/tutorials/arduino-nano-iot/arduino-nano-33-iot-scd40-co2-sensor */ #include <Arduino.h> #include <SensirionI2cScd4x.h> #include <Wire.h> // The SCD40 uses the I2C bus of the Arduino Nano 33 IoT: SDA = A4, SCL = A5 SensirionI2cScd4x scd4x; char errorMessage[64]; int16_t error; // Turn a CO2 value (ppm) into a simple air quality word const char* airQuality(uint16_t co2) { if (co2 < 1000) return "Good"; if (co2 < 2000) return "Moderate - open a window"; return "Poor"; } void setup() { Serial.begin(9600); while (!Serial) { delay(100); // wait for the Serial Monitor to open } Wire.begin(); scd4x.begin(Wire, SCD40_I2C_ADDR_62); // Clean start: wake up, stop any running measurement, reload settings scd4x.wakeUp(); // may return an error if the sensor is already awake, that is OK scd4x.stopPeriodicMeasurement(); scd4x.reinit(); // Start periodic mode: a new reading every 5 seconds error = scd4x.startPeriodicMeasurement(); if (error) { errorToString(error, errorMessage, sizeof errorMessage); Serial.print("Cannot start measurement: "); Serial.println(errorMessage); return; } Serial.println("SCD40 started, the first reading comes in about 5 seconds..."); } void loop() { bool dataReady = false; uint16_t co2 = 0; float temperature = 0.0f; // read too, but not printed in this example float humidity = 0.0f; // read too, but not printed in this example // Ask the sensor if a new reading is ready error = scd4x.getDataReadyStatus(dataReady); if (error) { errorToString(error, errorMessage, sizeof errorMessage); Serial.print("Data ready error: "); Serial.println(errorMessage); delay(1000); return; } if (!dataReady) { delay(500); // not ready yet, check again soon return; } // Read the new measurement error = scd4x.readMeasurement(co2, temperature, humidity); if (error) { errorToString(error, errorMessage, sizeof errorMessage); Serial.print("Read error: "); Serial.println(errorMessage); return; } Serial.print("CO2: "); Serial.print(co2); Serial.print(" ppm -> "); Serial.println(airQuality(co2)); }

Detailed Instructions

If you are new to the Arduino Nano 33 IoT, be sure to check out our Getting Started with Arduino Nano 33 IoT tutorial. Then, follow these steps:

Wire the SCD40 module to the Arduino Nano 33 IoT as shown in the diagram.
Connect the Arduino Nano 33 IoT board to your computer using a USB cable.
Open Arduino IDE on your computer.
Select the Arduino Nano 33 IoT board and its COM port.
Click the Library Manager icon on the left side of the Arduino IDE.
Search for SCD4x and find the Sensirion I2C SCD4x library by Sensirion.
Click the Install button.

Search for Sensirion I2C SCD4x created by Sensirion and click the Install button.

Newbiely | Arduino IDE 2.3.8

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Library Manager

Sensirion I2C SCD4x

All

Sensirion I2C SCD4x by Sensirion

Enables you to use the SCD4X sensor family via I2C. More info

1.1.0

INSTALL

Newbiely.ino

···

1 void setup() {

Output

Serial Monitor

Ln 1, Col 1

Arduino Nano 33 IoT on COM15

A window asks to install the dependency Sensirion Core. Click Install All.
Copy the code above and paste it into the Arduino IDE.
Click the Upload button to compile and send the code to the board.
Open the Serial Monitor. The first value shows up after about 5 seconds.
Breathe gently toward the sensor and watch the CO2 value go up.

Newbiely | Arduino IDE 2.3.8

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Newbiely.ino

···

8 Serial.println("Hello World!");

Output

Serial Monitor

Message (Enter to send message to 'Arduino Nano 33 IoT' on 'COM15')

New Line

9600 baud

SCD40 started, the first reading comes in about 5 seconds... CO2: 612 ppm -> Good CO2: 608 ppm -> Good CO2: 615 ppm -> Good CO2: 1184 ppm -> Moderate - open a window CO2: 1047 ppm -> Moderate - open a window CO2: 873 ppm -> Good CO2: 721 ppm -> Good CO2: 659 ppm -> Good

Ln 11, Col 1

Arduino Nano 33 IoT on COM15

The jump to over 1,000 ppm in the middle is from breathing on the sensor. It drops back as the air around it mixes again.

Reading Temperature and Humidity from the SCD40

The SCD40 has more to give than CO2. Each time you call readMeasurement(), it also returns the temperature (in °C) and the relative humidity (in %). The first code already reads them, but it does not print them. The code below prints all three values on one line, so the Arduino Nano 33 IoT becomes a small indoor climate monitor.

Keep in mind that the sensor heats itself a little while it works. If the temperature looks one or two degrees higher than your room thermometer, this is normal.

/* * This Arduino Nano 33 IoT code was developed by newbiely.com * * This Arduino Nano 33 IoT code is made available for public use without any restriction * * For comprehensive instructions and wiring diagrams, please visit: * https://newbiely.com/tutorials/arduino-nano-iot/arduino-nano-33-iot-scd40-co2-sensor */ #include <Arduino.h> #include <SensirionI2cScd4x.h> #include <Wire.h> // The SCD40 uses the I2C bus of the Arduino Nano 33 IoT: SDA = A4, SCL = A5 SensirionI2cScd4x scd4x; char errorMessage[64]; int16_t error; void setup() { Serial.begin(9600); while (!Serial) { delay(100); // wait for the Serial Monitor to open } Wire.begin(); scd4x.begin(Wire, SCD40_I2C_ADDR_62); // Clean start scd4x.wakeUp(); // an error here is normal if the sensor is already awake scd4x.stopPeriodicMeasurement(); scd4x.reinit(); error = scd4x.startPeriodicMeasurement(); if (error) { errorToString(error, errorMessage, sizeof errorMessage); Serial.print("Cannot start measurement: "); Serial.println(errorMessage); return; } } void loop() { bool dataReady = false; uint16_t co2 = 0; float temperature = 0.0f; float humidity = 0.0f; error = scd4x.getDataReadyStatus(dataReady); if (error || !dataReady) { delay(500); // no new data yet (a reading comes every 5 seconds) return; } // One read gives all three values error = scd4x.readMeasurement(co2, temperature, humidity); if (error) { errorToString(error, errorMessage, sizeof errorMessage); Serial.print("Read error: "); Serial.println(errorMessage); return; } Serial.print("CO2: "); Serial.print(co2); Serial.print(" ppm | Temperature: "); Serial.print(temperature, 1); Serial.print(" °C | Humidity: "); Serial.print(humidity, 1); Serial.println(" %"); }

Upload it the same way as before, then open the Serial Monitor:

Newbiely | Arduino IDE 2.3.8

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☐

Newbiely.ino

···

8 Serial.println("Hello World!");

Output

Serial Monitor

Message (Enter to send message to 'Arduino Nano 33 IoT' on 'COM15')

New Line

9600 baud

Ln 11, Col 1

Arduino Nano 33 IoT on COM15

Additional Knowledge

How does the SCD40 compare with the MQ2 gas sensor?

	SCD40	MQ2
What it measures	true CO2 in ppm	a mix of flammable gases and smoke
Output	digital (I2C)	analog and digital (threshold)
Extra values	temperature and humidity	none
Warm-up	short	long (heater needs time)

It is evident that the SCD40 is the right choice for checking indoor air and ventilation, while the MQ2 is meant for detecting gas leaks and smoke.

Video Tutorial

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