CO₂ sensors are essential for determining if a room is too 'stuffy' - high CO₂ makes humans grumpy and tired. That's why it's always nice to take a deep breath outside where CO₂ is about 400 ppm. However, until now, CO₂ sensors were always really chunky (like the SCD-30) or pricey (like the SCD-40). Inexpensive sensors like the SGP30 approximate the CO₂ using VOC gas concentration, but they don't actually measure CO₂.

The STCC4 is a tiny (4 x 3 x 1.2mm) sensor that can measure CO₂ gas and fit in just about any enclosure. It uses thermal conductivity (TC) instead of NDIR or photoacoustic measurements. TC is based on the inherent thermal conductivity of all gases. With a thorough understanding of the gas composition in ambient environments, subtle changes in gas concentrations can be detected. The measurement principle is based on heating the air within a measurement cavity and sensing the heat transfer with a temperature sensor. For more details check out the EYE ON NPI video we did on this sensor!

The trade-off for the small size and lower cost is that STCC4 is that it's really tuned just for indoor air measurement applications - not for scientific CO₂ sensing or where the air gas composition is 'abnormal'. That said, the vast majority of use cases we see are indoor air monitoring to reduce illness and stuffiness and for that purpose the STCC4 is great!

One thing to note, like all true CO₂ sensors, it must be self-calibrated by having it exposed to outdoor air once a week, to get a baseline 400 ppm measurement - in practice that means opening a window or making sure fresh air gets to the sensor.

To round the STCC4 out and make it a perfect mini indoor air quality sensor, we've added an SHT41 as well to improve the CO₂ measurements. The SHT41 has an excellent ±1.8% typical relative humidity accuracy from 25 to 75% and ±0.2 °C typical accuracy from 0 to 75 °C. Note it is connected to the secondary I2C port on the STCC4 so that you only have to query the STCC4 to get measurements rather than talk to both chips with two libraries.

Such a lovely pair of chips - so we spun up a breakout board with the STCC4 + SHT41 and some supporting circuitry such as pullup resistors and capacitors. To make things even easier, we've included SparkFun Qwiic compatible STEMMA QT connectors for the I2C bus so you don't even need to solder! QT Cable is not included, but we have a variety in the shop.

If you prefer working on a breadboard, each order comes with one fully assembled and tested PCB breakout and a small piece of header. You'll need to solder the header onto the PCB, but it's fairly easy and takes only a few minutes even for a beginner.

We've written both Arduino and CircuitPython/Python library code for this chip, so you can use it with just about any microcontroller or single-board computer like Raspberry Pi.

The default I2C address is 0x64.

Power Pins

• VIN - this is the power pin. To power the board, give it the same power as the logic level of your microcontroller - e.g. for a 5V microcontroller like Arduino, use 5V. For a 3.3V microcontroller, use 3.3V.
• 3Vo - this is the 3.3V output from the voltage regulator, you can grab up to 100mA from this if you like.
• GND - common ground for power and logic.

I2C Logic Pins

• SCL - I2C clock pin, connect to your microcontroller I2C clock line. This pin is level shifted so you can use 3-5V logic, and there's a 10K pullup on this pin.
• SDA - I2C data pin, connect to your microcontroller I2C data line. This pin is level shifted so you can use 3-5V logic, and there's a 10K pullup on this pin.
• STEMMA QT - These connectors allow you to connect to dev boards with STEMMA QT connectors or to other things with various associated accessories.

Address Pin and Jumper

• ADDR - The address pin for setting the I2C address. You can chain up to two of these boards together on one I2C bus. Leave this pin low for default I2C address 0x64 or tie it high for I2C address 0x65.
• Addr Jumper - On the back of the board is the address jumper, labeled Addr. You can leave this jumper open (low) to keep the board on the default I2C address 0x64. Solder the jumper closed (high) to change the I2C address to 0x65.

Power LED and Jumper

• Power LED - In the upper left corner, above the STEMMA QT connector, on the front of the board, is the power LED, labeled on. It is a green LED.
• LED jumper - This jumper is located on the back of the board and is labeled LED on the board silk. Cut the trace on this jumper to cut power to the "on" LED.

It's easy to use the STCC4 with Python or CircuitPython, and the Adafruit_CircuitPython_STCC4 module. This module allows you to easily write Python code that allows you to read the STCC4 CO2, temperature and humidity sensor. You can use this sensor with any CircuitPython microcontroller board or with a computer that has GPIO and Python thanks to Adafruit_Blinka, our CircuitPython-for-Python compatibility library.

CircuitPython Microcontroller Wiring

First, wire up an STCC4 to your board exactly as shown below. Here's an example of wiring a Feather RP2040 to the STCC4 with I2C using one of the handy STEMMA QT connectors:

You can also use standard 0.100" pitch headers to wire it up on a breadboard:

Python Computer Wiring

Since there's dozens of Linux computers/boards you can use, below shows wiring for Raspberry Pi. For other platforms, please visit the guide for CircuitPython on Linux to see whether your platform is supported. 

Here's the Raspberry Pi wired to the sensor using I2C and a STEMMA QT connector:

Finally, here is an example of how to wire up a Raspberry Pi to the sensor using a solderless breadboard:

Python Installation of STCC4 Library

You'll need to install the Adafruit_Blinka library that provides the CircuitPython support in Python. This may also require enabling I2C on your platform and verifying you are running Python 3. Since each platform is a little different, and Linux changes often, please visit the CircuitPython on Linux guide to get your computer ready!

Once that's done, from your command line run the following command:

• pip3 install adafruit-circuitpython-stcc4

If your default Python is version 3, you may need to run pip instead. Make sure you aren't trying to use CircuitPython on Python 2.x, it isn't supported!

CircuitPython Usage

To use with CircuitPython, you need to first install the STCC4 library and its dependencies into the lib folder on your CIRCUITPY drive. Then you need to update code.py with the example script.

Thankfully, we can do this in one go. In the example below, click the Download Project Bundle button below to download the necessary libraries and the code.py file in a zip file. Extract the contents of the zip file, and copy the entire lib folder and the code.py file to your CIRCUITPY drive.

Your CIRCUITPY/lib folder should contain the following folder and file:

• adafruit_bus_device/
• adafruit_stcc4.mpy

Python Usage

Once you have the library pip3 installed on your computer, copy or download the following example to your computer, and run the following, replacing code.py with whatever you named the file:

python3 code.py

Example Code

If running CircuitPython: Once everything is saved to the CIRCUITPY drive, connect to the serial console to see the data printed out!

If running Python: The console output will appear wherever you are running Python.

# SPDX-FileCopyrightText: Copyright (c) 2026 Liz Clark for Adafruit Industries
#
# SPDX-License-Identifier: MIT

import time

import board

import adafruit_stcc4

i2c = board.I2C()
sensor = adafruit_stcc4.STCC4(i2c)
print("Starting continuous measurement...")
sensor.continuous_measurement = True

while True:
    co2 = sensor.CO2
    print(
        f"CO2: {co2} ppm | "
        f"Temperature: {sensor.temperature:.1f} °C | "
        f"Humidity: {sensor.relative_humidity:.1f} %"
    )
    time.sleep(1)

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# SPDX-FileCopyrightText: Copyright (c) 2026 Liz Clark for Adafruit Industries
#
# SPDX-License-Identifier: MIT

import time

import board

import adafruit_stcc4

i2c = board.I2C()
sensor = adafruit_stcc4.STCC4(i2c)
print("Starting continuous measurement...")
sensor.continuous_measurement = True

while True:
    co2 = sensor.CO2
    print(
        f"CO2: {co2} ppm | "
        f"Temperature: {sensor.temperature:.1f} °C | "
        f"Humidity: {sensor.relative_humidity:.1f} %"
    )
    time.sleep(1)

In the example, the STCC4 sensor is instantiated on I2C. Then, in the loop, the CO2, temperature and humidity readings are printed to the serial console every second.

Using the STCC4 CO2, temperature and humidity sensor with Arduino involves wiring up the sensor to your Arduino-compatible microcontroller, installing the Adafruit_STCC4 library and running the provided example code.

Wiring

Wire as shown for a 5V board like an Uno. If you are using a 3V board, like an Adafruit Feather, wire the board's 3V pin to the STCC4 VIN.

Here is an Adafruit Metro wired up to the STCC4 using the STEMMA QT connector:

Here is an Adafruit Metro wired up using a solderless breadboard:

Library Installation

You can install the Adafruit STCC4 library for Arduino using the Library Manager in the Arduino IDE.

Click the Manage Libraries ... menu item, search for Adafruit STCC4, and select the Adafruit STCC4 library:

If asked about dependencies, click "Install all".

If the "Dependencies" window does not come up, then you already have the dependencies installed.

Example Code

// Continuous measurement example for Adafruit STCC4 CO2 sensor

#include <Adafruit_STCC4.h>

Adafruit_STCC4 stcc4;

void printStatus(uint16_t status) {
  Serial.print(F("Status: 0x"));
  Serial.print(status, HEX);
  Serial.print(F(" ("));

  bool first = true;
  if (status & STCC4_STATUS_VOLTAGE_ERROR) {
    if (!first) Serial.print(F(", "));
    Serial.print(F("VOLTAGE_ERROR"));
    first = false;
  }
  if (status & STCC4_STATUS_DEBUG_MASK) {
    if (!first) Serial.print(F(", "));
    Serial.print(F("DEBUG"));
    first = false;
  }
  if (status & STCC4_STATUS_SHT_NOT_CONNECTED) {
    if (!first) Serial.print(F(", "));
    Serial.print(F("SHT_NOT_CONNECTED"));
    first = false;
  }
  if (status & STCC4_STATUS_MEMORY_ERROR_MASK) {
    if (!first) Serial.print(F(", "));
    Serial.print(F("MEMORY_ERROR"));
    first = false;
  }
  if (status & STCC4_STATUS_TESTING_MODE) {
    if (!first) Serial.print(F(", "));
    Serial.print(F("TESTING_MODE"));
    first = false;
  }
  if (first) {
    Serial.print(F("OK"));
  }
  Serial.println(F(")"));
}

void setup() {
  Serial.begin(115200);
  while (!Serial) delay(10);

  Serial.println(F("Adafruit STCC4 test"));

  if (!stcc4.begin()) {
    Serial.println(F("Failed to find STCC4 chip"));
    while (1) delay(10);
  }

  Serial.println(F("STCC4 found!"));

  if (!stcc4.reset()) {
    Serial.println(F("Failed to reset STCC4"));
    while (1) delay(10);
  }
  Serial.println(F("Reset successful"));

  // Test getProductID function after reset
  uint32_t productID = stcc4.getProductID();
  Serial.print(F("Product ID: 0x"));
  Serial.println(productID, HEX);

  // Uncomment to perform factory reset (clears calibration history)
  // Serial.println(F("Performing factory reset..."));
  // if (stcc4.factoryReset()) {
  //   Serial.println(F("Factory reset complete"));
  // } else {
  //   Serial.println(F("Factory reset failed"));
  // }

  if (!stcc4.enableContinuousMeasurement(true)) {
    Serial.println(F("Failed to start continuous measurement"));
    while (1) delay(10);
  }
  Serial.println(F("Continuous measurement started"));

  // Uncomment to perform conditioning (takes 22 seconds)
  // Serial.println(F("Performing conditioning..."));
  // if (stcc4.performConditioning()) {
  //   Serial.println(F("Conditioning complete"));
  // } else {
  //   Serial.println(F("Conditioning failed"));
  // }
}

void loop() {
  uint16_t co2;
  float temperature, humidity;
  uint16_t status;

  if (stcc4.readMeasurement(&co2, &temperature, &humidity, &status)) {
    Serial.print(F("CO2: "));
    Serial.print(co2);
    Serial.print(F(" ppm, Temp: "));
    Serial.print(temperature);
    Serial.print(F(" C, Humidity: "));
    Serial.print(humidity);
    Serial.print(F(" %, "));
    printStatus(status);
  } else {
    Serial.println(F("Failed to read measurement"));
  }

  delay(1000);
}

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// Continuous measurement example for Adafruit STCC4 CO2 sensor

#include <Adafruit_STCC4.h>

Adafruit_STCC4 stcc4;

void printStatus(uint16_t status) {
  Serial.print(F("Status: 0x"));
  Serial.print(status, HEX);
  Serial.print(F(" ("));

  bool first = true;
  if (status & STCC4_STATUS_VOLTAGE_ERROR) {
    if (!first) Serial.print(F(", "));
    Serial.print(F("VOLTAGE_ERROR"));
    first = false;
  }
  if (status & STCC4_STATUS_DEBUG_MASK) {
    if (!first) Serial.print(F(", "));
    Serial.print(F("DEBUG"));
    first = false;
  }
  if (status & STCC4_STATUS_SHT_NOT_CONNECTED) {
    if (!first) Serial.print(F(", "));
    Serial.print(F("SHT_NOT_CONNECTED"));
    first = false;
  }
  if (status & STCC4_STATUS_MEMORY_ERROR_MASK) {
    if (!first) Serial.print(F(", "));
    Serial.print(F("MEMORY_ERROR"));
    first = false;
  }
  if (status & STCC4_STATUS_TESTING_MODE) {
    if (!first) Serial.print(F(", "));
    Serial.print(F("TESTING_MODE"));
    first = false;
  }
  if (first) {
    Serial.print(F("OK"));
  }
  Serial.println(F(")"));
}

void setup() {
  Serial.begin(115200);
  while (!Serial) delay(10);

  Serial.println(F("Adafruit STCC4 test"));

  if (!stcc4.begin()) {
    Serial.println(F("Failed to find STCC4 chip"));
    while (1) delay(10);
  }

  Serial.println(F("STCC4 found!"));

  if (!stcc4.reset()) {
    Serial.println(F("Failed to reset STCC4"));
    while (1) delay(10);
  }
  Serial.println(F("Reset successful"));

  // Test getProductID function after reset
  uint32_t productID = stcc4.getProductID();
  Serial.print(F("Product ID: 0x"));
  Serial.println(productID, HEX);

  // Uncomment to perform factory reset (clears calibration history)
  // Serial.println(F("Performing factory reset..."));
  // if (stcc4.factoryReset()) {
  //   Serial.println(F("Factory reset complete"));
  // } else {
  //   Serial.println(F("Factory reset failed"));
  // }

  if (!stcc4.enableContinuousMeasurement(true)) {
    Serial.println(F("Failed to start continuous measurement"));
    while (1) delay(10);
  }
  Serial.println(F("Continuous measurement started"));

  // Uncomment to perform conditioning (takes 22 seconds)
  // Serial.println(F("Performing conditioning..."));
  // if (stcc4.performConditioning()) {
  //   Serial.println(F("Conditioning complete"));
  // } else {
  //   Serial.println(F("Conditioning failed"));
  // }
}

void loop() {
  uint16_t co2;
  float temperature, humidity;
  uint16_t status;

  if (stcc4.readMeasurement(&co2, &temperature, &humidity, &status)) {
    Serial.print(F("CO2: "));
    Serial.print(co2);
    Serial.print(F(" ppm, Temp: "));
    Serial.print(temperature);
    Serial.print(F(" C, Humidity: "));
    Serial.print(humidity);
    Serial.print(F(" %, "));
    printStatus(status);
  } else {
    Serial.println(F("Failed to read measurement"));
  }

  delay(1000);
}

Upload the sketch to your board and open up the Serial Monitor (Tools -> Serial Monitor) at 115200 baud. You should see that the sketch has found your connected STCC4 sensor. Then, you'll see the CO2, temperature, humidity and status readings printed to the Serial Monitor every second.

What is WipperSnapper

WipperSnapper is a firmware designed to turn any WiFi-capable board into an Internet-of-Things device without programming a single line of code. WipperSnapper connects to Adafruit IO, a web platform designed (by Adafruit!) to display, respond, and interact with your project's data.

Simply load the WipperSnapper firmware onto your board, add credentials, and plug it into power. Your board will automatically register itself with your Adafruit IO account.

From there, you can add components to your board such as buttons, switches, potentiometers, sensors, and more! Components are dynamically added to hardware, so you can immediately start interacting, logging, and streaming the data your projects produce without writing code.

If you've never used WipperSnapper, click below to read through the quick start guide before continuing.

Wiring

Wire up the STCC4 exactly as follows. Here it's shown connected via our convenient StemmaQT cable, or alternatively wired up on a prototyping breadboard:

Usage

Connect your board to Adafruit IO Wippersnapper and navigate to the WipperSnapper board list.

On this page, select the WipperSnapper board you're using to be brought to the board's interface page.

If you do not see your board listed here - you need to connect your board to Adafruit IO first.

Next, make sure the sensor is plugged into your board and click the I2C Scan button.

You should see the STCC4's default I2C address of 0x64 pop-up in the I2C scan list.

With the sensor detected in an I2C scan, you're ready to add the sensor to your board.

Click the New Component button or the + button to bring up the component picker.

Adafruit IO supports a large amount of components. To quickly find your sensor, type STCC4 into the search bar, then select the STCC4 component.

On the component configuration page, the STCC4's sensor address should be listed along with the sensor's settings.

The Send Every option is specific to each sensor's measurements. This option will tell the Feather how often it should read from the STCC4 sensor and send the data to Adafruit IO. Measurements can range from every second to every 24 hours.

For this example, set the Send Every interval to every 30 seconds (scroll down to see the CO2 setting).

Your device interface should now show the sensor components you created. After the interval you configured elapses, WipperSnapper will automatically read values from the sensor(s) and send them to Adafruit IO.

To view the data that has been logged from the sensor, click on the graph next to the sensor name.

Here you can see the feed history and edit things about the feed such as the name, privacy, webhooks associated with the feed and more. If you want to learn more about how feeds work, check out this page.

Files

• STCC4 Datasheet
• SHT41 Datasheet
• EagleCAD PCB files on GitHub
• Fritzing object in the Adafruit Fritzing Library

Schematic and Fab Print