Signal, not guesswork
See stale air before you feel it
Rising CO₂ often means exhaled air is building up. That's when rooms get stuffy, focus drops, and the "I need fresh air" feeling kicks in. KnowCO₂ makes that visible.
Portable CO₂ monitor · Fully open source
See your air. Know your CO₂.
KnowCO₂™ is a pocketable, open-source CO₂ monitor built on ESP32-S3. Ships with a high-precision SCD41 on a swappable sensor architecture — SCD4x and SCD30 supported today, more sensors in testing, and an open driver API that lets anyone add support for new CO₂ sensors. Designed for makers, educators, and anyone who wants to understand indoor air quality in real time.
Wi-Fi telemetry ready
MQTT · HTTP · Adafruit IO · CSV export
Certified Open Source Hardware
Independently verified open by the Open Source Hardware Association
Hardware (CERN-OHL-S v2), firmware (MIT) & documentation (CC BY-SA 4.0) — design files, source, and docs are all openly licensed.
View certification ↗
Why CO₂ matters
CO₂ is more than a number. It's a proxy for ventilation, crowding, and how awake your brain feels in a room.
Thresholds that talk
Color-coded alerts at a glance
Tunable thresholds map directly onto the TFT display and rolling chart. Three display modes — text summary, big number, and live graph — so you pick what fits your space.
Open by design
Trustable hardware & firmware
Built using the ESP32-S3 Feather with a pluggable CO₂ sensor layer — SCD41 by default, SCD4x and SCD30 supported, Senseair Sunrise and Sensirion STCC4 in testing. Adding a new sensor is one driver file. All firmware, PCB files, and 3D enclosure models are open so you can inspect, modify, and improve them.
Connected everywhere
MQTT, HTTP, Adafruit IO & more
Native Home Assistant integration (HACS) plus MQTT auto-discovery — sensors appear in HA with zero YAML. Or push to any MQTT broker, post JSON to a custom HTTP endpoint, or stream to Adafruit IO — all from the built-in Wi-Fi portal.
Field-ready
Battery power + OTA updates
Runs on USB-C or an optional Li-Po battery with live fuel-gauge display. Update the firmware wirelessly over Wi-Fi — no cables, no reflashing tools needed.
Built for everyone
22 languages, full accessibility
The web configuration portal is fully translated into 22 languages. Built with ARIA roles, keyboard navigation, and reduced-motion support throughout.
How it works
From sensor readings to graphs and Wi-Fi telemetry in a few simple layers.
Step 1
Sense
The CO₂ sensor on I²C captures CO₂, temperature, and humidity every few seconds using high-accuracy NDIR technology. Ships with SCD41; the firmware's pluggable driver layer auto-detects SCD4x and SCD30, with more sensors in testing.
Step 2
Visualize
The ESP32-S3 updates the TFT display with a smooth rolling chart, color-coded by your configured thresholds.
Step 3
Connect
Built-in Wi-Fi lets you spin up an access point for configuration, or join your local network and push JSON to a cloud endpoint or local server.
Step 4
Share
Log readings, pipe them into dashboards, or contribute anonymized data to collaborative air-quality projects. Your device, your data pipeline.
For builders & educators
KnowCO₂ is a kit-friendly platform that's simple enough for a first hardware project, but extensible enough for serious telemetry.
Reference build
The default build uses off-the-shelf parts so anyone can assemble and flash the device without special tools.
MCU
Adafruit ESP32-S3 TFT Feather
Dual-core
USB-C
Wi-Fi + BLE
Sensor
SCD41 (default) — swappable: SCD4x & SCD30 supported, more in testing
NDIR
CO₂ + Temp + RH
Pluggable drivers
Power
USB-C power + optional Li-Po battery (not included)
Portable
External battery support
Enclosure
3D-printable
Screw-together case
Sensor airflow path
Example JSON telemetry
Point the device at your API or MQTT bridge. Each reading arrives as a small JSON payload you can drop into any stack.
POST https://api.knowco2.com/v1/ingest
{
"device_id": "co2-node-1",
"ts": 1781200402,
"co2": 742,
"temp_c": 22.3,
"rh": 46.1,
"battery_pct": 87,
"battery_v": 4.02,
"scd_serial": "12AB34CD5678",
"board_id": "adafruit_feather_esp32s3_reverse_tft"
}
Ready for AWS, Influx, Home Assistant & more.
FAQ
A few quick answers. You can always dive deeper in the docs and GitHub issues.
Is this a finished product?
Yes — KnowCO₂ is a complete, ready-to-use device. What makes it different is that it's also fully repairable, extensible, and open. Every part can be replaced or 3D-printed, the firmware updates over Wi-Fi, and it integrates out of the box with Home Assistant, MQTT brokers, Adafruit IO, and any custom API. You own it completely — including the right to modify, repair, and extend it however you like.
What accuracy should I expect?
KnowCO₂ ships with an SCD41 — a professional-grade NDIR CO₂ sensor. The firmware's sensor driver registry also supports the full SCD4x family and the SCD30, with the Senseair Sunrise and Sensirion STCC4 in testing — and adding a new sensor takes one driver file, so supported hardware keeps growing. Calibration docs are in the repo.
Can I change the CO₂ thresholds and chart behavior?
Yes. Thresholds, chart scale, alert modes, and more are configurable via the Wi-Fi settings page and in firmware. The JSON configuration is stored on the device so you can tweak and re-deploy.
How does Wi-Fi configuration work?
The device can create its own access point for first-time setup. From there, you can scan for networks, set the password, and configure your API endpoint or API key. If you skip Wi-Fi, it still works as a fully offline CO₂ display.
Is the hardware certified open source?
Yes. KnowCO₂ is certified by the Open Source Hardware Association (OSHWA) under UID US002828. OSHWA independently verifies that the hardware design files, firmware, and documentation are all openly licensed — hardware under CERN-OHL-S v2, software under MIT, and documentation under CC BY-SA 4.0 — so anyone is free to study, modify, build, and even sell the design.
Repair & self-service
KnowCO₂ is designed to be fully user-serviceable. Every part is replaceable, every file is published, and you never need a specialist to fix or upgrade it.
Every part is replaceable — drag to explore
3D-printable parts
Print your own replacement parts
All enclosure and sensor bracket files are published on GitHub. Print a replacement or customise the design for your own build.
- Main enclosure body & lid
- Sensor bracket with airflow cutout
- FDM-friendly — PLA or PETG, no supports needed
STL & 3MF on GitHub
Screw-together assembly
Sensor swap
Replace or upgrade your CO₂ sensor
The sensor uses a STEMMA QT cable over I²C — no soldering needed. SCD30, SCD40, SCD41 and other SCD4x variants are supported today; Senseair Sunrise and Sensirion STCC4 are in testing, and the open driver API means new sensors can be added with a single file.
- Power off the device
- Unplug the STEMMA QT cable from the old sensor
- Plug in the new sensor and reassemble
- Power on — sensor model is detected automatically
No soldering
Auto-detected on boot
More sensors coming
Firmware updates
Update firmware over Wi-Fi
No USB cable or flashing tool needed. Upload a new firmware file directly from your browser while the device is on your local network.
- Connect the device to your Wi-Fi network
- Open the device web portal (see Local Web Interface)
- Go to Settings → Update, upload the new firmware file
- Device reboots automatically — settings are preserved
No USB required
Settings preserved
CircuitPython runtime
Powered by open-source software, all the way down
KnowCO₂ runs on CircuitPython — a free, open-source programming language maintained by Adafruit and a global community. That means the software powering your device is fully auditable, improvable, and never locked to a vendor.
- If you ever need to refresh the runtime, it takes about two minutes with a USB-C cable
- Plug in, double-press Reset, and drag the new runtime file onto the drive that appears
- Reinstall the firmware from GitHub and you're back up
Open source runtime
Active community
USB-C cable only
Local web interface
Access the device on your network
Once connected to Wi-Fi, the device is reachable by name via mDNS — no need to find its IP address. The 4-character ID is shown on the device display.
- Connect device to your Wi-Fi (see Getting Started)
- Open knowco2-xxxx.local in any browser on the same network
- Configure thresholds, Wi-Fi, MQTT, and cloud settings
- Export a full CSV of readings from the portal
mDNS — no IP needed
CSV export built in
Using the device
Buttons & display modes
Three physical buttons control the display. No app or phone needed for day-to-day use.
- A — toggle between °C and °F · hold 2 s for low-power mode
- B — cycle display: summary → big number → live graph
- C — switch between readings and Wi-Fi info / QR code
- C (hold) — toggle between Wi-Fi client and access point mode
3 display modes
AP & client Wi-Fi
QR code for setup