This repo contains the hardware and software required to manufacture and assemble a small seven segment CO2 monitor.
The monitor is built around a 4 digit seven segment display (with automatic dimming based on ambient light) that displays the current atmospheric CO2 concentration as measured by an Adafruit SCD-40 module over i2c.
In practice, this project can serve as a generic platform for any i2c module that might benefit from a 4 digit display. Potentially, other projects could be built around this architecture.
There is a BOM included: the total cost per unit, as of when it was made in early 2025, was ~90 USD. Most of the cost comes from the SCD-40 module used -- directly integrating one of the modules onto the board should lower that price.
A rough block diagram is below:
The system has two power domains: a 5V domain is used for all digital logic, while the 3V domain is reserved for the seven-segement characters and associated dimming circuit. This separation of power supplies was chosen to both allow for a soft startup sequence (putting less strain on the overall power supply), and to remove some voltage headroom for the LEDs.
The final design was manufactured using JLCPCB's services. A BOM is included with this repo, as well as manufacturing files.
The board can be programmed using the Arduino IDE and a separate ISP over the SPI lines.