Making a CO2 Concentration Alert Keychain with Infrared SCD41 Photoacoustic Sensor

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Carbon dioxide emissions seem to be a major culprit of environmental pollution. Not only do they cause a decrease in air quality, but they can also lead to symptoms such as headaches, drowsiness, and decreased attention. I don't want to become a lazy panda due to poor indoor air quality, do you?

If you're like me and worried about your indoor environment quality, you may need a portable carbon dioxide detection device. A detection card made using an infrared light sound carbon dioxide sensor should be able to help us achieve this. This type of detection card can help us understand the concentration of carbon dioxide in the surrounding environment. That way, we can determine when to ventilate or go outside for fresh air. No more complaining about dizziness and blurred vision due to insufficient oxygen indoors! With this small device, we can always be aware of the indoor air quality.


1 Beetle ESP32-C3
1 Infrared carbon dioxide sensor
1 1.47" 172×320 IPS LCD Display
1 3.7V battery
1 RGB LED strip
1 3D printed shell
1 Acrylic board
1 Switch

Connection Schematics 



ESP32-C3 -- Infrared Light Sound Carbon Dioxide Sensor

3.3V   -   VCC

GND  -   GND

9        -    SCL

8        -    SDA


ESP32-C3 -- 1.4 Inch High-definition Display

3.3V   -   VCC

GND  -   GND

4        -   SCLK

6        -   MOSI

7        -    CS

2        -   RES

1        -   DC

10      -   BLK


ESP32-C3 -- RGB LED Strip

3.3V   -   VCC

GND  -   GND

20      -   DI

Shell Design

To create a 3D shell, first measure the spatial layout and size of the hardware.


I used Solidworks to draw the model, which consists of a 3D printed shell and an acrylic board cover, with reserved positions for the type-C interface, switch, and probe hole.


Slice and print the 3D model.

Cut the acrylic cover:

According to the size of the shell, use CAD software to draw a cutting diagram. The blue and black lines in the drawing are used to distinguish processing priority, with black parts being processed first followed by blue parts.


I used 1mm black semi-transparent acrylic board.


Hardware Soldering


The hardware that needs to be soldered includes: ESP32-C3, infrared light sound carbon dioxide sensor, 1.4 inch high-definition display, 3.7V battery, RGB LED strip, and switch.


Complete the circuit soldering according to the circuit connection diagram. Be sure to note that pin 10 of the ESP32-C3 is on the back.



Then, the soldering work is done.


Hardware Assembly




Secure the switch with hot glue.


Secure the battery with double-sided tape on the shell.


Use the protrusion points on the model and the sensor's hole position to secure the infrared carbon dioxide sensor, and apply a small amount of hot glue at the four points of contact.


Install the ESP32-C3 and align the type-C interface with the reserved position.


Use M2 screws to fasten the main control board to the mounting bracket to avoid displacement of the main control board when inserting the data cable.


Paste double-sided insulating tape on the back of the LED strip for easy fixing and to prevent short circuits caused by contact with solder joints.


Use hot melt glue to fix the screen.



Finally, apply UV glue to the outer shell and install the acrylic cover.



Uploading Program


Next, connect the device to the computer using a USB cable and upload the program.

In the program, the data read by the infrared light sound carbon dioxide sensor is displayed on the screen.

The trend line of changes in carbon dioxide concentration is displayed on the screen.

The number of LED beads/light strip breathing frequency is lit by the RGB LED strip to reflect the size of the carbon dioxide concentration.





When you see the keychain light up red, remember to open the window for ventilation or go outside for fresh air.

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