What is a "Salud"?
You have before you a carbon dioxide (CO2) air quality monitor. Salud is a Spanish toast "to your health." The Salud CO2 Monitor is a gift to help you understand and improve the air quality in your environment. It can also help identify situations where fresh air should be introduced to reduce the spread of airborne illnesses.
It measures the CO2 parts per million (ppm) in the air you are breathing. CO2 is a greenhouse gas, and is a large component of what humans exhale with every breath. With every breath you take, you exhale about 100 times more CO2 than you inhale. The level of CO2 in fresh air is growing over time and is one of the measures of the climate crisis. In Seattle, you can get measurements from the Space Needle thanks to NOAA.
Fresh air is about 415 - 430 ppm. Good quality indoor air is below 1000 ppm.
Between 1000 and 2000 ppm is a level of concern. It would be nice to get fresh air if you can. You are re-breathing a lot of CO2 that you exhaled. If other people are in the room, this is a good measure of your risk of exposure to airborne illnesses.
At above 2,000 ppm you may suffer some advserse effects. 5,000 ppm is the extreme limit for exposure in the workplace. More info from OSHA and the CDC.
How do you correct an environment with too much CO2? There are only three practical methods available: fewer people, bigger room, or introduce fresh air.
Why should you care? Don't take my word for it. Learn more:
New York Times: The Hot New Back-to-School Accessory? An Air Quality Monitor.
UK Health and Safety Executive: Identifying poorly ventilated areas by using CO2 monitors
GOV UK: All schools to receive carbon dioxide monitors
Most consumer CO2 meters do not measure above 2,000 ppm. This one goes to 5,000.
The display is e-ink like a Kindle. It refreshes slowly, but requires no power when inactive.
The buttons on the front don't do much. They bring up a menu with the QR code that brought you here and an option to perform a fresh air calibration.
The fresh air calibration is not needed very often, maybe a few times a year? Fresh air has a known and relatively stable CO2 ppm. In Seattle it is about 425. This provides a reference point for the sensor to calibrate its measurements. The calibration requires about 6 minutes in fresh air.
The button next to the USB port is the reset button. It should not be required in normal operation. If you connect it to a computer and press the reset button, a USB storage device should appear on your computer where you can edit the code or change configuration parameters. You could also change the graphic elements if you don't like cats.
This project is designed for a long battery life. To conserve battery, the Salud enters a power-saving deep sleep for five minutes between measurements. Above the display you will notice a small hole for an ambient light sensor. When it is dark or the battery is low, the sleep interval is increased to 15 minutes. The Salud will display a message when the battery requires charging. You can recharge it with any USB C charger. The battery should last at least five days. Maybe up to a week. The voltage is displayed on the front and will range from about 4.1v to 3.4v. Once it is below 3.6v it is almost dead.
Is this thing any good? Yes, I think it is, within the limits of reasonable expectations. I have set it to only report in increments of 25ppm. I am guessing it is dead-on accurate to +/- 50ppm, which is good enough to know if you have a problem and which way things are going. The measurements are very consistent across the five units built so far, and agree with a commercial CO2 monitor's readings. It uses the best CO2 sensor I could find for use in a project like this, the Sensirion SCD41 as packaged and sold by Adafruit. The electronics gubbins for this project are really very simple and not subject to being screwed up during implementation: a battery, an Adafruit MagTag, and the SCD41 module.
If you want to see a big number or are wondering if it is working, just breath in the general area of the Salud while it is taking one of its periodic measurements. This can be tricky to time with the five minute sleep interval. You should see a number well over 2500. Or just sleep in a sealed bedroom and take a gander at the high level of CO2 you wake up to in the morning.
You can force an immediate measurement and refresh of the display. To do this, press either of the left two buttons to wake from sleep and display the QR code. Then exit. This will force a fresh measurement and refresh the display.
Important: keep it out of direct sun, and always protect the sensor from foreign material or moisture.
Many monitors like this have a summary "at a glance" icon to display the current status. Red, Yellow, Green. Or smiley/frowny faces. But how conventionally boring is that? Not for the likes of us! Instead...CATS!
If you see Louie the tuxedo cat, the CO2 level is not a cause for concern. You can adopt Louie's attitude to life: chill, carefree, calm, relaxed. Tranquilo.
If you see Vera the tabby cat, you should be like she is and have an elevated level of concern, perhaps verging on the paranoid. jk. Kinda. Alerta!
And then at the highest levels of CO2, the cat jokes stop and you will see a scary biohazard symbol.
When you plug it into a computer and press the reset button, you should get a new storage folder called CIRCUITPY.
The file secrets.py has wifi settings should you wish to track your sensor's history and trends using the free Adafruit IO dashboard. There are also credentials for the Adafruit IO paltform in there, that you can change if you create your own free Adafruit IO account. Wifi is disabled by default.
The file config.py has settings you might want to edit to change things like the level of CO2 ppm that triggers an alert or change of display, as well as environmental parameters such as the elevation, local barometric pressure, and adjustments to the sensor's temperature readings. You can also change the sleep interval settings if you want to experiment with extending the batttery life further.
There are also .bmp image files with the background images if you want smiley and frowny faces instead of cats. Authoring these bmp images in the grayscale required for the e-ink display is an exercise left to the reader.