Air Quality Monitoring Powered By Cat Ears

To Lüften, or not to Lüften? That is the CO₂ question. Let’s check the ears.

If you’ve met me in person any time in the last few years, you might have seen me whip out a little gadget and stare at it obsessively as various strange graphics appear on the screen. But that’s enough about my phone.

This is a post about a Do-It-Yourself air quality monitoring badge that I use when I’m out and about to help me understand what the air quality is like. If you would like to try making your own badge, there should be enough info here to get you started.

Moodily lit photo of an electronic badge, displaying an ISO 7010 standard safety sign which depicts a person’s head in white on a blue background. The person is wearing a respirator and what appear to be Personal Protective Cat Ears. Under the graphic is some white text indicating that the CO₂ (Carbon Dioxide) reading is 1542 PPM and the temperature is 19°C with 60% relative humdity. There is also an event lanyard with a design that may be familiar to some readers. Let’s just say it Might Cause Happiness.

This is something like the fifth or sixth generation air quality monitoring badge that I’ve made in the years since COVID entered our lives. It’s also the one which (I hope) is least likely to result in an unexpected trip to the Special Room if you bring it through a border security check. No guarantees, though!

As you may have surmised I had decided to display some colour coded images to make it easier to tell at a glance what the air quality was like. Helpfully, Wikimedia Commons has a whole section of public domain ISO 7010 safety signs, like these:

Three ISO 7010 inspired safety signs. The first one shows a lifebuoy with a light and smoke generator (E068) on a green background. It looks distinctly as though the lifebuoy has arms which are holding these up. The second shows a person sat on the floor leaning slightly forwards with lots of little black dots in the air around them (W041) on a yellow background. The third (M017) we have already discussed. Hmm, were those cat ears part of the ISO standard?

Here are some action shots:

Two photos of the badge running the air quality monitoring code. The first photo shows the reading in a well-ventilated lecture theatre space where the CO₂ is 657 PPM and the badge is displaying the lifebuoy graphic. The second photo shows a stuffier setting where the CO₂ level is 1227 and the badge shows the person sat on the floor surrounded by black specks in the air.

Incidentally, if you like would like a regular drip feed of international standard safety signs in your feed (and if we’re honest, who wouldn’t?), Sam from Hacks’R’Us has set up a lovely fediverse ISO 7010 safety signs bot.

Follow the ISO 7010 safety signs bot

So how does an air quality monitor work? We’ll need a sensor, a microcontroller to read and process the sensor data, and some way to communicate to the user. Oh, and a power source, like a rechargeable battery or a USB cable with a power source to plug it into. For this device I was interested in monitoring CO₂ levels, which are a useful way to understand how well ventilated an indoor space is. Poorly ventilated spaces are statistically more likely to be sources of respiratory infections, so it’s good to be able to quickly check. Other sensors are available for things like particulates (PM2.5 and PM10) and volatile organic compounds (VOCs).

There are two sorts of CO₂ sensor - some of them estimate the CO₂ levels, and some are “true CO₂ sensors” which claim a much higher degree of accuracy. I used a version of the Sensirion SCD41 true CO₂ sensor which comes with a STEMMA QT / Qwiic connector, made by Pimoroni, Sheffield’s Primary Provider for Piratical Projects. To keep things simple I plugged the sensor into Pimoroni’s Tufty2040 badge, which handily combines a Raspberry Pi 2040 microcontroller with a screen, buttons and STEMMA/Qwiic connector:

Two photos of the "finished" badge. The first photo shows how the air quality sensor and the battery attach to the Tufty 2040. The second photo shows the back of the device. I've 3D printed a back plate to protect and contain the badge's delicate innards, including holes for attaching a lanyard. There are also some magnets to hold the case closed, and someone has added a Good Enough quality control sticker. The air quality sensor protrudes through a crudely fashioned hole cut into the back plate.

If you wanted to make one of these yourself, you’d need:

Pimoroni Tufty 2040 badge
Pimoroni Sensirion SCD41 sensor breakout
JST-SH to JST-SH cable to connect the sensor to the badge
Battery with JST-PH connector
Optional quality control sticker, magnets, back plate etc

Be aware that the Tufty 2040 doesn’t have Wifi, Bluetooth or an SD card, so your options are limited if you wanted to save, upload or further process the sensor readings. Other boards are also available :-)

My memory is hazy about the 3D printed backplate, but I think I adapted a model created by Henrique Salvador to make it a bit thinner. There are quite a few printable cases for the Tufty 2040 out there on teh interwebs, if this one doesn’t quite work for you.

And here’s my hacky Python script to read the sensor data output the readings and choose which image to display. It also incorporates a little animated bar graph widget which I found somewhere in the Tuftyverse. As ever, this is not intended to be correct or complete, and I’m confident there are a myriad of ways it could be done better, simpler etc…

from picographics import PicoGraphics, DISPLAY_TUFTY_2040, PEN_RGB332
import time, math, random, gc
from pimoroni import BREAKOUT_GARDEN_I2C_PINS
import breakout_scd41
from pimoroni_i2c import PimoroniI2C
import jpegdec
import gc
    
i2c = PimoroniI2C(**BREAKOUT_GARDEN_I2C_PINS)
breakout_scd41.init(i2c)
breakout_scd41.start()

display = PicoGraphics(display=DISPLAY_TUFTY_2040,rotate=90)
jpg = jpegdec.JPEG(display)

BACKLIGHT = 0.5
display.set_backlight(BACKLIGHT)

WIDTH, HEIGHT = display.get_bounds()
WHITE = display.create_pen(255, 255, 255)
BLACK = display.create_pen(0, 0, 0)

YELLOW = 8
ORANGE = 9
RED = 10
MAGENTA = 11
VIOLET = 12
BLUE = 13
CYAN = 14
GREEN = 15

def bargraph(ox, oy, w, h, value, colors, pips=32):
    step = int(pips / len(colors))
    filled = int(value * pips)
    pip_width = int(w / pips)
    for i in range(pips):
        p = colors[int(i / step)]
        display.set_pen(p)
        x = i * pip_width
        if i > filled:
            display.rectangle(ox + x, oy + h - 1, pip_width - 2, 1)
        else:
            display.rectangle(ox + x, oy, pip_width - 2, h)

co2 = temperature = humidity = 0
    
while True:
    t = time.ticks_ms() / 1000.0
    
    display.set_pen(display.create_pen(50, 50, 70))
    display.clear()
    
    TEXT_COLOUR = WHITE

    filename = old_filename = "ISO_7010_E068.jpg"
    jpg.open_file("ISO_7010_E068.jpg")
    jpg.decode(0,0)
    display.set_font("bitmap8")
    display.set_pen(TEXT_COLOUR)

    if breakout_scd41.ready():
        print("sensor ready")
        try:
            co2, temperature, humidity = breakout_scd41.measure()
        except:
            print("sensor error - skipping")
            
    display.text("CO2:", 20, 255, 22)
    display.text(str(int(co2)), 60, 255, 22)
    display.text("Temp:", 120, 255, 18)
    display.text(str(int(temperature)), 180, 255, 18)    
    display.text("Hum:", 120, 275, 18)
    display.text(str(int(humidity)), 180, 275, 18)

    if co2 <= 400:
        filename = "ISO_7010_E068.jpg"
        print("co2: %d ppm - let's be honest, you're probably outside right now!" % (co2))
    elif co2 > 400 and co2 <= 800: 
        filename = "ISO_7010_E068.jpg"
        print("co2: %d ppm - good for a well ventilated indoor space" % (co2))
    elif co2 > 800 and co2 <= 1500:
        filename = "ISO_7010_W041s.jpg"
        print("co2: %d ppm - poor air quality, need to get some ventilation" % (co2))
    else:
        filename = "ISO_7010_M017_cat.jpg"
        print("co2: %d ppm - danger level, N95 mask recommended (cat ears optional)" % (co2))

    if filename != old_filename:
        jpg.open_file(filename)
        jpg.decode(0,0)
        old_filename = filename
        print("changed image")
    
    print("updated")
    for v3 in range(1,30):
        bargraph(10, 300, 230, 8, (v3/30), [VIOLET, BLUE, CYAN, GREEN])
        display.update()
        time.sleep(1)

    display.set_pen(BLACK)
    display.rectangle(10, 300, 230, 8)
    display.set_pen(WHITE)
    gc.collect()

And if you got this far and are still wondering about Lüften, this is the German practice of airing out rooms by opening all the windows for a short time. Yes, of course the Germans have a word for it! See also Querlüften (cross-ventilation) and my personal favourite: Stoßlüften - shock ventilation.

So basically: Katzenohren? ⇒ Stoßlüften!