VOC Sensors
Sensors that detect volatile organic compounds
(VOCs) come in various types that can detect
alcohols, aldehydes, methane, carbon monoxide,
and other gases or vapors.A VOC sensor is
sometimes called an eCO
2
sensor because it
estimates an equivalent calculated CO
2
level
using an algorithm based on the presence of
other gases.
The Bosch BME688 sensor, available from
Adafruit on a Stemma-format board, is $20.For
our purposes, VOC sensors don’t effectively
measure CO
2
but they’re great for calculating the
indoor air quality (IAQ) index, as VOCs are emitted
by paint, carpets, hand sanitizers, etc.
Figure
G
shows a plot of a day in the life of
Guido Burger, as measured by a CO
2
sensor and
a VOC sensor.The orange graph on top follows
the CO
2
sensor readings while Guido was sitting
in his office. Around noontime, he has a phone
call and starts talking, and CO
2
levels rise to
about 1000ppm, and then he opens the window
and the value drops. The purple graph shows the
VOC sensor, which doesn’t change with the rise
of CO
2
in the orange graph.At times there seems
to be a relation between the two, but they’re each
measuring different things and both can be worth
looking at.For instance, the VOC sensor detects
changes in air quality while Guido is sleeping even
though the CO
2
levels in the room remain steady.
A Warning System
A CO
2
device could simply display the ppm
reading from the CO
2
sensor, but that doesn’t
provide much context for what the reading
means.Guido used three colors of LEDs to create
a CO
2
Traffic Light (Ampel in German):“So green
light means everything is okay,” said Guido.
Sensirion, Guido Burger, WinSen
“Yellow means you should open the window. And
red light is the ultimate warning.”
•
Red = Above 2000ppm
•
Yellow = Between 1000 and 2000ppm
•
Green = Below 1000ppm
Consider changing the threshold values if you
wish to be cautious and provide an earlier
warning for rising levels.For instance, you might
trigger red above 1000ppm, yellow between 800
and 1000ppm, and green below 800ppm.
While individual readings create a simple
indicator, we might want to visualize the data
over time on a graph, which can show the rate
of change.In Figure
H
, Guido added a matrix
display that shows the data as a curve.Now
we can see the readings change over time and
begin to see how quickly the exchange of fresh
air comes in and changes the reading.“This
allows us to see ventilation as a dynamic system,”
explained Guido.
Carter Nelson’s RGB Matrix Portal Room CO
2
Monitor (learn.adafruit.com/matrix-portal-room-
co2-monitor) used a large display that anyone in
a room could read and understand. The device
presents a concise public-health message using
29
make.co
Comparison of the graphs for a CO
2
sensor (orange)
and a VOC sensor (purple).
Guido Burger’s CO
2
Traffic Light, with optional graphical display of CO
2
readings over time.
G
H
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