Using Cheap Asian Electronic Modules
By Jim Rowe
PAS CO2
Air Quality
Sensor Module
Continuing our series of articles describing low-cost air quality sensors
(LCAQS), this month, we take a close look at a sensor module based on
photo-acoustic spectroscopy, or PAS for short. This time it’s the Infineon
XENSIV PAS CO2 mini-board.
S
ensors that make use of
PAS (photo-acoustic spectroscopy) take advantage of the way
molecules of a particular gas absorb
specific IR wavelengths according to
the Beer-Lambert law.
In PAS sensors, the degree of absorption is measured using a phenomenon Alexander Graham Bell discovered in 1880. When a thin metal disc
is exposed to pulses of sunlight (Bell
used a rotating slotted wheel), it emits
sound. Later, Bell showed that materials exposed to the non-visible wavelengths in sunlight (like infra-red/IR
and ultraviolet/UV) also emit sound.
The basic structure of a PAS sensor
is shown in Fig.1. On the left is the
pulsed IR light source (generally, an
array of LEDs), with an optical filter
passing only the wavelengths absorbed
by the gas to be detected – in this example, 4.2μm for the detection of CO2.
At the far end of the chamber, there
is a MEMS microphone optimised to
detect low audio frequencies. When
the detected sound level is amplified, it can be converted into a figure
corresponding to the amount of CO2
present in the cell. The whole sensor
is enclosed in an acoustic insulation
layer, to reduce the influence of external sound.
LCAQS sensors using the PAS principle have only appeared in the last
couple of years because their development has depended on MEMS
technology. The only one currently
available seems to be the XENSIV
PAS CO2 sensor from Infineon Technologies (an offshoot of Siemens in
Munich, Germany).
This comes on a very compact module measuring only 14 x 13.8 x 7.5mm,
combining the PAS sensor with a dedicated microcontroller unit (MCU)
running advanced compensation
algorithms and providing a choice of
three different data interface ports. It
is currently available from suppliers
like Farnell/element14 or DigiKey for
around £20-£25.
Fig.1: the basic structure of a PAS sensor. A pulsed IR LED emits light through a
filter leaving only wavelengths of light that the gas to be detected can absorb. A
MEMS microphone then detects low-frequency audio that is emitted by the gas,
which can be measured to provide the amount of gas in the cell.
Practical Electronics | August | 2023
Inside the module
Fig.2 shows a functional block diagram of the XENSIV PAS CO2 sensor
module. At the top is the PAS measurement cell, with its gas inlet pipe
on the right, the MEMS IR emitter in
the centre and the MEMS LF microphone on the left.
In the lower part of the diagram are
the microcontroller and memory, the
light-source driver and the circuit that
measures the voltage of the external
12V DC supply that is used to power
the IR emitter.
As mentioned above, the PAS CO2
sensor mini-board provides a choice
of three different data interfaces for
communicating with an external MCU:
I2C, asynchronous serial (UART) and
PWM (pulse-width modulation). The
one to be used is chosen by setting the
logic level on the PSEL and PWM_DIS
control pins.
To use the I2C interface, the PWM_
DIS and PSEL pins must be pulled
down to GND. For the UART interface,
Labels for the pin connections are
available on the module underside.
41
Fig.2: a functional block diagram of the XENSIV PAS sensor module.
PWM_DIS is pulled down while
PSEL is pulled up to logic high (3.3V)
instead. Finally, if you want to use the
PWM interface, the PWM_DIS pin is
pulled to logic high (3.3V).
When the I2C interface is selected,
the SDA/TX pin is used for the data
line and the SCL pin for the clock
line. When using this interface, both
the SDA/TX and SCL pins need to be
fitted with 10kW pull-up resistors to
the +3.3V supply.
When the UART interface is selected,
the SDA/TX line is used as the serial
data output and the RX pin for serial
data input. But when the PWM interface is selected, the width-modulated
pulses emerge from the PWM pin.
The INT pin is an output to allow the
internal MCU to indicate when it has
finished a measurement. I could not
find much information on this item,
but I suspect that it is only functional
when the UART or PWM interfaces
are being used.
The actual pin connections for
the PAS CO2 mini-board are shown
in Fig.3, which is a simplified top
view of the module. There are six
pins on each side, but the two lowest pins, labelled SWD and SWCLK,
are for testing during manufacture
and should not be connected when
the module is being used. All of the
remaining pins correspond to those
shown in Fig.2.
Trying it out
Once I had obtained a sample XENSIV
PAS CO2 mini-board module, the challenge was to discover how to connect
it to a standard low-cost MCU like an
Arduino Uno. Fortunately, I found this
information on the Infineon website.
Although Infineon only provides
specific information on connecting
the module to either a PSoC 6 WiFi-BT
Pioneer Kit or an up-market Arduino
Due, I was able to glean enough from
the latter option to work out how to
connect it to an Uno or similar.
This turned out to be relatively
straightforward, as you can see from
Fig.3, which shows how to connect
the module to an Arduino Uno via
I2C. The 3.3V logic supply comes
from the +3.3V output of the Uno,
while the SCL and TX/SDA pins connect to the Uno’s SCL and SDA pins
and a pair of 10kW pull-up resistors.
The PWM_DIS and PSEL pins are
tied to ground for I2C mode, as mentioned earlier.
Since the module also needs a 12V
DC supply to provide power for the
IR LED, this can come from a separate
plugpack supply. It can be a small supply, since the average current is less
than 600μA with brief pulses of around
20mA. Three bypass capacitors on the
12V supply line provide smoothing.
Of course, we need a software
library to send commands to and
receive data from the sensor, plus a
sketch to use the library.
After a bit of searching on the Arduino website in the ‘reference/en/
libraries’ section and then in the list
of 900-odd contributed libraries for
communicating with sensors, I found
one called ‘PAS CO2 Arduino Library
v1.0.3’.
When I clicked on that one, it took
me to github.com, where I discovered
that the library was provided by and
maintained by Infineon! So it was
obviously the right one to download.
Fig.3: connecting the PAS sensor to an Arduino board is straightforward. Note:
we tied the PWM_DIS and PSEL pins to GND so that the module is in I2C mode.
Useful links
Reproduced by arrangement with
PAS modules:
SILICON CHIP magazine 2023.
• https://bit.ly/pe-aug23-pas1
www.siliconchip.com.au
• https://bit.ly/pe-aug23-pas2
• https://bit.ly/pe-aug23-pas3
Software libraries (or download through the Arduino IDE Library Manager):
• https://bit.ly/pe-aug23-pas-sw1
• https://reference.arduino.cc/reference/en/libraries/xensiv-pas-co2/
Photoacoustic spectroscopy:
• https://w.wiki/4wsX
42
Practical Electronics | August | 2023
I downloaded the library zip file
and added it to my Arduino IDE’s list
of installed libraries. I then discovered
that it came with 12 example sketches
– four of which are for using the module’s PWM interface mode, while the
other eight are for the serial interface
modes (ie, I2C or UART). The latter had
the following titles:
• serial-alarm
• serial-api-test
• serial-calibrate
• serial-device-id
• serial-diagnosis
• serial-oneshot
• serial-periodic
• serial-reset
Fig.4: sample output
15:37:04.303 -> pas co2 serial intialized
15:37:09.505 -> co2 ppm value : 633
15:37:14.520 -> co2 ppm value : 623
15:37:19.487 -> co2 ppm value : 621
15:37:24.502 -> co2 ppm value : 611
15:37:29.516 -> co2 ppm value : 609
15:37:34.483 -> co2 ppm value : 610
15:37:39.498 -> co2 ppm value : 649
15:37:44.466 -> co2 ppm value : 1018
15:37:49.480 -> co2 ppm value : 1255
15:37:54.448 -> co2 ppm value : 1254
15:37:59.462 -> co2 ppm value : 1256
15:38:04.477 -> co2 ppm value : 1317
15:38:09.444 -> co2 ppm value : 1409
15:38:14.459 -> co2 ppm value : 1418
15:38:19.426 -> co2 ppm value : 1405
15:38:24.441 -> co2 ppm value : 1392
I decided to try serial-oneshot.ino,
and when I loaded it, compiled it and then uploaded it
to the Arduino and opened virtual serial port COM3, it
all sprang into life.
Fig.4 shows the output in the Arduino IDE Serial Monitor following the startup of the serial-oneshot sketch. The
first line indicates that the PAS CO2 and its I2C serial port
have been initialised, while the following lines show the
measured CO2 levels in ppm (parts per million). These
measurements are about five seconds apart, as you can see
from the timestamps.
The other thing to note from Fig.4 is that the initial seven
readings are all between 610ppm and 649ppm, whereas the
eighth reading suddenly jumps up to 1018ppm and then
following readings move up to 1418ppm before starting
to fall again.
At about 15:37:40, I exhaled towards the PAS CO2 sensor. So it was responding to the sudden increase in CO2
level, as it’s supposed to.
Encouraged by this initial success, I then tried loading,
uploading and running the serial-calibrate.ino example
sketch. This sketch ran very quickly, simply giving a ‘sensor now calibrated’ message before ending.
Summary
Despite being very compact, the Infineon XENSIV PAS CO2
sensor mini-board is a good performer. As it uses a standard
I2C interface, it is compatible with just about any microcontroller, including virtually all Arduinos.
No doubt it would work with a Micromite as long as it
was set up to send the correct I2C commands.
Although it is priced higher than the MOS sensors we’ve
looked at previously, and it needs a 12V supply, it is a
good choice if you want a small and accurate CO2 sensor.
Your best bet since
MAPLIN
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Visit: www.cricklewoodelectronics.com
Or phone our friendly knowledgeable staff on 020 8452 0161
Components • Audio • Video • Connectors • Cables
Arduino • Test Equipment etc, etc
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The PAS sensor measures 14 x 13.8mm, making it tiny in
comparison to the enlarged photo shown here.
Practical Electronics | August | 2023
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