CO2meter: A Python interface to USB CO2 monitor devices

CO2meter

CO2meter is a Python interface to the USB CO2 monitor with monitoring and logging tools, flask web-server for visualization and Apple HomeKit compatibility.

Installation

Prerequisites

OSX

Necessary libraries ( libusb , hidapi ) could be installed via Homebrew :

brew install libusb hidapi

Linux (including Raspberry Pi)

libusb could be retrieved via apt-get

sudo apt-get install libusb-1.0-0-dev libudev-dev

If the script is not intended to be started under root , proper permissions for the device should be set. Put the following two lines into a file /etc/udev/rules.d/98-co2mon.rules :

KERNEL=="hidraw*", ATTRS{idVendor}=="04d9", ATTRS{idProduct}=="a052", GROUP="plugdev", MODE="0666"
SUBSYSTEM=="usb", ATTRS{idVendor}=="04d9", ATTRS{idProduct}=="a052", GROUP="plugdev", MODE="0666"

and run sudo udevadm control --reload-rules && udevadm trigger .

Windows

For installation of hidapi package Microsoft Visual C++ Compiler for Python is required.

Installation of python package

Then installation of co2meter could be done via the pip utility:

pip install hidapi co2meter

Optionally, if pandas package is available then the data will be retrieved as pandas.DataFrames rather than list of tuples.

Note 1: there could be a potential name conflict with the library hid . In this case the import of the module in python will fail with the error AttributeError: 'module' object has no attribute 'windll' (see here ). If this happens, please try uninstalling hid module (executing pip uninstall hid in the console).

Note 2: there were reports on issues with installation on Raspbian Stretch Lite (#5), where build failed with and error code 1 in gcc . Most likely the reason is in missing dependencies. Possible solution is described in the comment .

Optional: flask web-server

In order to be able to start monitoring web-server a few extra packages are needed. Basic web-server will allow reading the current value in browser, downloading data in CSV/JSON and monitoring the status with a simple dashboard. Dependencies could be installed via pip:

pip install -U flask pandas

Optional: Apple HomeKit compatibility

In order to be able to add co2monitor to Apple Home application (iPhone/iPad) HAP-python of version 1.1.5 is required (it was reported that the co2meter is incompatible with the newer version ):

pip install HAP-python==1.1.5

In case when the "hosting server" is running on OSX no extra libraries are needed. For Linux (e.g. Raspberry Pi) servers you will need Avahi/Bonjour installed (due to zeroconf package):

sudo apt-get install libavahi-compat-libdnssd-dev

Note: Setup was tested on Python 3.5. homekit compatibility might not be available on Python 2.7 (see #7).

General usage

Basic use

The interface is implemented as a class:

import co2meter as co2
mon = co2.CO2monitor()

Standard info of the device which is connected:

mon.info

Read CO2 and temperature values from the device with the timestamp:

mon.read_data()

If pandas is available, the output will be formated as pandas.DataFrame with columns co2 and temp and datetime-index with the timestamp of measurement. Otherwise tuple (timestamp, co2, temperature) will be retured.

Continuous monitoring

The library uses threading module to keep continuous monitoring of the data in the background and storing it in the internal buffer. The following command starts the thread which will listen to new values every 10 seconds:

mon.start_monitoring(interval=10)

After this command, python will be free to execute other code in a usual way, and new data will be retrieved in the background (parallel thread) and stored in an internal property data . This property will be constantly updated and the data could be retrieved at any point. For example, if pandas is available, then the following command will plot saved CO2 and temperature data

mon.data.plot(secondary_y='temp')

The data could be at any point logged to CSV file (NB! pandas required). If the file already exists, then only new data (i.e. with timestamps later than the one recorded in the last line) will be appened to the end of file:

mon.log_data_to_csv('log_co2.csv')

The following command stops the background thread, when it is not needed anymore:

mon.stop_monitoring()

Plotting data

The data that was logged to CSV file could be read usig function read_csv() :

old_data = co2.read_csv('log_co2.csv')

CO2 and temperature data could be plotted using matplotlib package with the function plot() of the module:

co2.plot(old_data)

Or the following command will plot CO2 data together with the temperature from the internal buffer (see "continuous monitoring"):

co2.plot(mon.data, plot_temp=True)

By default all data is smoothed using Exponentially Weighted Moving Average with half-life of approximately 30 seconds. This parameter could be changed, or smoothing could be switched off (parameter set to None ):

co2.plot(mon.data, plot_temp=True, ewma_halflife=None)

Note, that both plotting and reading CSV files requires pandas package to be installed.

Apple HomeKit compatibility

It is possible to start use your CO2 Monitor with Apple HomeKit. In order for doing that run the following command in the terminal:

co2meter_homekit

which by default will launch the HomeKit Accessory service listening to the local IP address on the port 51826.

Once the co2meter_homekit script is up and running, device could be added to the Apple Home app on iPhone/iPad. For this first make sure that both the iPhone and the machine where co2meter_homekit are in the same network (e.g. that iPhone is connected to the WiFi) launch the Home app -> + -> Add accessory -> Don't have a code or can't scan? -> CO2 Meter . When asked for PIN code type 800-11-400 (it is hardcoded in homekit.py ).

Monitoring web-server

The web-server is started by the following command in the terminal

co2meter_server

It will start server by default on the localhost on the port 1201 and saving the log to the file logs\co2.csv . Both host, port and the log file name could be configured, for example:

co2meter_server -H 10.0.1.2 -P 8000 -N "Living room"

Once started, it could be accessed via browser at a given address ( http://127.0.0.1:1201 in the first case and http://10.0.1.2:8000 ). The main page will show last readout from the sensor and links to the log history in CSV and JSON formats and a dashboard with the recent charts:

CO2/temperature readings are stored in the logs folder. By default (if -N parameter of the command line is not specified), all values will be appended to the single log file ( co2.csv ), however if there's a need to have separate logs (e.g. in case when device is used in several places and logs are not to be confused), the name could be set up from the command line. Dashboard allows to check history of all available logs. In order to change name of the log on a running server, use the following GET call: http://host:port/rename?name=new_name .

Finally, HomeKit and web-server could be combined:

co2meter_server_homekit

which will start homekit accessory and flask web-server on the local IP address.

Running web-server as a service

The low-key solution for running server on the remote machine (e.g. on Raspberry py) is to call

nohup co2meter_server_homekit -N default_room &

Another alternative is to register it as a service in the system . For this create the file /etc/systemd/system/co2server.service under sudo (be sure to provide an appropriate full path to co2meter_server_homekit and choose proper WorkingDirectory ):

[Unit]
Description=CO2 monitoring service
After=network.target

[Service]
ExecStart=/home/pi/.virtualenvs/system3/bin/co2meter_server_homekit -N default_room
WorkingDirectory=/home/pi/home/co2
Restart=always
User=pi

[Install]
WantedBy=multi-user.target

Then the server could be started:

sudo systemctl start co2server.service

as well as stopped ( systemctl stop ), restarted ( systemctl restart ) or its terminal output could be displayed ( systemctl status ). If you'd like to start it automatically on reboot, call:

sudo systemctl enable co2server.service

Notes

• The output from the device is encrypted. I've found no description of the algorithm, except some GitHub libraries with almost identical implementation of decoding: dmage/co2mon , maizy/ambient7 , Lokis92/h1 . This code is based on the repos above (method CO2monitor._decrypt() ).
• The web-server does not do caching (yet) and was not tested (yet) over a long period of up-time.
• The whole setup is a bit heavy for such simple problem and (in case someone has time) could be simplified: e.g. talking to the device (in linux) could be done via reading/writing to /dev/hidraw* , parsing of the CSV and transformations could be done without pandas .

Resources

Useful websites:

• CO2MeterHacking with brief description of the protocol
• ZG01 CO2 Module manual (PDF)
• USB Communication Protocol (PDF)
• Habrahabr.ru blog-posts with the description, review and tests of the device: part 1 , part 2 , part 3 (Russian, 3 parts)

Scientific and commercial infographics:

• Results of Berkeley Lab research studies showed that elevated indoor carbon dioxide impairs decision-making performance. Original research paper U. Satish et al. in Environmental Health Perspectives, 120(12), 2012 and a [feature story] ( https://newscenter.lbl.gov/2012/10/17/elevated-indoor-carbon-dioxide-impairs-decision-making-performance/ ):
• Commercial infographics on CO2 concentration and indoor air quality ( from tester.co.uk ):
• Commercial infographics on CO2 concentration and productivity ( from dadget.ru ):

License

CO2meter package is released under the MIT license.