Accessing custom I2C devices from Python


[I originally posted this in Hardware because it has to do with I2C but after working on it pretty hard I’ve come to realization that this is probably a docker issue, so I’m reposting here]

I have a custom PCB with several I2C devices (ADS7830 a/d converters, TCA8534 IO expanders, etc.) that I’d like to integrate into my system. The hardware is known working (I’m converting from OpenHAB to HA after the whole Insteon debacle). I’m running on a Pi 3B+ with Home Assistant OS 8.0.

I followed the instructions here for enabling I2C and it appears to be working:

➜  ~ ll /dev/i2c*
crw-------    1 root     root       89,   0 Mar 11 02:33 /dev/i2c-0
crw-------    1 root     root       89,   1 Mar 11 02:33 /dev/i2c-1
crw-------    1 root     root       89,  10 Mar 11 02:33 /dev/i2c-10
crw-------    1 root     root       89,  11 Mar 11 02:33 /dev/i2c-11
➜  ~ lsmod | grep i2c
i2c_dev                20480  0
i2c_mux_pinctrl        16384  0
i2c_mux                16384  1 i2c_mux_pinctrl
i2c_bcm2835            16384  0

My plan is to write something in AppDaemon that periodically scans the I2C devices and connects the I2C endpoints to HA objects either directly or through MQTT (is this the best approach?). Currently though I’m having trouble accessing the I2C bus from within HA using Python. For example, when I try from the Python console I get the following:

➜ python3
Python 3.9.7 (default, Nov 24 2021, 21:15:59)
[GCC 10.3.1 20211027] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from smbus2 import SMBus
>>> bus = SMBus(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.9/site-packages/smbus2/", line 280, in __init__
  File "/usr/lib/python3.9/site-packages/smbus2/", line 310, in open
    self.fd =, os.O_RDWR)
PermissionError: [Errno 1] Operation not permitted: '/dev/i2c-1'

I get a similar error from within AppDaemon.

I thought that maybe HA had the I2C bus open but wouldn’t that increase the reference count on the i2c_dev module? Is this a protection mode issue?

Any help would be greatly appreciated.

Update 1:

  • Turning off Protection mode on the terminal has no effect
  • Tried it on a Pi4 with the same results
  • On Raspian I tried having two simultaneous open i2c devices and that worked OK so it’s not a problem of HA already having the device open.

Update 2:
I’m fairly convinced that the host device is not being added to the container resulting in the Permission Error. I’m guessing that this is supposed to happen as a result of putting the rpi-i2c.conf file in CONFIG/modules on the boot partition. I believe that I did this correctly. Is there any way to check that this is the issue? Why didn’t it work?

I finally figured out how to make this work. The solution was to create a custom add-on. Creating a custom add-on is easy, you can get one up and running by adding just four files in your add-ons folder. The new add-on runs in its own docker container and by configuring this container correctly you can get the required permission to access the I2C device. Here’s an overview of what I did.

  1. Install HA on an uSD card.
  2. Follow instructions here for enabling the i2c device. Be sure to reboot twice. After doing this you should see /dev/i2c-1 in the file system.
  3. Follow instructions here for creating the add-on but use the file contents shown below to enable the i2c device in the add-on container.
  4. Install the add-on as described and then build and start it. In the log you should see “I2C test starting…” followed by “it worked!”.
  5. If you get this far you have a working i2c bus and the full power of Python (with imports) available to you. Be sure to modify the Dockerfile for each import you use.
  6. After making changes to the python script click Rebuild. HA doesn’t always immediately notice changes to config.yaml. To force it, uninstall the add-on, then do ‘Check for updates’ in the add-on store and re-install. Change the version number so that the update is more obvious.
File #1: Dockerfile
RUN apk add --no-cache python3 py3-pip
RUN pip3 install smbus2
RUN chmod a+x /
CMD [ "/" ]
File #2:
import smbus2
bus = smbus2.SMBus(1)
print('it worked!')
File #3: config.yaml
name: "I2C Test"
description: "Opens the I2C device"
version: "1.0.0"
slug: "i2c_test"
init: false
  - aarch64
  - amd64
  - armhf
  - armv7
  - i386
log_level: info
startup: services
  - /dev/i2c-1
device_tree: true
File #4:
#!/usr/bin/with-contenv bashio

echo "I2C test starting..."

@412jim - many thanks for this.
I was running around in circles trying to find something like this.
Super helpful post

s6-rc: info: service s6rc-oneshot-runner: starting
s6-rc: info: service s6rc-oneshot-runner successfully started
s6-rc: info: service fix-attrs: starting
s6-rc: info: service fix-attrs successfully started
s6-rc: info: service legacy-cont-init: starting
s6-rc: info: service legacy-cont-init successfully started
s6-rc: info: service legacy-services: starting
s6-rc: info: service legacy-services successfully started
I2C test starting...
it worked!
s6-rc: info: service legacy-services: stopping
s6-rc: info: service legacy-services successfully stopped
s6-rc: info: service legacy-cont-init: stopping
s6-rc: info: service legacy-cont-init successfully stopped
s6-rc: info: service fix-attrs: stopping
s6-rc: info: service fix-attrs successfully stopped
s6-rc: info: service s6rc-oneshot-runner: stopping
s6-rc: info: service s6rc-oneshot-runner successfully stopped```

Thank you for the very helpful post! I thought that I had found the solution access the i2c bus on my pi4 running HA OS, sadly not…

Have enabled i2c and is visible, following instructions above I get below error when attempting to install the addon

ERROR (SyncWorker_0) [supervisor.docker.addon] Invalid build environment, can’t build this add-on!

I have also installed i2c-tools (apk get i2c-tools) but i2cget and i2cset both come back with Operation not permitted

Can i2c devices actually be used when running HA OS?