I replaced the fan with a Noctua fan to get better airflow and quality. But if the fan does not even spin, it makes no difference. For the fan to work, a PWM signal must be sent out on GPIO13 (pin 33) on the Raspberry Pi 4.

Which can be seen in the documentation for the X735 V2.5. GeekWorms wiki-page contains links to python code to control the fan that can be downloaded from github.

OK, good documentation and good with finished code as well.
But…
I installed Home Assistant OS and I do not know how to run a python script in Home Assistant.
This script should be running all the time, so the best solution might be to have it as an add-on. But I feel that it’s a bit over my head, for now 🙂 Instead, it will be Google again that gets the question “Home Assistant fan regulating with raspberry pi PWM pin” and I find this page. Somewhere I also found this page which was the basis for me being able to figure out how to measure the CPU temperature.

The result was the following code in configuration.yaml:

sensor:
  - platform: command_line
    name: CPU Temperature
    command: "cat /sys/class/thermal/thermal_zone0/temp"
    # If errors occur, make sure configuration file is encoded as UTF-8
    unit_of_measurement: "°C"
    value_template: "{{ value | multiply(0.001) | round(1) }}"

switch:
  - platform: rpi_gpio
    ports:
      13: RPI Cooling Fan # PWM fan control

climate:
  - platform: generic_thermostat
    name: RPI Cooling Fan Controller
    heater: switch.rpi_cooling_fan
    target_sensor: sensor.cpu_temperature
    min_temp: 0
    max_temp: 85
    ac_mode: true
    target_temp: 45
    cold_tolerance: 0.1
    hot_tolerance: 0.1
    min_cycle_duration:
      seconds: 30
    keep_alive:
      minutes: 5
    initial_hvac_mode: "cool"

The sensor part is a command line readout from the system to be able to get the current temperature on the CPU.
The switch part is specific to the Raspberry Pi in being able to set the GPIO pins to the desired values.
Climate creates a virtual thermostat that takes in the temperature and sets GPIO 13 (switch) ON if the temperature becomes too high, i.e. above “target_temp”. This solution actually works really well. But…
The fan only has ON (max speed) and OFF (off), which makes it sound quite a lot from the chassis when the fan starts. I want it to start at a lower speed and then increase if the temperature increases. But it is not possible with switch, only ON and OFF. But it is easy to change target_temp with a climate control in the Lovelace panel.

To be continued…