The Geekworm KVM-A3 kit includes a 30mm fan integrated into the X630-A3 Hat. The fan is hard-wired to 5V, and runs continuosly at full speed. It's not too noisy, but it's not too quiet either - certainly in my quite office space. I looked for a true PWM controlled 30mm fan, but was unsuccessful, so I set out to convert it to a PWM controlled fan that would track the CPU temperature and spin accordingly.
The results were very satisfactory. This modification is probably applicable to most any PiKVM clone with a fan, or perhaps even the branded PiKVM devices. I don't have any, so I can't say for sure.
The KVM-A3 uses a RPi-4b mainboard, and this modification is specific to that board, but could be tweaked to work with other RPi boards.
The 30mm fan that comes with the kit is a 5V 2-wire fan, so implementing PWM control is not as simple as connecting it to the RPi PWM control interface. The fan sinks more current than is safely available from the RPi GPIO drivers, and requires 5V instead of the GPIO 3.3V output. An intermediary component is required to provide the necessary voltage and current.
I settled on the EZ Fan2, though there are probably more to choose from. This device is very small, and is easy to tuck into the tight space within the enclosure. I ordered mine with headers, but no connector wires or heat-shrink.
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Re-wire the fan to go through the controller
I cut and modified the cable connecting the fan to the Hat, and wired in the fan controller between them. You can tuck it all in right next to the fan on the hat.
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Wire the GPIO pin of the EZFan controller to GPIO 12 of the RPi.
I used a 100 ohm resistor in series to limit the current draw from the RPi GPIO pin, but that's not totally necessary unless you plan on using other GPIO pins on your KVM (there is a limit to the total current available to all GPIO pins). Note, at these higher frequencies you have to consider the AC current draw through the parasitic capacitance across the Gate to Drain/Source of the EZFan transistor.
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Modify /boot/config.txt to enable PWM
With the 4b, you only have two choices as to which set of PWM outputs to use; GPIO 12&13 or GPIO 18&19 (pins 32&33, pins 12&35). For the Geekworm KVM-A3 hat, GPIO 18 is already used (for audio I belive), so that leaves only 12&13.
I chose pin 12 (pin #32 PWM0), as that's the default for kvmd-fan. Don't confuse GPIO numbers with pin numbers. RPi-4 pinout
# PWM on GPIO-12 dtoverlay=pwm-2chan,pin=12,func=4,pin2=13,func2=4 -
Get/modify a version of kvmd-fan to "do the right thing".
The stock version of kvmd-fan runs the PWM signal in balanced mode, and a high (> 9MHz) signal on the PWM output pin. This results in a lot of acoustic noise (for the two fans I tried).
I made changes to set the frequency to ~25KHz (industry standard for small fans), and enabled mark-space mode which produces a uniform square-wave. My modified version is available here - github.com/agspoon/kvmd-fan. Note, you want the "pwm_mode_freq" branch in my forked repo.
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Configure kvmd-fan with the following content in /etc/conf.d/kvmd-fan (tweak as needed for your fan, environment, and ears).
You will need to establish the lowest speed at which the fan will still spin reliably (--speed-idle).
KVMD_FAN_ARGS=" --speed-idle=40 --speed-low=50 --speed-high=90 --temp-low=30 --temp-high=60" -
Enable the kvmd-fan service at boot, and start it,
[root@pikvm ~]# systemctl enable kvmd-fan.service [root@pikvm ~]# systemctl start kvmd-fan.service
You can see the status of the fan PWM state in the PiKVM UI, System->About->Hardware
If you don't see this, make sure "/etc/kvmd/fan.ini" contains the following stanza, and is referenced in the "kvmd-fan" service file ("--config=")
[server]
unix = /run/kvmd/fan.sock
unix_rm = 1
unix_mode = 666
This UNIX socket is where kvmd-fan publishes the temperature and PWM fan state You can read it with the following command,
[root@pikvm ~]# curl -s --unix-socket /run/kvmd/fan.sock http://localhost/state
My A3 based KVM now sits next to me, and is nearly silent. It will spin up if it has to work hard, but almost never runs at full speed. Enjoy the peace and quiet! :)
Other than the kvmd-fan mods above to make the fan quieter and smoother, I wanted to see the PWM state on the OLED screen. So I modified the kvmd-oled app to add the current PWM duty cycle to the display.
Cold 
Hot 
These changes are in my github kvmd fork here - OLED Mod, but beware I'm not necessarily keeping this code up to date with the upstream changes. Though I will see if the maintainers would take some form of this change.
First steps are to make sure that the PWM controller is working correctly. If you connect the EZFan GPIO signal wire, that would normally go to GPIO 12, to GND (or leave floating) the fan should stop. If connected to 3.3v (e.g. RPi pin 1) it should spin at full speed.
If that's working, then you can try manually controlling the PWM signal through the Linux PWM sysfs interface. Connect the EZFan controller to GPIO 12, and stop the kvmd-fan service ("systemctl stop kvmd-fan"). Then use the sysfs interface to modify the PWM signal as follows, pwm.txt
[root@pikvm ~]# cd /sys/class/pwm/pwmchip0
[root@pikvm pwmchip0]# ls -l
total 0
lrwxrwxrwx 1 root root 0 Jan 28 07:39 device -> ../../../fe20c000.pwm
drwxr-xr-x 2 root root 0 Jan 28 07:39 power
lrwxrwxrwx 1 root root 0 Jan 28 07:39 subsystem -> ../../../../../../class/pwm
--w------- 1 root root 4096 Jan 28 07:39 export
-r--r--r-- 1 root root 4096 Jan 28 07:39 npwm
-rw-r--r-- 1 root root 4096 Jan 28 07:39 uevent
--w------- 1 root root 4096 Jan 28 07:39 unexport
[root@pikvm pwmchip0]# echo 0 > export
[root@pikvm pwmchip0]# ls -l
total 0
lrwxrwxrwx 1 root root 0 Jan 28 07:39 device -> ../../../fe20c000.pwm
drwxr-xr-x 2 root root 0 Jan 28 07:39 power
drwxr-xr-x 3 root root 0 Jan 28 07:40 pwm0 (new controller)
lrwxrwxrwx 1 root root 0 Jan 28 07:39 subsystem -> ../../../../../../class/pwm
--w------- 1 root root 4096 Jan 28 07:40 export
-r--r--r-- 1 root root 4096 Jan 28 07:39 npwm
-rw-r--r-- 1 root root 4096 Jan 28 07:39 uevent
--w------- 1 root root 4096 Jan 28 07:39 unexport
[root@pikvm pwmchip0]# cd pwm0
[root@pikvm pwm0]# ls -l
total 0
drwxr-xr-x 2 root root 0 Jan 28 07:40 power
-r--r--r-- 1 root root 4096 Jan 28 07:40 capture
-rw-r--r-- 1 root root 4096 Jan 28 07:40 duty_cycle
-rw-r--r-- 1 root root 4096 Jan 28 07:40 enable
-rw-r--r-- 1 root root 4096 Jan 28 07:40 period
-rw-r--r-- 1 root root 4096 Jan 28 07:40 polarity
-rw-r--r-- 1 root root 4096 Jan 28 07:40 uevent
[root@pikvm pwm0]# echo 40000 > period (this is 25KHz)
[root@pikvm pwm0]# echo 1 > enable (start the signal)
[root@pikvm pwm0]# echo 40000 > duty_cycle (100% duty cycle)
[root@pikvm pwm0]# echo 20000 > duty_cycle (50% duty cycle)
[root@pikvm pwm0]# echo 0 > duty_cycle (stop)
[root@pikvm pwm0]# echo 15000 > duty_cycle (minimum my fan will spin)
[root@pikvm pwm0]# echo 0 > enable (stop the signal)
[root@pikvm pwm0]# cd ..
[root@pikvm pwm0]# echo 0 > unexport (removes pwm0)
If everything above works as expected, then my modified kvmd-fan app should control things as expected.
You can watch how kvmd-fan is reacting, by enabling Debug mode to see all the state transitions. Stop the kvmd-fan service, and run it by hand with the desired options;
[root@pikvm ~]# /usr/bin/kvmd-fan --debug --speed-idle=40 --speed-low=50 --speed-high=90 --temp-low=30 --temp-high=60
You can add additional debug information from the WiringPi library by setting the WIRINGPI_DEBUG environment variable prior to invocation;
[root@pikvm ~]# WIRINGPI_DEBUG=1 /usr/bin/kvmd-fan --debug --speed-idle=40 --speed-low=50 --speed-high=90 --temp-low=30 --temp-high=60