I have researched this question for about 8 years and I could not find a satisfactory answer. In short, the brightness keys do not work on the Aspire 7740 with KDE Plasma. I'm sure this has been a nuisance for many owners of the excellent laptop if they want to use Linux on it.
If I do $: ls / sys / class / backlight the result is:
$: video0 acer_wmi intel_backlight
Therefore, I added the kernel parameter acpi_backlight = vendor to / etc / default / grub after the "silent splash" that resulted in "silent splash acpi_backlight = vendor"
After this, I did sudo update-grub
I restarted on the graphic desktop again. Reran ls / sys / class / backlight and the result was that video0 had been left to acer_wmi and intel_backlight. The brightness keys fn still did not work. Then, I blacklisted the module acer_wmi by adding it to the file /etc/modprobe.d/blacklist.conf: "blacklist acer_wmi" (at the end)
Restarted again to x, and wala … the fn keys work for brightness.
I think this could certainly help anyone who was frustrated by this (as I was). However, I'm not sure of the purpose of the acer_wmi module. Do I need it for something?