This is an mopidy extension to control volume and playlists just with scroll devices like mouse-wheel or dedicated rotary encoders.
It is available in Arch Linux User Repository: trizen -Sa mopidy-headless
Add the configuration to mopidy.conf. The default configuration uses the first input device and assumes it is a mouse:
[headless]
enabled = true
volume_device = /dev/input/event2
volume_axis = REL_WHEEL
playlist_device = /dev/input/event2
playlist_axis = REL_X
mute_device = /dev/input/event2
mute_key = BTN_LEFT
If using the rotary-encoder device driver as mentioned above, the configuration might look as follows:
[headless]
enabled = true
volume_device = /dev/input/event0
volume_axis = REL_WHEEL
playlist_device = /dev/input/event2
playlist_axis = REL_WHEEL
mute_device = /dev/input/event0
mute_key = BTN_LEFT
shutdown_command = "/usr/bin/sudo /usr/bin/systemctl poweroff"