XRandrBashCtl is a simple script to adjust the brightness of the external display.
$ git clone https://github.com/UlyssesApokin/XRandrBashCtl.git
$ cd XRandrBashCtl
$ chmod +x xrandrbashctl
Bind hotkeys to given script commands in your window manager
Increase brightness
~/XRandrBashCtl/xrandrbashctl inc
Decrease brightness
~/XRandrBashCtl/xrandrbashctl dec