3 of 4 added 8 characters in body

Bash + Xdotool, 36 bytes

for((;;)){ xdotool key 66;sleep .5;}

Just execute it in a bash shell. It needs to be in a graphical environment. Infinite loop from here. Changed Num_Lock to 66 to save 6 bytes, and thanks to @Michael Kjörling for 2 bytes.