The function of the NPN transistor (2N3904) is
similar to that of a valve, it controls the flow of the
current through the LED. The flow of this current will
depend on the base voltage of the transistor. This
base voltage in turn depends on the loudness and
pitch of your voice. Thus, the light intensity of this
LED will vary as you speak into the microphone.
This encoded light signal will then be transmitted to
the receiver through a fiber optic cable.