Use cases question

Hi,
I am reading "UML 2 for dummmies" (for me :) ). It says when I have collected the use cases and describe it as seen by the actors. After that says I represent the system as a class with use cases as operations.
My problem is the system I am modeling has three subsystems:
-Reader: Reads data.
-Visualizer: Show data on screen.
-Timer: Generates events to Reader and Visualizer subsystems. For exampe: orders to read every minute a refresh visualizer every twenty seconds.

Here exists use cases that user doesn't see (those use cases where Timer is the actor).

I need to represent every subsystem as a class with its use cases as operations? or only one big class representing entery system? I last, then I need to put operations (use cases) that are generated by Timer subsystem??

Any ideas? Thanks.

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