First: The cat will go where it is warm, and be drawn to that. You will notice that the fur between its eyes and its ears is thinner than the rest of the fur on the skull - this is where cats sense infra-red - and it is directional just as with their sight and hearing. A weight-actuated heating pad will be cold when not in use, and so not attractive to the cat. Second: If you put the pad inside a sheltered area or box, over a piece of thick, stiff foam, that will help retain the heat, and make the timer a more reliable option both for safety and for the cat. Lastly, there are fixed-value thermostats that will hold a temperature (hence the suggestion of a sheltered area or box) at whatever you want it to be.
If you really want the timer function, you may put the fixed-value stat in series with it. If you want there to be minimal warmth whatever the timer wants, put them in parallel, but use a lower temp fixed stat. You have options.
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KISS
Peter Wieck Melrose Park, PA