Linear Time Varying Parameter

Hi Guys,

I am getting stuck on how to vary a parameter of my first order filter as a function of Simulink's simulation time. My filter suppose is 1/(a*s+1). So for example I would like a = 1 for t < 2, and for t >= 2, a = 2.

Is there a way to do this? I have searched matlab and the web for info on this but not much out there. Any help would be great! thanks in advance.

-t