What kind of mate would I need to solve this?

Hello,

I was wondering if you could help me with a solution to a problem. I have an assembly where a box when it sits down on a lever causes the lever to move and when the box is released the lever falls back.

In short I need a collision mate where the lever moves only when the box touches it and when weight is taken off it reverts to original position. I'm using Solidworks 2013 professional.

SolidworksAssemblies