Dectect collision, but not actually stop the object?

I was wondering if it was possible to make an object that detected a collision when something hit the collider, but it also allowed the object to pass through it unhindered. I am trying to get a charactercontroller to walk through a particle system, and activate a script in doing that (I have a collider on the particle system, and that is how the script current works). If there is another way to activate the script when the charactercontroller touches it, or a way to allow things to go through a collider, that would be appreciated.