Use Keyframe as trigger

Hi,

Does anybody know whether there is anyway to use a keyframe as a trigger for an animation in an expression?

What I'm talking about is, that you have a layer that's supposed to be doing a small repeating animation (i.e. pulsating / increasing and decreasing it's size) from time to time. The starting points of this animation are supposed to be triggered manually and than stop until the next trigger comes.

I was thinking about keyframing the animation once inside the object and then use a slider object that would be increased in value, every time I need the animation. The difficult thing is to link the two, because first of all the animation will have to start as soon as the value of the slider object increases and than stop until the value rises again.

I know that I could just copy and paste the keyframes but there has to be some way to solve this with an expression.
Does anybody have an idea how? Or do I need scripting to solve this problem (I do have some feeling for expressions but I have to admit that I'm quite a noob when it comes to scripting).

Thanks a lot in advanced,

Lisa