ActionScript 3.0 :: Get A Script That Would Play An Animation Consisting Of 4 Frames For Say 20 Seconds?

I need a script that would play an animation consisting of 4 frames for, say 20 seconds. The order of those frames needs to be random but it has to stop after 20 seconds.Next, frames need to change each, say, half a second. But, as much as I MIGHT be able to figure this one out, here's what made me post here. The four frames are four keys on a keyboard. Each time frame changes to a corresponding key drawing, one has to hit the right key on the keyboard to make a different animation on the stage, animate.