You may be interested in a 20-minute power point timer that I just added to the Resource Library. You can search Resources for 20-Minute Timer, or follow this link:
I found this on line a couple of years ago, but just this year have had the technology in place to use it. What a difference it makes in my classroom! Students are more focused because they know how much time they have left at an easy glance. There are also audio and visual reminders when it is time to complete their reading logs (@ 2 min.) and computer log (@ 1 min.). Then, students have 1 minute of music to transition from one rotation to another. You can add your own music clips as well. I have it set to loop, so I don't have to worry about restarting it.