Karen http://karenwillis.edublogs.org/ shared this wonderful resource http://www.online-stopwatch.com/eggtimer-countdown/full-screen/
There are rotational activites that occur in my classroom twice a week. Currently a timer that explodes with a bang goes off when its time to move to the next group. I also like this resource as it gives children more of a concept about time with more detail.
The students in my room do however love the bomb as it usually catches the teachers off guard as they are too involved in their groups activities so it creates a bit of a laugh amongst the class. It certainly gave me a fright on my first day!