We use this simple rotation timer during our math rotations. This timer rotates through 20 minute stations giving students a 10 minute, 5 minute, and 2 minute warning.
At the 2 minute warning I usually ring my bell, although due to the darker slide change, most students begin picking up without my alert.
The timer then repeats 3 times, but can be changed to fit your needs by deleting or copying additional slides. There is an optional rotate slide you can choose to delete and instead have your students wait for your (or your station managers!) signal!
I welcome any feedback or questions and am more than happy to help you edit or change anything you might need for this to be a success in your class!
Please email me at firstname.lastname@example.org should you need anything!