Na serveru Slunečnice.cz je zajímavá iOS aplikace:

Ring the Bell 1.1

A virtual bell is all you need when you want your group's attention. You might be a teacher assembling your students, a child playing a game or a host for a friendly bingo or quiz night, as long as your phone is loud enough, you can just wave it around (tap the bell) and indicate that time's up. …

Adresa: https://www.slunecnice.cz/sw/ring-the-bell/