Fifty-five minutes is the class period that already subtracted the walk to the next room. Spend the opening on the example, then keep students in one problem until the last few minutes of collection.
Do not start a second activity in the scraps. Collection is the ending, not a bonus task. Press space to start or pause, R to reset, and F for fullscreen. The screen can stay awake after you start, using the wake lock your browser allows. Pick the alarm before you begin. A bell, a chime, a beep, or a short digital pulse all come from the Web Audio API, so there is no sound file to wait on.
This page is the 55 minutes countdown and nothing else is running underneath it. Related lengths are linked under the guide if you want a neighbor, and the timer itself stays above the fold.
Questions
How do I run a 55 minutes timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. Do not start a second activity in the scraps. Collection is the ending, not a bonus task.
What is 55 minutes actually good for?
Fifty-five minutes is the class period that already subtracted the walk to the next room.
Will the countdown drift if I switch tabs?
No. The end is a timestamp. The display is the gap between that timestamp and now, so a background tab does not lose seconds the way a ticking counter would.