Thirty-eight minutes is close to a standard class period once you subtract arrival. Use it for the lesson itself. Two halves of nineteen work if the lesson is input then practice.
Show the timer. Hidden time makes students ask how long is left, which spends the minutes you were saving. 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 38 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 38 minutes timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. Show the timer. Hidden time makes students ask how long is left, which spends the minutes you were saving.
What is 38 minutes actually good for?
Thirty-eight minutes is close to a standard class period once you subtract arrival. Use it for the lesson itself.
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.