Thirty-nine minutes is three thirteens: read, try, explain. It suits a lab step that has a wait inside it. Start the timer when the wait starts, not when you wish it had.
Label the thirds on the board so the room can tell which part they are in without asking. 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 39 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 39 minutes timer on a projector?
Press start, then F or the fullscreen button. Classroom mode hides the writing so the digits fill the wall. Label the thirds on the board so the room can tell which part they are in without asking.
What is 39 minutes actually good for?
Thirty-nine minutes is three thirteens: read, try, explain.
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.