Classroom instrument · Fullscreen timer
Classroom Timer
Set a time, press Start, and project it: digits sized for the back row, one-tap presets for common class blocks, and a soft three-beep chime at zero. Everything runs locally in your browser — nothing you do here is sent anywhere.
Fullscreen scales the digits to fill a projector screen. Press Esc or tap the Exit button in the corner to leave.
How this timer works
This is a wall-clock countdown instrument. You give it a duration — a preset button, or a custom entry in minutes, minutes and seconds, or hours — and it displays the time remaining until that duration has elapsed, as m:ss, or h:mm:ss for blocks over an hour. The readout strip reports the instrument's state, the duration you set, and the clock time the countdown will reach zero; the large digits carry what is left.
The countdown is anchored to your device's clock rather than to a ticking counter. Pressing Start stores a single target — the current time plus the remaining duration — and every refresh recomputes the difference between that target and now. This matters because browsers throttle background timers hard: a hidden tab may run its update once a minute, and a timer that counted its own ticks would silently drift. Anchored to the clock, a delayed update costs nothing — whenever the next refresh happens, the subtraction still yields the true remaining time. Pausing stores the remainder; resuming sets a fresh target from it. The Fullscreen button uses the browser's Fullscreen API to enlarge just the timer panel, and the end chime is synthesized with the Web Audio API from your Start press, which is why the browser permits it to sound minutes later.
What this page cannot survive is the device itself stopping. A laptop that sleeps or a tablet that locks its screen suspends the page completely: the display freezes and the chime cannot fire until the device wakes, although the recomputed time will again be correct at that point. While running, the page requests a screen wake lock to reduce that risk, but the lock only holds while the tab is visible and some browsers refuse it. The end of the countdown is also only as accurate as your device's own clock — this page has no reference time source of its own.