Classroom timer or interval timer?

Choose by whether the timer repeats

A classroom timer suits one timed activity: five minutes for a starter task, twenty for a test, or ninety seconds to pack up. The large countdown gives the room a shared end point. On a projector, everyone needs to read the time left for this task at a glance, without picking it out from the rest of the page.

An interval timer cycles through work and rest phases for a fixed number of rounds. The readout shows which phase you are in and how much time is left in it; the round count helps you pace your effort across the session. Using a single countdown for that job means resetting it by hand between phases and keeping track of rounds yourself. Choose the repeating timer when the next interval should start without another button press.

When a tab is hidden or the device sleeps

Background throttling can affect any browser timer. Browsers may delay the callbacks that update a timer when a tab is hidden or the device is under resource pressure. A timer that treats each callback as a fixed amount of elapsed time can then fall behind. The scheduling policy depends on the browser and its state, so code set to a one-second update interval can run less often.

Both Device Bench timers work out the remaining time from wall-clock timestamps, so when the browser resumes them after a delay, the display catches up to the right time. The screen does not update while the device sleeps, though, and a cue due in that gap can sound late. To see how your setup behaves, compare a five-minute run with an independent clock after switching tabs or letting the display sleep.

If a cue has to sound on time, keep the page visible and test it on the actual device and browser first: audio can also be delayed or blocked in a background tab or before you have interacted with the page. The classroom timer requests a screen wake lock while running. A browser can refuse the lock, it holds only while the tab is visible, and it cannot override a manual lock, aggressive battery saving, or administrator policy, so set the device's sleep settings as a backup.

Sound cues, without flashing

The classroom timer chimes at zero; the interval timer sounds a short tone at each transition. Neither uses flashing as a cue. Full-screen color flashing can be hazardous for people with photosensitivity, including children watching a projected timer or someone following intervals during exercise. The color changes here are held states: a brief tone and a steady color mark each change.

If you will follow the timer by ear, test the sound before you rely on it. Pressing Start lets the page request audio, but browser policy or device settings can still leave it silent. For a silent interval session, uncheck “Audio cue at each change” before starting. The classroom timer has no separate sound switch; mute the device if you need silence. Either timer can be followed visually while muted, provided you keep watching for the end or the next phase.

Check the screen and settings before starting

For a projected timer, check readability from the furthest seat. Physical text size depends on projector scaling, output resolution, browser zoom, screen size, and throw, not on a “4K” label alone. Use Fullscreen on the actual setup if it is available, walk to the back, and adjust the view before the lesson. Try the exit too: confirm Escape works and that you can leave by touch using the classroom timer's Exit button or a tap on the interval timer's fullscreen countdown.

For intervals, pick a preset by the work, rest, and round values it displays. Preset names are labels for convenience; before pressing Start, compare the numbers the timer will run with the plan or source you intend to follow.