I am looking for a way to project a countdown timer onto a screen so that
meeting attendees can know how much time is left during scheduled breaks.
Use PowerPoint (which you may already be using for presentation).
Create a deck in which each slide shows a large number, possibly with clip art,
colors, and whatever.
Arrange the slides in descending order. Click Slide Show > Slide Transition.
Choose to advance the slide automatically after 01:00 (that is, one minute), and
click the Apply to All Slides button. When you run the slide show, it will start
with the first slide (the highest number) and go to the next slide one minute
later, then the next, etc.