In the Harry Potter books and movies, the Weasley family has a special clock that uses magic to track where family members are at any given time.
That clock has nine hands, one for every member of the household: each hand points to a location marked on the clock to track the movements of each individual. Of course, as an actual clock, the thing was useless, as marked by Harry Potter, but at least family members knew if others were at home, school, lost or in mortal peril.
Although the clock first gets mentioned in the novel version of Harry Potter and the Goblet of Fire, Dumbledore comments on the clock in the Weasleys' living room in The Half-Blood Prince, noting that Molly Weasley always knows where her family is and what each member is up to: "Of course she will already know, that ingenious clock of hers." However, the clock made its first appearance in the movies in Harry Potter and the Chamber of Secrets.
A Harry Potter fan recently recreated a working real-world version of that clock that uses technology, instead of magic, to track people.
This real-world clock uses location data from family members' phones to track where they are at any given time. The hands are LED lights that light up when a family member is within a specified radius that falls under one of the location categories on the clock's face. This just goes to show how science and magic are sometimes almost the same thing.
This particularly brilliant Harry Potter fan began with an old broken clock, cut and laser-etched the face's design on wood and then used some advanced engineering know-how to add the tech. According to Reddit, it took the fan about a month to complete the clock just in time for the holidays last year.
So, what happens when a family member's location falls under "mortal peril?" That depends on a family member's definition of mortal peril. For example, the builder explained that his sister defined that as weather below a certain temperature, but for him, it's the "radius of our rival school."
For anyone willing to tackle creating such a clock for their own families, there are detailed instructions for a similar project here. Of course, coding knowledge is a must, so this probably isn't a project for beginners.
