The main reasons to connect the iPhone/iPad to an external monitor, TV, or projector are either to present the running time or to show the finish-view. When you connect an iPhone you normally get a mirror of the screen, so also with SprintTimer. In two situations, however, will the app send something different to the external screen: When viewing a photo finish you will get a much broader version of the finish-view that fills out the whole screen. And during the race will the external motion display the time in a large font. The latter means that you set up a display so the audience can get a live view of the running time.
Essentially any type of display can be used and it doesn’t have to have the highest possible resolution. Especially if you want to display the time is an old computer monitor enough. After connecting the screen (see below) it will usually start to display a mirror of the iPhone screen. If not, open the Control Center and tap on “Mirror screen”. When you go to the start view the external screen will automatically switch to a time display. If you are using Video- or Motion finish it will also react to the detected motion by displaying the time in yellow. See video.
You can connect your display directly to the iPhone/iPad through an adapter that plug into the lightning port. They usually have an hdmi connector on the other side, so you need a cable with hdmi on one side and whatever your display can take on the other. One choice is the original Apple adapter. It worked fine on all the TV:s and monitors that I tested on. There are also a number of third party adapters on the market, and they typically costs around one third of the Apple one. I bough a couple that had gotten decent reviews on Amazon. They worked well in some cases but refused to display anything on other displays. So if you buy one, make sure that you can send it back if there is a problem.
The Apple TV costs more than an adapter (but you might have use for it at home
Newer iPad Pros has a usb-c port that can be connected directly to a monitor. But you still need an appropriate cable, e.g. usb-c to hdmi.
Some newer (2018-2019) TV:s from Samsung and LG support Airplay. Sony and Vizio have also announced support. This means that you can display the SprintTimer screen on a TV without any extra hardware. But on the other hand, you might not want to drag your new 65″ UHD-TV out in the field 🙂