Turns out that I need to add a doorbell to the back door of our house and want it to just be a HomeKit doorbell. You can certainly buy HomeKit doorbells, but they invariably are camera doorbells, and in this case we don't need that. Well, there's a HomeBridge plugin that can turn a GPIO pin into a HomeKit doorbell input, and I had a spare Pi Zero and one of these boards, so I've used the sensor side of the board for this. It works just fine, and if I can come up with a use for the relay later, well, it'll be there.