Build notes
Two infrared gates detect the dart passing. A Pico 2 W measures the time between them and calculates its speed from the distance between the gates.
The sensor signal passes through an LM393 comparator and a 74HC14 Schmitt trigger to give the Pico a clean signal. The demo mounts the assembly on a Rhino blaster; the next idea is more beams per gate so the dart doesn't have to pass through one perfect line.