As simple as this project may sound like for anyone who has a bunch of these little Arduino-compatible modules lying around and has probably made something similar to this in their spare time ...

Inside [Klaus]’ device is an Arduino Uno, an HC-SR04 ultrasonic distance sensor, and an Adafruit Wave Shield.

Runner and Arduino enthusiast Nikodem Bartnik has created a DIY 100m sprint timer which is placed at the finish line and signals the start of timing via an audible bleep from its integrated speaker.