Color sensing (as well as detecting empty spaces) settled on IR-based sensors commonly used in line-following robots. You can watch the clock in action in the video embedded below just under the ...

Besides helping you test prototypes of your wacky Arduino ideas ... or another temperature sensor. Likewise, ultrasonic and ...

Thanks to an IR proximity sensor, [romain] can interrupt the cycle ... If you want to make your own version, this project is wide open, though the hardest part would be making it look as clean ...