You could put the Arduino in deep sleep, if you wanted to and that LED will still blink. With a little work, you could probably adapt this idea to any number of circuits out of the 555 playbook ...

Fastest finger first indicators (FFFIs) bring precision to quiz games, measuring contestants’ reaction times with accuracy.

To actually power and control the entire circuit, [Martyn] used an Arduino loaded up with a program ... and warning indicators working with the LED bar array / tachometer, everything will look ...