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 ...

Droplet and StackAR, developed by MIT’s Media Lab, allow users to set timers or program an Arduino using light-based communication. By pressing the tool against a screen, flashes of light can ...

[Scott-28] recently sent in a particularly well-documented project that turned an antique rotary phone into a digital kitchen timer using an internal Arduino.

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 ...