You have a timer and clock in just about every single device out there these days, but if you want to build you own, Instructables user GeraldF6 shows you how to do so with a Arduino.

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 ...

A single button controls operation of the timer, with short presses incrementing each digit and long presses moving on to the next digit.