An incoming mail alert system, solar powered and transmitting mail arrival notifications wirelessly to an "inside home" micro base station.
An incoming mail detection system needs a sensor - I decided to start prototyping with an infrared LED and sensor: the SeeeStudio Grove "line-finder" that I bought here and a Teensy LC that I got from http://hackaday.io some weeks ago (inside a 2015HackadayPrize giveaway!).
It worked just out of the box, using a code that can be found on my GitHub (link in this project page). You can see a video of the first test below: