A small 5 cm hollow sphere which has a base and a hole for a 5mm LED. I used mine with an RGB led and Arduino, and made it into a Pomodoro timer. It starts green, fades to red over 25 minutes, then for 4 minutes glows blue, signalling a stretch break. Detailed instructions for creating the Pomodoro timer are below. Print out the Sphere in a neutral or white colour. Insert 5mm LED of your choice into the bottom and power it however you like! If you want to build the Pomodoro timer which I did, follow this instructable for building the RGB LED circuit for the Arduino, which is what I used: http://www.instructables.com/id/Fading-RGB-LED-Arduino/. It give clear steps and materials required. The code I wrote for the Pomodoro specific logic is here: