Happiness Hat, a project by Lauren McCarthy, is a wearable conditioning device that detects if you’re smiling and provides pain feedback if you’re not. Frowning creates intense pain but a full smile leaves you pain free.
An enclosed bend sensor attaches to the cheek and measures smile size, a servo motor moves a metal spike into the head inversely proportional to the degree of smile. Through repeated use of this conditioning device you can train your brain to smile all the time. The device runs on Arduino.

http://lauren-mccarthy.com/happinesshat/