The Self-Adjusting Electro-Mechanical Sunglasses were a project for Burning Man 2009. They are built around overlapped linearly polarized sheets, one of which can be rotated by means of the gear train to change the opacity. It is driven by a miniature RC servo, which is controlled from a custom electronics package which includes a light sensor for automatic action.
The control electronics were chosen to enhance the clunky, retro aesthetic. (If you have an excuse to use a knife-switch, do it!) The circuit inside is based around a 555 timer chip to generate PWM signals to control the servo. The light sensor and potentiometer vary the timing constants, thus changing the PWM signal and moving the servo.
The gears are mounted on a hand-cut 1/4" aluminum frame. The only power tool used in the creation of the glasses was a drillpress. This was partially due to a lack of shop resources in Vancouver, and partially to get into the spirit of the aesthetic.
The sunglasses and control electronics are housed in a handsome leather box with velvet-lined compartments for safe and convenient storage.