SADbot (The Seasonally Affected Drawing Robot) is a solar powered, interactive drawing machine for the Eyebeam window gallery in New York City. It uses two 18.5'' x 13'' solar panels to power two stepper motors which allow the control of a pen in two dimensions.
SADbot also takes input from people walking outside, by using a set of sensors which can tell how much light they're getting (photocells) and putting them up against the inside of the window, SADbot knows if someone is covering up one of the sensors, and can change its drawing behavior accordingly.
The window gallery doesn't get any direct sunlight, so we set up an array of mirrors on the roof of Eyebeam to direct sunlight to a fixed mirror hanging off the roof to reflect light down to the solar cells in the gallery window. The solars cells, in turn, power the motors to run the drawing machine.