Here’s an Android powered pen plotter that does it all. It was built by [Ytai Ben-Tsvi] to take with him to Maker Faire. He’s the creator of IOIO, a hardware interface module designed to communicate with an Android device via USB (host or OTG are both supported).
The physical hardware is simple enough. He draws on a pad of white paper using a felt-tipped marker. Located at the top of the easel are two wheels with stars etched on them. They are reels which spool and dole-out string to control the pen’s movements. The pen tip can be lifted by a ball bearing mounted just below it.
But the project really takes off when you watch [Ytai's] demonstration. The Android tablet controlling the device captures a picture of an object — in this case it’s a toy truck. The app then processes it using edge detection to establish how to plot the image.