Hack a Crayola ColorStudio HD Pen and load it up with your own Arduino sketch. The pen uses a PIC microcontroller to pulse a reed relay which is picked up by the capacitive screen on the tablet. An app on tablet decodes the pulses, and changes the color of the paint brush.
Rob pried it open and replaced the PIC12F505 microcontroller with a ATtiny 44 pin compatible chip. He loaded it up with an Arduino sketch and got it to emulate the original device, as can be seen in the video above.
Having already made some custom PCB that use capacitive communication I immediately recognized the familiar sound of a reed relay being turned on and off when using the pen. Knowing that the device contains a relay it becomes clear how the app works, by pulsing this electromechanical component at a constant frequency this is detected by the tablet as a series of touches and distinguished from a standard finger touch. Having used the same technique in my boards I was able to reprogram the pen to be able to transmit data as is shown in the video.
Via Hacked Gadgets.