Paul Bishop has developed a simple light pen input device for use with the Arduino (Atmega328). The hardware consists of a CDS photocell connected between +5 volts and Analog 0, and the code uses the TVout library for video output through several resistors to an RCA plug.
To increase the pen’s sensitivity or adjust response, a simple voltage divider could be created by adding a variable resistance between Analog 0 and Ground with the light sensor.
Via MAKE.