The AutoGraph will operate using the same g-code instructions as a laser engraver. Instead of interpreting the "S" g-code command as laser power, the AutoGraph will interpret it as the pressure between the pencil and paper. The 0-5V "laser power" signal from a Arduino Uno running GRBL will be connected to an Arduino Nano that controls pressure between the pencil and paper (via control of the z-axis stepper motor). 

Unlike a laser engraver, a graphite pencil wears down during use, requiring that graphite be periodically dispensed from the mechanical pencil. To account for this, the Arduino Nano controlling pencil pressure will also track the length of graphite currently at the tip of the pencil. When it detects that additional graphite should be dispensed, a feed hold will be commanded to the Arduino Uno via the feed hold hardware pin. During the feed hold, the Arduino Nano will command a graphite dispense operation. Afterwards, the feed hold will be released and the drawing will continue.

The LaserGRBL application will be used to convert bitmap images to laser engraver g-code.