LCD
The LCD for our project displayed slightly different graphics depending on the system's current mode of operation. In relative drawing mode, the LCD displays the line that has been drawn on the touchscreen during a period of constant pressure on the touchscreen. Once pressure is removed from the touchscreen (i.e. you lift your stylus off the touchscreen) the LCD resets and displays a graphic of an Etch-A-Sketch with the words "Touch Me!" printed in the middle of the screen. In absolute mode, the LCD displays all lines drawn on the touchscreen while in absolute mode, without resetting after removing pressure from the touchscreen.
Touchscreen
The touchscreen is basically a resistor network overlaid on top of the LCD display. To read coordinates off of the display, we used 4 transistors as switches (X+, X-, Y+, Y-) as you can only read one set of coordinates at a time (either X or Y).
Servos
We used continuous rotation servos to rotate the two knobs of the Etch-A-Sketch. These servos operate using pulse-width modulation, rotating counter-clockwise at full speed with a pulse-width of 1.4 ms on a 20 ms interval, stopping with a pulse-width of 1.5 ms, and rotating clockwise at full speed with a pulse-width of 1.6 ms.
High Level Component Diagram
