This is a very simple drawbot I've built with our Lego Mindstorms set.
It's quick to build and program and once you've got it up and running you'll be printing one page after the other with it.
Step 1: Make a detailed construction plan
Just kidding, this is my sketch I made at work.
Step 2: Building the gears
At first I didn't build a gearbox. It worked, but the pen was flying over the page. I have the smallest gear directly on the motor and the largest connected to the small one. That makes it a lot slower and gives the pen time to leave a trail on the paper.
Step 3: The pen holder
As you can see it's just a rubber band holding the pen. Makes it easy to change the pens.
Step 4: Program it
I know the screenshot is a little blurry. But there is not a whole lot to do. Just let the two motors rotate.
Just make sure that both motors are NOT turning at the same speed. Otherwise they will draw a nice shape but will keep on repeating it over and over.
Step 5: Let it draw
Place a sheet of paper under the pen, start the program and watch in awe. ;)
There are three things you can do to alter the shape of the drawings.
Changing one of them will be enough
Step 6: More Videos
A general overview how the drawbot works and where and what to change to get different shapes