Building Robots With Lego Mindstorms

446 Chapter 23 • Drawing and Writing

Looking at the bottom, you can see the front skid plate. (Figure 23.3).

Using a technique described in Chapter 11, we placed a rubber band to make

the mechanism bi-stable, so that when the pen is down it tends to stay down, and

vice versa (Figure 23.4).

The pen is a non-LEGO part, a common marker with its body wrapped in

adhesive tape so as to make it fit tightly into the 2 x 2 studs squared hole

reserved for the purpose. It stays there with nothing but friction.

The pen control mechanism is a swinging assembly operated by a third motor

(Figure 23.5).

Now the turtle is ready. Place a large piece of paper on the floor ...