Hardware Reference
In-Depth Information
HERE IS AN
idea I bet nobody has thought of before: Use two rotary controls to steer the
path of a point on the screen. Where the point has been, it leaves a trail so you can sketch a
picture using two knobs mounted on a box. In a crazy twist, how about if the picture was
erased when you turned the box upside down and gave it a shake. Way out? I know, but it
might just catch on, especially when you can post your artistic eforts straight to Facebook
for all your friends to see.
The Concept
Well, maybe I have been beaten to a patent on this idea, but it is an interesting project from
many angles - not the least of which is the requirement for a rotary control. In the old days
of analogue, most rotary controls were
potentiometers,
or
pots
as they are called for short.
hese were potential dividers where a ixed resistance had a tap of point or wiper, controlled
by a rotary knob or a slider control. he rotary type tended to have a travel of somewhere
between 180 and 270 degrees; there were some special continuous rotary types, but they
were made mainly for servo motor positioning control and were quite expensive. What is
more, there was a dead spot where the wiper had to wrap round from one side of the ixed
resistor to the other. An extra complication in using this sort of control with a computer is
the fact that the pot produced a changing voltage output, and this has to be digitised with an
analogue to digital converter (A/D) before a computer could make any use of it.
Although it is perfectly possible to build an A/D, it is often much simpler to keep everything
in the digital domain. So, in modern designs where you want a continuously rotating control,
a component is used called a
rotary shaft encoder.
hese come in many diferent implementa-
tions, but by far the cheapest is the switched type. Another type is an optical encoder where
the rotary movement is detected by something interrupting a beam of visible or infrared
light. Optical encoders were widely used in the old type of computer mouse, the ones that
had a ball in the base. A much newer type of rotary encoder utilises magnetic sensing to
detect changes, which is covered in much more detail in the next chapter, “he Pendulum Pi,
a Harmonograph”.
Rotary Encoder Types
he switched rotary encoder is at the heart of this project, and you have the choice of several
diferent types. Perhaps the most distinguishing feature is whether the encoder has detents
or not. A
detent
is a mechanical stop on a rotary spindle that divides the rotation into a set
number of increments. When you twist an encoder with detents you feel multiple clicks as
you twist. his is ideal for a control because you get a good positive feedback of its movement
and when you release the control it stays where it is.
