www.servomagazine.com/index.php?/magazine/article/november2011_Ramirez
FIGURE 3. The low
cost red plug-and-play
VEX potentiometer
which interfaces to the
microcontroller. It is a
very useful VEX sensor
since it can be used to
measure angles of arm
and leg joints from zero
to 250 degrees, and
can easily be attached
using VEX hardware.
FIGURE 4. There are other kinds of resistive sensors that work
similarly to potentiometers, including CDS cells (light sensors where
the resistance varies proportionally to the brightness).
FIGURE 2. The schematic
symbol for a potentiometer.
Notice that it has three terminals.
volts, respectively, for most VEX applications.
The low cost red plug-and-play VEX potentiometer
which interfaces to the microcontroller (sold by IFI) is shown
in Figure 3. It is a very useful sensor since it can be used to
measure angles of arm and leg joints from zero to 250
degrees, and can easily be attached using VEX hardware.
Other applications for it include accurate speed control
where the motor speed is proportional to the resistance or
voltage at the center terminal (wiper) on the potentiometer.
The August article mentioned that the potentiometer could
be substituted for the quadrature optical encoder which
was used as a strobe light frequency control. This time,
we’ll explore more of this sensor’s capabilities and how it is
similar to other resistive-based sensors.
There are other kinds of resistive sensors that work
similarly to potentiometers, including CDS cells (light
sensors where the resistance varies proportionally to
brightness) shown in Figure 4, thermistors (resistors that
vary with temperature), and a flexible resistor shown in
Figure 5 that is otherwise known as a flex sensor (a resistor
whose resistance varies with the degree of flex or bend).
Note that it only has two terminals and requires a 10K pull-up resistor to +5V in order to form a voltage divider. Since
these sensors are all analog in nature, they can be used in a
similar manner. These kinds of sensors are read using the
microcontroller’s analog
inputs which are connected
internally to the 10-bit ADC
(Analog-to-Digital Converter).
In this experiment, we will have some fun making
our glove using flexible resistors which work very similarly
to potentiometers. Take a look at the bill of materials
(Table 1) needed to carry out this experiment. Note that
you don’t need to purchase all five flexible resistors; instead,
you can keep the cost down and still get good results with
one of these resistors (which cost around $13 each from
SparkFun). This glove can easily be assembled in a couple of
hours. I designed this glove for carrying out my own VEX-based Telepresence and telerobotics experiments. It can be
used to liven up any magic show, musical production, or
rock concert when the “star” uses it to cue the beginning
of a dazzling light, laser, or pyrotechnics show to wow the
FIGURE 5. A flexible resistor otherwise known as a flex sensor (a resistor whose resistance varies with the degree of flex or bend).
Note that it only has two terminals and requires a 10K pull-up resistor to +5V in order to form a voltage divider.
SERVO 11.2011 49
