Accessories for
the CheapBot- 14
By Paul Verhage
I really enjoy using the
CheapBot- 14 robot
controller that I described
in my SERVO article in the
June ‘09 issue. It’s a nice
entry-level robot controller
and I think it makes a
great robotics kit (the
Lawrence, KS 4H also
seems to agree). However,
if the CheapBot- 14 is
going to behave like a
robot, then it needs to
sense the world around it.
So, this time I’ll describe
some of those sensors.
Line Follower
After you can program your robot to do a little dance,
it’s time to give it a real task. One of the first ones for the
beginning roboticist is to have their robot follow a black line
on a white board. Conceptually, line following is a simple
task, but it’s challenging enough for the beginner to be
satisfying. Best of all, it results in a behavior that’s easy to
evaluate for success.
The line follower presented in this article is easy to
interface to both the robot body and the robot controller
(and it’s not just limited to the CheapBot- 14). It consists of
two infrared LED-phototransistor pairs spaced four inches
apart on a single PCB (printed circuit board). By reflection,
each infrared LED-phototransistor pair detects the presence
— or absence — of a black line beneath it. If the area
beneath the phototransistor is black (i.e., it’s above the
black line), then light from the infrared LED (IRED) doesn’t
reflect up to the phototransistor. Without IR, the
52 SERVO 08.2009
phototransistor doesn’t conduct and there’s no current path
to ground. So, the robot controller detects five volts — or a
logic high — from the phototransistor’s pull-up resistor.
If, however, the space beneath the phototransistor is
white (there’s no black tape beneath the phototransistor),
then light from the IRED reflects up to the phototransistor
causing it to conduct. The conducting phototransistor sends
current from the pull-up resistor to ground rather than to
the robot controller. As a result, the robot controller
detects a logic low. By monitoring the voltage at the two
phototransistors, the microcontroller can determine if
the black line lies beneath one, both, or neither of the
phototransistors.
Assembly Directions
Assembly is pretty easy. Bend the leads of the resistors
and solder them into place. There’s one jumper wire on the