Servo - August 2009

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.