CHAPTER 25 ■ LAUNCHING THE LINE-FOLLOWER
Improving Line-Following
Cadmium-sulfide photoresistors react fairly slowly to changes in light. Phototransistors respond 1000
times more quickly. That suggests that substituting phototransistors for photoresistors would improve line
following.
Figure 25-10 shows the photoresistors (R3, R4, R5, and R6) replaced with phototransistors, such as part
#120221 from Jameco for $2.75 each. R2 must then be replaced with a 500 kW trimpot to retain the same
levels of voltage division.
Figure 25-10. Phototransistors replace photoresistors on Sandwich’s brother
Surprise! After substitution, no noticeable improvement in line following was observed. That doesn’t
mean that the phototransistors didn’t improve the responsiveness of the sensor circuit, it just means that
something else was the critical factor in steering. Hmmm. What would that be?
Analyzing Cornering, Frame By Frame
To determine how the robot handles steering, I shot a video of the robot and analyzed each frame
(see Figure 25-11). I apologize for the poor centering of the robot in the video, but the relevant material is
still visible.