CHAPTER 26 ■ SMOOTHER, SMALLER, CHEAPER
Size is not the only benefit of surface-mount parts. Because these parts are used in high volume
for commercial electronics, they are cheaper than similar through-hole varieties, which are no longer
commonly used in production.
More significantly, using surface-mount parts provides access to advanced technologies that are simply
not being released in older packages. For example, the elegantly named NSS12200LT1G transistor can
provide 4000 mA of current to the motors, compared to 600 mA for the classic 2907A transistor.
Looking Tiny
The basic operation of the robot is unchanged. White LEDs reflect light off the floor into light-sensitive
sensors. The brain compares the voltages of the sensors against each other and then turns on transistors to
power the motors.
The underside of the board (Figure 26-8) features Vishay TEMT6000X01 phototransistors and 2-PLCC white
LEDs. Notice that the sensors are smaller than the head of the 4-40 screw that holds the board to the robot’s case.
Figure 26-7. Finished board with hand-soldered surface-mount parts
Figure 26-8. Line-following sensors soldered onto the bottom of the board