CHAPTER 27 ■ ENCORE
The simplest sense of touch is a pushbutton of some form. Appropriate pushbuttons for touch sensing
have long lever arms that activate with little force (see top right of Figure 27-13). The pushbutton can
electrically disconnect a motor, connect it in reverse, or signal the event to the brains.
Figure 27-14. Metal-encased infrared photodetector to suppress false signals due to electrical noise (left) and
standard Vishay TSSP4038 infrared photodetectors (right)
Figure 27-13. Touch sensors: vibration sensor (top left), snap-action level switch (top right), and Jameco
#150551 $12.95 flex sensor (bottom)
Flex sensors are variable resistors (see bottom of Figure 27-13). As they bend, they change resistance.
You can see this in action on a multimeter in W mode. Flex sensors work well with comparators or
microcontrollers. Surrounding the robot with a ring made of flex sensors would inform the robot of the
slightest warping from any side.
One form of a vibration sensor is a stubby flex sensor with a weight attached (see top left of Figure 27-13).
You could put a couple of these inside a robot at different angles and determine the gross direction in which
the robot is oriented. Or, knocking the robot on the head could make it say “ouch” or start moving.
Object Detection and Infrared Remote Control
A popular technique for detecting the presence of an object is to use an infrared LED to blink a signal and
simultaneously check an infrared detector to see if the signal bounces back. If the signal doesn’t bounce
back, the area in front of the infrared LED is clear of obstacles.
There are a couple of commercially manufactured parts that perform the most difficult part of detecting
a signal (see Figure 27-14). These parts usually have only three pins: one pin for positive power, one pin for
negative power, and the last pin, which turns on or off depending on the presence of the signal.