Build Your Own Combat Robot

FIGURE 13-7
Mini sumo body
assembly
Chapter 13: Robot Sumo 287
receives, at least until it is fully saturated. Because different colors absorb different
amounts of infrared light, different colors will reflect different amounts of infrared
light (surface texture will affect the amount of reflected light, and some materials
allow infrared light to pass through).
By placing an infrared detector pair near a surface, the infrared light from the
infrared LED will reflect off the surface toward the infrared phototransistor. Because
the amount of current that flows through the phototransistor is a function
of the amount of infrared light it receives (reflected infrared light from the surface),
this type of arrangement can be used to detect surface color changes. Figure 13-8
shows you a simple schematic of this type of sensor. This circuit was first demonstrated
in a mini sumo by Bill Harrison of SineRobotics.
When the detector pair is over the black portion of the sumo ring, the signal out
from the sensor is high. This is due to the 10 kΩ pull-up resistor and that the transistor
is not conducting any current. When the sensor passes over the white sumo ring
edge, the output signal from the detector pair will go low because the transistor is
not conducting the current straight to ground. The potentiometer is used to adjust
the intensity of the infrared LED, adjusting the sensitivity of the detector pair.
The relative distances between the infrared LED, the phototransistor, and the
surface will have an effect on the sensitivity of this circuit. The reflective sensors
from Optek P/N OPB706A and QT Optoelectronics P/N QRD-1114 have both the
infrared LED and infrared phototransistor built into a single small package. Both of
these sensors operate well at distances from 0.04 to 0.20 inches from the surface.