Page 85 - Robots Androids and Animatrons : 12 Incredible Projects You Can Build

Light sensors (sight)
There are a large variety of light sensors: photoresistive, photo-
voltaic, photodiodes, and phototransistors. Light sensors can be
used for navigation and tracking. Some robots use an infrared light
source and detector to navigate around obstacles and avoid crashing
into walls. The infrared source and detector are placed in front of the
robot facing in the same direction. When the robot encounters an ob-
stacle or wall, the infrared light is reflected off the surface causing an
increase in the infrared light detected. The robot’s CPU interprets
this increased radiation as an obstacle and steers the robot around it.
Filters can be placed in front of light sensors to inhibit their response
to some wavelengths while enhancing their response to others. One
example of the use of filters is as flame detectors used in fire-fighting
robots. One would try to enhance the response to light from a fire
while inhibiting the response to light from other sources.
Another example is the use of colored gels as filters to promote
color response. One could imagine a robot that separates or picks
ripened fruit based on the fruit’s skin color.

Photoresistive
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Cadmium sulfide (CdS) sensors (see Fig. 5.5) are photoresistors that
can read ambient light. The CdS cells response to the light spectrum
is in close approximation to that of the human eye (see Fig. 5.6).
These are semiconductor sensors without the typical PN junction.
The CdS cell displays its greatest resistance in complete darkness.
As the light intensity increases, its resistance decreases. Measuring
its resistance provides us with an approximation of ambient light.

Photoresistive light switch
Figure 5.7 shows a basic light switch. Because the CdS cell is a
resistive-type transducer, it can be placed as a resistor in a voltage
divider. When the light intensity increases, the resistance of the

Photo-Resistor(CdS Cell) 5.5 Cadmium sulfide cell

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Chapter five