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Chapter 15    ■  Surveillance Robot     201


        milliseconds and forwardOrProximity moves            Another effect of this design is that the sensor is
        forward for the time specified in its argument, or  not linear, but rather follows a parabolic curve.
        until the proximity sensor detects an obstacle. If an  Fortunately for us, we just want to know when we
        obstacle is detected, the robot buzzes twice and   are close to an obstacle, not how far away it is.
        then reverses.
                                                           Driving Motors
        IR Distance Sensor                                 Most robots need motors of some description and
        The IR distance sensor uses triangulation to detect  many will need to drive them from a microcontroller
        the distance of an object (Figure 15-19).          like the Arduino. Just turning a motor on and off

           A bright IR LED with a focusing lens projects a  is quite straightforward. MOSFET transistors (see
                                                           the “Theory” section of Chapter 5) are often used
        dot. This will be reflected by any object, and the
                                                           for this because of their excellent switching
        reflected light then falls on a linear CCD array of
                                                           characteristics—that is, very low “on” resistance
        photocells. The most brightly illuminated cells will
                                                           and very high “off” resistance. They also take very
        vary according to the distance of the reflected
                                                           little current to turn them on and off.
        object. The control electronics then provide an
        analog voltage that indicates the distance.          Figure 15-20 shows the basic schematic for
                                                           controlling a motor using a single N-channel
           One side effect of the way this works is that the
                                                           MOSFET.
        sensor has both a maximum and minimum distance
        it can sense. So, if the robot becomes too close to
                                                                                                Motor
        an object, it will not be able to tell that an object is                               positive
        close.                                                                                  supply









                                                                                                 M














                                                           Microcontroller
                                                           Digital Output

                                                                                             N




                                                            GND

         Figure 15-19  IR distance sensor                   Figure 15-20  Controlling a motor with a MOSFET
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