Page 211 - Robots Androids and Animatrons : 12 Incredible Projects You Can Build

Vcc
4.7K –
3.9 6 8 7
K
5
+

–
2
1
3 4
4.7 3.9 +
K K

8.19 Dual op-amp neural comparator circuit

photoresistor voltage divider is fed into the inverting input of one
op-amp and the noninverting input of the other op-amp.
190 Two other voltage dividers are needed. They are symmetric but
opposite. One voltage divider has a 3.9K-ohm resistor connected
to Vcc and a 4.7K-ohm resistor connected to ground. The second
voltage divider uses the same value resistors, in opposite positions.

When both photoresistors are evenly illuminated, neither LED is
lit. Cover one or the other photoresistor and the corresponding
LED will light.
Each op-amp acts like a simple electronic neuron. When the elec-
trical stimulus falls above or below (depending upon which op-amp
we’re talking about) a threshold (determined by the 3.9K-ohm and
4.7K-ohm voltage dividers), the neuron fires. The firing of the neu-
ron (or outputs of the op-amp) can be used to turn on a DC motor
using an NPN transistor (see Fig. 8.20). The DC motors in turn pro-
vide movement and direction for the photovore robot.

To create a simple photovore robot, a chassis is designed that has
two gearbox DC motors (see Fig. 8.21). When both motors are
powered, the robot moves forward in a straight line. When one
motor is turned off, the motor that still receives power will turn the
robot left or right.
For our photovore robot, we need both motors to be powered
when the two photoresistors are evenly illuminated. Running the
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