Comparator Circuits Line Following Robot
To truly understand the comparator’s operation, let’s look at a few circuits. First we will build
a “low battery” circuit. This circuit will illuminate an LED whenever the 12 volt line goes
below 11 volts. See the schematic below.
A reference voltage is established at the inverting input to the comparator by R3 and the 6.8
volt zener. R3 serves to limit the current through the 6.8 volt zener to about 10mA. The voltage
developed across the zener is fairly stable over a wide range of zener current. To see why,
go back and look at the steepness of the zener V-I curve in the reverse breakdown region.
The next circuit we will look at is a “photovore” circuit. If used with the Tekbot base and
motor control board, the circuit causes the robot to drive towards the area of greatest light in a
room. The sensors that allow light sensing are Cadmium Sulfide cells. These are essentially
light sensitive resistors. They change their value more than two orders of magnitude from a
“dark” to “light” conditions. When they are in the dark they present about 100K ohms to their terminals. When in bright light the resistance drops to about 1k ohms.
To understand the operation of the circuit, imagine that there is a CdS cell on each side of the
robot. If the brightest area is directly in front of the robot, both cells are illuminated equally.
This causes the voltage at the junction of the two cells to be 2.5 volts. This being the case, the
upper and lower comparator’s outputs will be at 5 volts causing both motors to go forward.
Note that the inverting and non-inverting terminals on the comparators have been switched to
make the schematic easier to follow. Also the power supply connections to the comparators
are not shown to lessen schematic clutter. Finally, note that two reference voltages are applied
to the comparator inputs. These may be generated by a voltage divider or other circuit but is
not shown.
Now suppose that the area of greatest light is to the left of where the robot is traveling. This
means that the left sensor will receive more light and the right senor less. This means the resistance
of the upper cell is going down and the lower cell is going up. This will raise the voltage
at the junction of the cells. Eventually, when the voltage exceeds 3 volts, the bottom comparator
will switch its output to 0 volts reversing the left motor and cause the robot to rotate toward
the light source. This will occur until the voltage at the junction of the cells is less than 3 volts.
Identical behavior (with opposite results) will occur when the robot encounters more light on
the right side. If a different orientation of terminals is chosen, the photovore can be changed into a photophobe.
