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This is a simplified portion of a solar tracking circuit. The potentiometer represents a light dependent resistor (LDR). As the Sun hits the LDR the resistance drops, creating a smaller resistance on the potentiometer shown, and thus a smaller voltage drop. This creates a lower voltage at the non inverting input terminal (+) of the operational amplifier as compared to the reference voltage at the input of the inverting terminal (-) created from the voltage divider at the bottom, 4.5 volts. This in turn turns the output low and thus off-red light, and the motor stops, solar panel stays facing Sun. As the sun moves, the light hitting the LDR decreases and thus increasing the resistance/voltage drop at the bottom portion of the potentiometer and thus creating a larger voltage at the non inverting terminal as compared to the inverting terminal- green light and the motor turns on and the tracker moves once again. The outputs would need to be raised using a mosfet circuit, or something to actually handle the current from the motors.
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