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See this track, there is a number of crossing. There are cuttings in the way and some extra lines are added in the track to mislead the robot. See what happens when you use the above circuits. The analog circuits above cannot take a decision. If they loose the way, they cannot come back to the track.
Then the speed will be reduced at the crossing. Analyze the circuit yourself. Sometimes your robot looses the way in crossings and cuttings and no way exist to bring back the robot back to the track. If you want to do this then you had to add extra circuits and complexity of circuit increase with the path. But if you use a microcontroller then this problem will be solved and the requirement of processing circuits also avoided. A microcontroller circuit can trace back the path which it followed. It can avoid the situation of external light. If we use ADC for the LDR readings then the external light problem will be avoided. External light affect the readings almost equally in all sensors, so it equally in ADC reading. We can subtract the reading to avoid this problem. Suppose if we use 4 sensors
on each side. Then from the following circuit diagram you can use PWM to control the H-bridge by connecting PWM output to enable of H-bridge. Then the PWM can be controlled by the following way. If ADC1 readings increase then PWM will be reduced from 100% to 90%. Assuming that ADC1 reading is the front LDR (ldr1). Suppose if ldr1 crosses line then we can detect it and we can reduce pwm to 80%. But in analog case this was not possible. We  can determine the instant position of the ldr's and take appropriate decisioning while that is not possible in analog circuits. This make the robot to get a proper movement. Suppose if we use microcontroller just like a parallel port and the sensor readings are inputted to the microcontroller and LM317 circuit is connected to another port. You can see that there are 2^4 combinations of voltage are possible with that circuit, but with the analog circuit we are able to use only about 4 voltages. We can use 16 different voltages using a
microcontroller. This 16 different voltages are applied to the motor using the ADC readings of the sensor(1-4). Thus we can get voltages from 1.25 to 12V with 1V difference and better control is possible and we can determine the position of sensors, so that we can get a robot with good speed by
adjusting voltage with better precision.