INDEX

Here I used relay. Suppose the above position show when LDR is in white strip(white background), then the motor will get 12V on one terminal and 0V on other terminal. When black comes then the relay looses contact and motor will get 12V on both terminals and breaking action occurs. So the
motor will stop abruptly and the speed obtained is somewhat improved compared to other one. The relay get activated when the LDR is in white strip and motor is in default position(in figure). The motor will run on white strip and stop on black strip.

DISADVANTAGES:
1. The switching speed of the relay is very slow about .1sec
2. Power transistor sometimes creates problem
3. It is very difficult to adjust according to different light situation
4. The job is very boring because we should adjust the pot to make the motor run.
5. The adjustment is done by the following way. First put the sensor on black strip and adjust the potentiometer such that motor will stop. See when you put on white strip motor will run. All these settings should be done when you kept the robot in ground, not on hand or table, i.e, in the arena
6. The adjustment will vary with outside light. As I told you there will be regions in between saturation and cut off of a power transistor where the current Ic will change. The other thing about
the above circuit is that it will be difficult to adjust because we should bring down the voltage to .5V, this is a big task and sometimes won't be practical. Although above circuits worked for me, but troubleshooting them is a headache. So let's go to comparator.