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Here it is somewhat good to adjust. You can connect motor directly at the collector instead of using relay. Here i had drawn the reference voltage using two 10Kohm, but you better use a potentiometer (variable resistor) . The 10Kpot series with LDR is varied to adjust. Here you can use a 9V or 12V relay. But using 12V relay will sometimes won't work because Vcc won't be exactly or greater than 12V.
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
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