well how to make a bot using logic chips.... I hopw you can use analog comparators as well (op amp).
anyway take 4 sensors. Light dependent resistors are the best. and then take 4 superbright red LED's. and make those LED's shine on the LDRs. \/ alignment (light reflects from floor). and also shield the whole system from all amb. light with cloth or tape or whatever...
Anyway that done let's move to the sensor reading part... Connect those sensors to the analog comparator inputs. right to comparator1(C next time) + input and one of the middle ones to C1-, then the other middle one to C2- and the left one to C2+ (you might want to switch the inputs. haven't tested it so don't know the outcome). and the other sensor legs (not connected to C1/2) to Vcc (+5v). then take a 4x10k resistors and connect them from GND to C1+- & C2+- legs (on all legs as pulldowns).
And you have the sensors done... Next take a good transistor (say BC618) and connect it to the C output. one for each motor. also connect a 180ohm (or alike) resistor between the transistor base and the Comparator output. (just in case). and then slamm the motors to the transistors and a way you go
a lot of trimming and tuning needs to be done, but this is ht funnest and most educational part of the robot building
And remember, this is just one way of doing it... you could use 100 other ways to do it. but I would appreciate, if you at least considered mine as it took so long to write
anyway good luck
If you have any questions, you know where to find me