This is a line-follower robot based on LM339 comparator.
The robot is totally analog i.e. without micro-controller or any programmable boards.
Line-edge sensors are made from two photoresistors in series with 10k resistors, forming an voltage divider, with an LED in-between.
The comparator compares the output voltage of the two voltage dividers (sensor: photoresistor+10k resistor), than, the corresponding DC motor is activated.
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Step 1: More Details
1. LM339 is quad-comparater i.e. it can compare 4 different voltages. We will use just two-comparators.
2. Connect the two sensor outputs (LDR1 and LDR2) to the two comparator inputs: LDR1 to input1+ and input2-, and LDR2 to input1- and input2+.
3.Connect output1 to right motor and output2 to left motor.
4. For power supply you can use 9 V with voltage regulator XX7805.