This wasn't my first attempt at a line follower, more is described in the robot car section, but having run into problems building what I thought would be a simple machine, decided to invest in a kit to do this task, with the side benefit that most of the components would be reusable.
This kit varies in price quite a lot on the internet, got mine from Amazon where it was also available for three times the price, but essentially consists of the same components wherever it's bought from, though someone might argue that theirs is higher quality. The instructions supplied were mostly in Chinese, which I don't speak or read, but were clear enough for anyone familiar with making kits to understand.
It took about an hour to assemble but basic testing showed it wasn't working, eventually tracked down to basic open circuit faults on the PCB. The following picture shows the simple rewiring done with component leads to fix this, but if the kit was being used as an educational project could be distracting and disappointing.
Extra: I bought a bag of kits for people to make while learning soldering which included several of these line follower kits, every one had the same fault on the PCB, even though it wasn't visibly obvious.
The sleeving on the wires was added by me.
So once corrected, it was fitted with batteries and put onto the test circuit on the rear of the instruction leaflet which worked very well. I already had a test track laid out but that had become worn, so I laid a new track for the test using electrical insulation tape. It worked fine, as can be seen in the video, but it's important to recognise the shuffling movement as just that.
This is an electronic mechanism which shuffles the chassis around the circuit, first driving one motor until interrupted, and then switching drive to the other motor. The spacing between the sensors makes this shuffling action very short and so the whole moves along the line being followed in a zig-zag fashion, there isn't any drive to keep going smoothly round a corner, or even to drive in a straight line, imagine being a passenger in a robot car which did this.
So it does it's job. Briefly, its an LM393 dual comparator IC which has as inputs, a resistance bridge made up of two variable resistors, to tune the response, and two light dependent resistors (LDRs), illuminated by a pair of LEDs pointed at the surface of the track. An imbalance in the reflected illumination, triggers the LM393 comparator to switch on one of two transistors connected to the motors (and the indicator LEDs) and drives the chassis one way or another, it's either one motor or the other switched on. The variable resistors allow the circuit to be tuned for different illumination conditions and variations in components. In practise I've assembled several of these and never had to adjust the resistors.
It's very successful. Next is the ESP32-CAM, a wifi enabled MCU with a camera and SD card reader.