eBay had a line follower cart kit.
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It has two motors and a two light sensors and LEDS and a LM393 comparator.
It has a circuit:
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It is important to consider the comparator circuit.
https://www.ti.com/product/LM393
The LM393 uses an open collector output that sinks current to turn on the motor.
The comparator needs the inputs to be about 1.5V below the positive supply.
This means it is more that a comparator.
There are two comparators, one for each motor, but the + and - inputs are wired contral making it look like a comparator with an inverter on the output.
However they are only comparators if the -ve input is 2V below the positive supply!
We can put together this truth table.
| sensor1 | sensor2 | motor1 | motor2 |
| dark | dark | on | on |
| bright | brighter | off | on |
| brighter | bright | on | off |
There are two trim pots to adjust for battery voltages, and ambient light levels.
Make sure it turns in circles on a bright surface, and goes straight on dark surface.
Now the fun starts!
You do not need to follow a line. I found a picture frame with a strong graphic contact and it had a lot of fun exploring the dark blue areas and going around in circles on the white borders.
https://www.ti.com/product/LM393
7.3 Feature Description The comparator consists of a PNP darlington pair input, allowing the device to operate with very high gain and fast response with minimal input bias current. The input Darlington pair creates a limit on the input common mode voltage capability, allowing the comparator to accurately function from ground to VCC– 1.5 V input. Allow for VCC– 2 V at cold temperature. The output consists of an open drain NPN (pull-down or low side) transistor. The output NPN sinks current when the negative input voltage is higher than the positive input voltage and the offset voltage. The VOL is resistive and scales with the output current. See Figure 6-3 for VOL values with respect to the output current.
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