What is a comparator and how to use it in IR sensor for a Line or Path follower Robot - IC LM358 & LM324

Comparator

Comparator is a device which compares two input voltages and gives output high/low. In circuit diagram it is normally represented by a triangle having- Inverting (negative) Input (-),Non Inverting (positive) Input(+), Vcc, Ground, Output.

What is an Infra Red (IR) Sensor and how it is used to distinguish between white & black color line in Line / Path follower robot

Infra Red Sensor

IR reflective sensors have one emitter (IR LED) and one receiver (Phototransistor or photo diode. If we have white surface it reflects the light and it will sensed by the receiver, similarly if we have black surface it absorbs the light and receiver can not sense light.

Photo diode has property that if IR light fall on it its electrical resistance comes down (i.e. it comes down from 150kΩ to 10kΩ if no noise present).