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).

 

                                 Figure:IRSensor                                                      

Sample Calculation:                                                             
Say Receiver has resistance
Rs=150kΩ without light (on black surface)
Rs=10kΩ with light (on white surface)

The voltage that goes to comparator Without light: (on black surface)
Vp=(Rs÷(Rs+R))Vcc=150÷(150+10))*5=4.6875V
With light: (on white surface)
Vp=(Rs÷(Rs+R))Vcc=10÷(10+10))*5=2.5000V

Thus we get variation of voltage that is sensed by comparator IC (LM324).
This gives logical high or low according to input.