Hi Sindhu
you are using ir sensors. if you calculate the voltage of ir sensor circuit
only then it gives you only for 0.3-0.9V and controller read as low logic. it is not
enough voltage to read controller as high voltage.therefore this circuit is used.
when sensor gives minimum of 1.8V then it give logic as high=5v
The purpose of this circuit is to increase the voltage. the question is how?
first, start with diode, diode is working in that place where it block negative
voltage only. if you dont want to connect the diode, then there is no effect on
the circuit but you must give pure dc supply.
Then the resistor is connect with BJT. you know BJT is depend on current to operate
therefore, resistor in series must must be connect. I tell you in first paragraph
ir sensors gives only 0.3-09 V only. here is use npn transistor, and sensor wire
is connect with base which is positive and to increase the positive voltage. you see
emitter is ground and positive base voltage voltage go to the controller with logic high and controller
take positive voltage only and now it won’t be confused.Bjt use for voltage increase from 0.9-
3.2V approximately.
high resistance about 4.7k is used because you want to increase voltage not current
v=ir when resistance is high, current reduce and voltage increase.