In all digital circuit boards we surely find Pull up/Pull down resistors in it. Here we are going to learn what it does, how we can use it in our circuit and calculations for finding values of these resistors.
What are Pull UP/ Pull Down resistors ?
As the name suggests pull up / pull down , these are the resistors which are used in digital circuits to put the digital inputs and voltage equivalent to the level of Vcc or to the ground(GND).
Image 1: Pull Up Resistor 
Image 2: Pull Down Resistor
Why to use it ?
We all know that all digital systems work on “0s” and “1s”. These 0s and 1s at input and output are determined by its Logic Level. Now Logic Level is the voltage range which decides how an output or input in a digital circuit in interpreted either as a 0 or 1.
And not all the digital devices work on same voltage levels which is dependent on the logic family used in that device (component). There are different logics like TTL, CMOS, RTL, DTL, ECL . Where TTL and CMOS family devices are famous.
And if the pins of these devices left open without any resistor, device gets effected by the EM waves available in the environment. Due to this an undefined or any floating voltage will appear on this pin which may be taken wrongly by logic devices.
As you can see in below image :
Here in the above image any value between 2.7V and 5V at output side or between 2V and 5V at input side will be read as HIGH means “1” or “ON”.
And any value between 0V and 0.5V at output side or between 0V and 0.8V at input side will be read as LOW means “0” or “OFF”.
If this not taken into consideration then our device may get ON or OFF or confusing state itself by reading some floating voltage in these ranges.
So to stop this from happening we use Pull Up / Pull Down Resistors.