Every thing related to Pull up and Pull down resistor along with its basic need with some practical example, with its circuit, its applications, the definitions etc.

1. PULL-UP & PULL-DOWN
RESISTORS
By Robotics Sir (Ashutosh Kumar)

2. WHAT IT IS?
When using any kind of “open” inputs with an
Arduino/Microcontrollers/logic gates such as switches,
push buttons, reed relays, and some sensors, a pull-up or
pull –down resistor is needed for its reliable operation.
These resistors hold the I/O pin at a known value until
the switch forces the I/O pin to a different known value.

3. DEFINITION
Pull up:- The Pull-up resistor pulls the signal to high state of
the input pin, unless it is driven low.
Pull-up defaults to a HIGH. It’s value is LOW when active.
Pull down:- A pull-down resistor pulls the signal to low state
unless it is driven high.
Pull-down defaults to a LOW. It’s value is HIGH when active.

4. DEFINITION (ANOTHER)
Pull up:- Pull-up resistors are resistors which are used to ensure
that a wire is pulled to a high logical level in the absence of
an input signal.
Pull down:- Pull-down resistors are resistors which are used to
ensure that a wire is pulled down to a low logical level in the
absence of an input signal..

5. WHY WE USE THEM?
To prevent accidental switching of digital circuits/ICs,
any of the unconnected inputs called “floating
inputs”, should be tied to a logic “1” or logic “0” as
appropriate for the circuit.

6. PRACTICAL EXPERIENCE
Normally the input pins of the Digital gates/ICs/
Controllers etc. behaves as a “floating pin (there value keep
varying, 0 or 1)”. Because of the presence of the EMI
waves/noise signal etc. in surrounding, the input value of these
pins constantly keep varying on their own. We can either use a
Multimeter or a CRO to visualize these variations. That’s why
we use Pull-up or down resistors to fix their values to 0 or 1.

7. CIRCUIT-PULL UP
As its name suggest “Pull-up” i.e. +ve , hence
the resistors should be connected to +ve i.e.
+5v. And its another end should be connected
to the input pin of the digital
device/components.

8. CIRCUIT-PULL DOWN
As its name suggest “Pull-down” i.e. -ve ,
hence the resistors should be connected to
-ve or ground i.e. 0v. And its another end should be
connected to the input pin of the digital
device/components.

9. FORMULA FOR (PULL UP)
Q. Suppose we have a logic circuit where
the supply source is 3.3V and the
acceptable logic high voltage is 3V, and we
could sink a current maximum of 30uA,
then we can choose the pull-up resistor
using the formula like this way-

10. FORMULA FOR (PULL DOWN)
Q. Now, If we consider the same example
stated in the previous question, where the
circuit accepts 1V as the maximum logic
Low voltage and could source up to
200uA of current then the Pull-down
resistor will be,

11. APPLICATIONS (1)
 Pull-up and Pull-down resistors are frequently used
in Interfacing devices like interfacing a switch to
microcontroller.
 These resistors are often used in A/D converters to
provide a controlled flow of current into a resistive sensor.

12. APPLICATIONS (2)
 Pull-up and pull-down resistors are used in I2C protocol
bus, wherein the pull-up resistors are used to allow a single
pin to act as an I/P or O/P.