Pull up & pull down resistors

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 , h ence 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 I nterfacing 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 .