A pull up resistor is a resistor that works the chip normally HIGH and change it to LOW when switched on, Pull-Up or Pull-Down resistor mostly they are connected to the pins of the chip so that they can define the state of the pins. The Pins of the chip can be in any of 3 state, they can be LOW, a voltage near 0v or below half of the voltage fed into the power supply of the chip is fed into the pin the pins can be high, meaning a voltage above half of the voltage fed into the chips power supply is fed into the pin, or pins can be floating, meaning they are unconnected to either a LOW or HIGH voltage. In this case the pin may go randomly between LOW or HIGH, pins may float because they have no clear defined voltage signal. Such as external noise may allow it to randomly take on both values.
It is undesired for pins to float, and this is why we normally want the signal going into a pin to be clearly defined. for sure whether it is LOW or HIGH, and this is why use to pull-up and pull-down resistors.
The behavior of Pull-Up Resistor is to Pull up the Chip pin to a HIGH state, and the Pull-Down resistor is to Pull-Down the Chip state to LOW.
When connected to a switch such push-button, the state of the pins can be switched to the other when we switched it on.
Below are the example how to connect a pull-up resistor to a pin of a chip so that a pin can be normally HIGH and then switched to LOW when a switch such as a push-button is pushed down.
The best circuit to test the PULL-UP and PULL-DOWN are logic chip circuit. Logic chips are chip that work purely base on LOW and HIGH signals.
A NAND gate is a logic chip that outputs a high signal only when one or both inputs of the gate are LOW, it will output a LOW signal if both of the inputs are HIGH.
4011 NAND Gate Chip
2x 10k Ohms Resistor
2x Push Buttons
330 Ohms Resistor
The 4011 NAND Gate Pin Out
Each NAND Gate has 2 input and 1 output. We’re going to use a push-button as out input to the NAND gate, and the values we will going to use for the pull-up resistors are 10k ohms. The other components required is out LED and the 330 Ohms resistor in series to limit the current going to the LED so that doesn’t burn out.
This Pull-Up resistor circuit can be used in all pull-up resistor circuit, the pin of the chip will be HIGH normally and when the switch is closed it will drop to LOW. Due to the open switch, all of the voltage from the power supply drops across the resistor, when the switch is closed the pin makes direct contact to ground and it will change to LOW state.
Schematic Diagram of NAND Gate circuit with 2 Pull-Up resistors at the input to the Gate
Wiring the 4011 with Pull-Up Resistor
You need to provide a +5v to pin 14 and connect the pin 7 to the GND. This will establish a power to the chip, next is connect the Pull-Up resistors to each of the inputs to NAND gate. A Pull-up resistor circuit first has a resistor connected to + voltage then a push button connected to ground. When the push-button is not push, each of the pins of the NAND gate will be HIGH. When push button is pressed down, it will make a direct contact to the ground and falls to LOW logic level. For a NAND gate logic, if one or both of the inputs are LOW, the output will be HIGH, when both push buttons are unpressed the output will be LOW and the LED will turn off. either both push buttons are pressed down the output will be HIGH and LED will turn on.
This illustrates the use of Pull-Up resistors, when working with chips, they allow a chip to have a pin that normally HIGH, and then which can be switched LOW.