Introduction
Today we learn the use of A Pull Up Resistor using 4011 NAND Gate Chip, A pull-up resistor is a commonly used electronic component in digital circuits to ensure that a signal remains in a known state when it is not actively driven by a device. It is especially useful in applications where you want to avoid floating or undefined states.
When a digital input is not actively driven by a signal source, it can be in a floating state, susceptible to noise and interference. By connecting a pull-up resistor, you provide a defined path for the current to flow, which helps establish a stable logic level.
Components Required
- CD4011 NAND Gate Chip
- (2)10KΩ Resistors
- (2)Pushbuttons
- LED
- 330Ω Resistor
Circuit diagram of Pull Up Resistor:
This pull up resistor circuit can be used exactly as is for every pull up resistor circuit.
The pin of this chip will be HIGH normally and when the switch is closed drops to LOW.
How this circuit works is the resistor is directly connected to the positive voltage. Due to the open switch, no current flows through the circuit; therefore, no voltage builds up across the resistor. All of the voltage is contained across the power supply. So the voltage at (either) end of the resistor is equal to the power supply voltage.
When the switch is closed, now the pin makes direct contact with ground and is now at a LOW state. Current flows through the circuit, so voltage is able to build up across the resistor. Being that practically all of the power supply voltage falls across the resistor, the voltage after the resistor is essentially 0V.
NAND Gate Circuit Using Pull Up Resistors
The schematic diagram of the Pull Up Resistor using 4011 NAND Gate Chip at the inputs to the gate is shown below.
Breadboard Schematic
Below is the breadboard circuit so that you can see the exact wiring of the circuit to the 4011 chip.
Working
First of all we give power to the chip. We will give +5V to pin 14 and we connect pin 7 to GND. This establishes power to the chip.
Now we connect pull up resistors to each of the inputs of the NAND gate. A pull up resistor circuit first has a resistor connected to positive voltage and then a pushbutton connected to ground. When the pushbutton is not pushed, each of the pins of the NAND gate will be HIGH. When a pushbutton is pressed down, the pin of the chip now makes direct contact with ground and falls to a LOW logic level. For a NAND gate logic, if one or both of the inputs are LOW, the output will be HIGH. So when both pushbuttons are unpressed, the output will be LOW and the LED in the circuit will be off. When either or both of the pushbuttons are pressed down, the output will be HIGH and the LED will turn on.
Now we use pull up resistors when working with chips, they allow a chip to have a pin that is normally HIGH, and then which can be switched LOW.
Applications
- Switches and buttons: Pull-up resistors are commonly used with switches and buttons to establish a default logic level. When the switch is open, the pull-up resistor pulls the voltage to a high logic level (e.g., Vcc or +5V), and when the switch is closed, it overrides the pull-up resistor and pulls the voltage to a low logic level (e.g., ground).
- Digital inputs: Pull-up resistors are used with digital inputs to ensure a defined logic level when no external signal is present. This is especially important in microcontroller-based systems where unused inputs can float and produce unpredictable results. The pull-up resistor pulls the input voltage to a high logic level when no external signal is driving it.
- I2C and SPI interfaces: Pull-up resistors are often used in I2C (Inter-Integrated Circuit) and SPI (Serial Peripheral Interface) communication interfaces. These interfaces use open-drain or open-collector lines, which require pull-up resistors to establish the high logic level. The pull-up resistors ensure proper communication by keeping the lines at the correct voltage level when no device is actively driving them low.