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Table of contents
What are Universal Gates?
NOR Gate
Realization of Logic Gates Using NAND & NOR Gates
Using NAND Gates
Using NOR Gates
Frequently Asked Questions
Why are NAND and NOR gates called universal gates?
Can complex digital circuits be built using only NAND or NOR gates?
Is there any advantage to using universal gates over other logic gates in circuit design?
Last Updated: Mar 31, 2024

Universal Gates

Author Rahul Singh
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Universal gates form the most important part of digital electronics, powering everything from the simplest gadgets to the most complex computing systems. These special gates have the unique ability to perform any logical function, making them indispensable in the world of circuit design. 

Universal Gates

In this article, we'll learn what universal gates are, focus on NAND and NOR gates, and demonstrate how they're used to construct other types of logic gates. 

What are Universal Gates?

A universal logic gate is a kind of logic gate that may be utilised to construct any kind of logic function without the need for additional logic gate types.These gates are called "universal" because with just one type of gate, you can make any other gate you need, be it an AND, OR, NOT, or any other logic gate. The two most important gates are the NAND and NOR gates. They are like the building blocks of digital electronics, allowing us to construct complex circuits from just these two types of gates.

A universal gate can take two or more input signals and, based on a specific rule, produce one output. The beauty of these gates lies in their simplicity and versatility. By combining them in different ways, you can create circuits that perform a wide range of logical operations. This capability makes universal gates incredibly important in designing and simplifying circuits in computers, gadgets, and all sorts of electronic devices.

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In the world of universal gates, let's start with the NAND gate, a true powerhouse in digital electronics. A regular AND gate, which gives a high output (1) only when all its inputs are high (1). Now, add a twist by inverting its output, and voilà, you have a NAND gate. This means if any of the inputs are low (0), the NAND gate outputs a high (1). The only time it gives a low output (0) is when all its inputs are high.

NAND gate

Here's a simple way to remember it: NAND is short for "Not AND." It's like the AND gate's rebellious sibling that says "no" when AND says "yes."

To give you a clearer picture, let's look at a basic example with a 2-input NAND gate:

  • If both inputs are 0 (0 & 0), the output is 1.
  • If one input is 1 and the other is 0 (1 & 0 or 0 & 1), the output is 1.
  • Only when both inputs are 1 (1 & 1), the output is 0.

This behavior makes the NAND gate incredibly versatile. With enough NAND gates, you can create any other logic gate or circuit you need, making it a cornerstone of digital circuit design.

Let's see a bit of code to simulate a 2-input NAND gate:

  • Python


def nand_gate(input1, input2):
# Perform an AND operation on the inputs
and_result = input1 and input2
# Invert the result to get the NAND output
nand_output = not and_result
return int(nand_output) # Convert boolean to integer (True to 1, False to 0)

# Example usage
input1, input2 = 1, 0 # Change these values to try different inputs
nand_output = nand_gate(input1, input2)
print(f"NAND Gate Output for inputs {input1} & {input2}: {nand_output}")


NAND Gate Output for inputs 1 & 0: 1

This simple Python function nand_gate takes two inputs and returns the output of a NAND gate. By changing input1 and input2, you can test different scenarios and see the NAND gate in action.

NOR Gate

Now, let's shift our focus to the NOR gate, another type of universal gate that plays a crucial role in digital circuits. If the NAND gate was the AND gate with a twist, the NOR gate is the OR gate turned on its head. In an OR gate, if any of the inputs are high (1), the output is high. But in a NOR gate, the output is high (1) only when all inputs are low (0). As soon as any input goes high, the NOR gate's output flips to low (0).

To put it simply, NOR stands for "Not OR". It's like the OR gate's strict counterpart that only agrees when everyone else disagrees.

Here's how a 2-input NOR gate works:

  • If both inputs are 0 (0 & 0), the output is 1.
  • If one input is 1 and the other is 0 (1 & 0 or 0 & 1), the output is 0.
  • When both inputs are 1 (1 & 1), the output is also 0.

This unique property makes the NOR gate a fundamental element in creating complex logic circuits and even combining multiple NOR gates to mimic any other logic gate's behavior.

Consider this example code for simulating a 2-input NOR gate:

  • Python


def nor_gate(input1, input2):

   # Apply OR logic to the inputs

   or_result = input1 or input2

   # Invert the result for NOR logic

   nor_output = not or_result

   return int(nor_output)  # Convert the boolean result to integer

# Test the NOR gate function

input1, input2 = 0, 0  # Change these values to test different scenarios

nor_output = nor_gate(input1, input2)

print(f"NOR Gate Output for inputs {input1} & {input2}: {nor_output}")


NOR Gate Output for inputs 0 & 0: 1

This Python function, nor_gate, takes two inputs and returns the NOR gate's output. You can adjust input1 and input2 to experiment with various input combinations and observe how the NOR gate responds.

Realization of Logic Gates Using NAND & NOR Gates

The magic of NAND and NOR gates doesn't stop at their individual functions. These gates are so versatile that they can be used to create any other logic gate. This is crucial in digital electronics, as it allows for the design of complex circuits while minimizing the variety of components needed.

Using NAND Gates

NAND gates can be cleverly arranged to mimic the behavior of AND, OR, and NOT gates, which are the basic building blocks of digital logic.

  • AND Gate with NANDs: Simply connect two NAND gates in series. The first one acts as a regular NAND gate, and the second one inverts the output back, giving you an AND operation.
  • OR Gate with NANDs: Use three NAND gates. The first two invert the inputs, and the third NAND gate acts on these inverted inputs. This setup gives you the OR functionality.
  • NOT Gate with NANDs: Connect both inputs of a NAND gate to the same input signal. The output will always be the inverse of the input, acting as a NOT gate.

Using NOR Gates

Similarly, NOR gates can be configured to replicate the AND, OR, and NOT operations

  • AND Gate with NORs: This requires three NOR gates. The first two NOR gates invert the inputs, and the third NOR gate gives the AND result by NOR-ing these inverted inputs.
  • OR Gate with NORs: Just like with NANDs, to get OR functionality, you need two NOR gates. The first NOR gate acts as the OR gate, and the second one inverts the output back.
  • NOT Gate with NORs: Connect both inputs of a NOR gate to the same signal. The output will be the opposite of the input, serving as a NOT gate.

These configurations demonstrate the flexibility and importance of NAND and NOR gates in digital circuit design. By strategically arranging these universal gates, we can achieve any logical function, paving the way for complex circuit construction with simplified components.

Frequently Asked Questions

Why are NAND and NOR gates called universal gates?

NAND and NOR gates are termed universal because they can be used independently to create any other basic logic gate (AND, OR, NOT, etc.), making them highly versatile in digital circuit design.

Can complex digital circuits be built using only NAND or NOR gates?

Yes, complex digital circuits can be constructed using only NAND or NOR gates. This is due to their ability to replicate the functions of all other basic logic gates, allowing for the design of any digital circuit, no matter the complexity.

Is there any advantage to using universal gates over other logic gates in circuit design?

Using universal gates like NAND and NOR can lead to simpler and more cost-effective circuit designs. They reduce the need for multiple types of gates, which can simplify manufacturing and increase reliability.


In this article, we looked into universal gates, focusing on NAND and NOR gates and their incredible versatility. We've seen how these gates work as the foundation of digital electronics, enabling the construction of every other type of logic gate and, by extension, complex digital circuits. 

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