NOR gate

Shihab Uddin Ahmed | February 7, 2017

In this series of ‘digital electronics’ I have already discussed about three basic gates and NAND gate, which is a widely used gate in digital electronics. Now my topic is NOR gate.

Definition of NOR operation

NOR operation is a Boolean operation where output is true if all operands are false.

In terms of NOR gate

NOR gate circuit is a logic circuit where output is always zero except when all inputs are zero.

Two input NOR gate symbol photo

Two input NOR gate symbol

Two input NOR gate truth table photo

Two input NOR gate truth table

Like NAND gate truth table there is an important point to note in NOR gate truth table also. At first we may consider OR operation on two operands (inputs) A and B, after that we are inverting the result with NOT operation. This is how NOR gate works.

NOR gate in simple words

NOR gate is like connecting an OR gate output to a NOT gate input like the figure below:

OR-gate-and-NOT-gate-make-NOR-gate photo

NOR gate may have three or more inputs

Three input NOR gate symbol photo

Three input NOR gate symbol

Boolean expression for NOR gate operation is given below which is read as x = not A+B,

Boolean-expression-for-NOR-gate-operation photo

Like NAND gate, NOR gate is also a universal gate, it’s possible to construct any kind of gate using only NOR gates.

Circuit diagram of a NOR gate using transistors and resistors photo

Circuit diagram of a NOR gate using transistors and resistors

The output will be high only if both transistors are at cut-off state. But if any one of the inputs goes high it will bias respective transistor which will take output at low state that is in line with NOR gate truth table.

NOR gate IC

Various NOR gate ICs are available in market, 7402 is one of them. It’s a 14 pin two input NOR gate IC where four NOR gates are packed. It works on 5 volt power supply.

7402 IC configuration photo

7402 IC configuration

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