What is Logic OR Gate & its Application

The OR gate is a digital logic gate that implements logical disjunction. The OR gate returns true if either or both of its inputs are true; otherwise, it returns false. The input and output states are normally represented by different voltage levels.

The output, Q of a “Logic OR Gate” only returns “LOW” again when ALL of its inputs are at a logic level “0”. In other words for a logic OR gate, any “HIGH” input will give a “HIGH”, logic level “1” output.

The logic or Boolean expression given for a digital logic OR gate is that for Logical Addition which is denoted by a plus sign, ( + ) giving us the Boolean expression of A+B = Q.

Digital Logic “OR” Gate Types

The 2-input Logic OR Gate

The 3-input Logic OR Gate

the OR function can have any number of individual inputs. However, commercial available OR gates are available in 2, 3, or 4 inputs types.

Multi-input OR Gate

The Boolean Expression for this 6-input OR gate will therefore be:

Q = (A+B)+(C+D)+(E+F)

2-input Transistor OR Gate

A simple 2-input inclusive OR gate can be constructed using RTL Resistor-transistor switches connected together as shown below with the inputs connected directly to the transistor bases. Either transistor must be saturated “on” for output at Q.


  • Used in Alarm Systems and Intrusion Detection Equipment
  • Used in Industrial Plants for some protective measures

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