The OR gate has two or more inputs and a single output. Ia performs logical addition. The OR gate produces aHIGH outputs when any one of the inputs is HIGH.  The output is  LOW only when all of the inputs are LOW The logic gate is a digital circuit with one or more inputs, with only one outputs. The gate can be constructed from diodes, transistors, and resistors. The logic gate is designed to perform a particular logical operation or function. Any logic gate operation can be easily understood with the help of truth table. A voltage in a digital circuit is said to be at the logic level 0 or logic level 1, depending on its actual numeric value. Sometimes logic 0 and logic 1 may be represented as,

 Logic  0 Logic 1 False True Off On Low High No Yes Open switch Close switch

Operation:
CASE 1. A = 0 and B = 0 then output is 0 Low.
CASE 2. A= 0 and B =1 then Y = 1.
CASE 3. A =1 and B =0 then Y = 1,
Case 4.  A =1 and B =1 then output Y =1 means HIGH means 5v Dc power supply. Input A Input B O/P=Y 0 0 0 0 1 1 1 0 1 1 1 1

Boolean Logic Expression:  The (TTL) Transistor Transistor Logic variables are A, B and the output variable is Y, the logic expression for and OR gate is Y =A+B.  For more than two input variables,OR expression is Y = A+B+C  Logic NOR
For inputs Y =A+B+C. The common application of an OR gate is an alarm system with instruction detection. IC 7432 is a 2 inputs and quad gates.