A Logic OR Gate or InclusiveOR gate is a type of digital logic gate that has an output which is normally at logic level “0” and only goes “HIGH” to a logic level “1” when one or more of its inputs are at logic level “1”.
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.
Then we can define the operation of a 2input logic OR gate as being:
“If either A or B is true, then Q is true”
A simple 2input logic OR gate can be constructed using RTL Resistortransistor switches connected together as shown below with the inputs connected directly to the transistor bases. Either transistor must be saturated “ON” for an output at Q.
Logic OR Gates are available using digital circuits to produce the desired logical function and is given a symbol whose shape represents the logical operation of the OR gate.
Symbol  Truth Table  
2input OR Gate

B  A  Q 
0  0  0  
0  1  1  
1  0  1  
1  1  1  
Boolean Expression Q = A+B  Read as A OR B gives Q 
Symbol  Truth Table  
3input OR Gate

C  B  A  Q 
0  0  0  0  
0  0  1  1  
0  1  0  1  
0  1  1  1  
1  0  0  1  
1  0  1  1  
1  1  0  1  
1  1  1  1  
Boolean Expression Q = A+B+C  Read as A OR B OR C gives Q 
Like the AND 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. Additional inputs will require gates to be cascaded together for example.
The Boolean Expression for this 6input OR gate will therefore be: Q = (A+B)+(C+D)+(E+F)
If the number of inputs required is an odd number of inputs any “unused” inputs can be held LOW by connecting them directly to ground using suitable “Pulldown” resistors.
Commonly available digital logic OR gate IC’s include:
In the next tutorial about Digital Logic Gates, we will look at the digital logic NOT Gate function as used in both TTL and CMOS logic circuits as well as its Boolean Algebra definition and truth table.
please help me….. on how to draw the truth table on this logic expression Y=bar A+B bar …
Very helpful
Help me Help Help me
How to create 4 bit OR Gate circuit ?
Please make a project file as a pdf of physics on topic or gate
how can i design a logic circuit using an or gate
In case of we consider
three inputs AND gate what is express would be
like example 2 input as we know
y=a.b (two input and gate)
y=???? (three input how it will be)
y = a.b.c
how the or gate true at a=1,b=1
a+b=2 then 2=10
then carry =1,sum=0
which one we consider for or gate sum or carry
please say answer
As it same logic notification only
two inputs for OR gate in series combination
I’m not sure I see why you would need a pulldown resistor, if you have inputs you don’t need to use. Why couldn’t you just connect directly to ground? Is the idea that there might be positive voltage flowing from the input, and you want to limit the current draining to ground?
if you do that then the route directly to the ground has clearly less resistance than the other route in your logic circuit that means it will have a short circuit if you do not use a pull down resistor
ok
Try to see whst my wife is really doing