The OR gate differs from the AND gate in that only ONE input has to be HIGH to produce a HIGH output. An easy way to remember the OR gate is that any HIGH input will yield a HIGH output.
The figure below shows the standard symbol for the OR gate. The number of inputs can vary according to the needs of the designer.
OR Gate Equivalent Circuit
The OR gate may also be represented by a simple circuit as shown in the figure below. In the OR gate, two switches are placed in parallel. If either or both of the switches are closed (view 1), the lamp will light. The only time the lamp will not be lit is when both switches are open (view 2).
Let’s assume we are applying two variables, A and B, to the inputs of an OR gate. For the circuit to produce a HIGH output, either variable A, variable B, or both must be HIGH. The Boolean expression for this operation is f = A+B and is spoken "f equals A OR B." The plus sign indicates the OR function and should not be confused with addition.
OR Gate Operation
Look at the figure below. At time t0, both A and B are LOW and f is LOW. At t1, B goes HIGH producing a HIGH output. At t2 when both inputs go LOW, f goes LOW. When A goes HIGH at t3, f also goes HIGH and remains HIGH until both inputs are again LOW. At t5, both A and B go HIGH causing f to go HIGH.
Using the inputs A and B, let’s construct a Truth Table for the OR gate. You can see from the discussion of the figure above that there are four combinations of inputs. List each of these combinations of inputs and the respective outputs and you have the Truth Table for the OR gate.
When writing or stating the Boolean expression for an OR gate with more than two inputs, simply place the OR sign (+) between each input and read or state the sign as OR. For example, the Boolean expression for an OR gate with the inputs of A, B, C, and D would be:
f = A+B+C+D
This expression is spoken "f equals A OR B OR C OR D."