The OR gate is a digital logic gate that implements logical disjunction – it behaves according to the truth table to the right. A HIGH output (1) results if one or both the inputs to the gate are HIGH (1). If neither input is HIGH, a LOW output (0) results.
The expression X = A + B reads as “X equals A OR B”.
The + sign stands for the OR operation, not for ordinary addition.
The OR operation produces a result of 1 when any of the input variables is 1.
The OR operation produces a result of 0 only when all the input variables are 0.
An example of three input OR gate and its truth table is as follows:
With the OR operation, 1 + 1 = 1, 1+ 1 + 1 = 1 and so on.
A common example of an OR gate circuit is a security system.
Assume that a room is protected by a system that watches three inputs:
a door open sensor, a glass break sensor, and a motion sensor. If none of these sensors detects a break-in condition, i.e., they all send logic 0 to the OR gate, the alarm is off (logic 0). If any of the sensors detects a break-in, it will send logic 1 to the OR gate which in turn will output logic 1 indicating an alarm condition.
It doesn’t matter what the other sensors are reading, if any sensor sends logic 1 to the gate, the alarm should be going off. Another way to describe the operation of this circuit might be to say, “The alarm goes off if the door opens or the glass breaks or motion is detected.” Once again, the use of the word “or” suggests that this circuit should be implemented with an OR gate.