In this article, we will learn about logic OR gate with its basic definition, symbol, logical expression, truth table, circuit diagram, and implementation. So, let’s get started with the basics of logic OR gate.
What is an OR Gate?
In digital electronics, the OR gate is a type of basic logic gate that produces a high or logic 1 output when at least one of its inputs is high or logic 1. It produces a low output or logic 0 when all of its inputs are low or logic 0.
The logic symbols of two-input and three-input OR gates are depicted in the following figure.
Logical Expression of OR Gate
The operation of the OR gate is represented by the ‘+’ symbol.
The logical expression of a two-input OR gate is given by,
Where A and B are the input variables, and Y is the output variable of the OR gate.
The logical expression of a three-input OR gate is given by,
Where, A, B, and C are the three input variables and Y is the output variable of the logic OR gate.
Operation of OR Gate
The operation of a two-input OR gate for different input combinations is described as follows:
- When A = 0 and B = 0, the output Y of the OR gate is 0 as per the logical expression given above.
- When A = 0 and B = 1, the output Y of the OR gate is 1.
- When A = 1 and B = 0, the output Y of the OR gate is 1.
- When A = 1 and B = 1, the output Y of the OR gate is 1.
The operation of a three-input OR gate for different possible input combinations is described below:
- When A = 0, B = 0, and C = 0, the output Y of the OR gate is 0 according to the logical expression mentioned above.
- When A = 0, B = 0, and C = 1, the output Y of the OR gate is 1.
- When A = 0, B = 1, and C = 0, the output Y of the OR gate is 1.
- When A = 0, B = 1, and C = 1, the output Y of the OR gate is 1.
- When A = 1, B = 0, and C = 0, the output Y of the OR gate is 1.
- When A = 1, B = 0, and C = 1, the output Y of the OR gate is 1.
- When A = 1, B = 1, and C = 0, the output Y of the OR gate is 1.
- When A = 1, B = 1, and C = 1, the output Y of the OR gate is 1.
Truth Table of OR Gate
The table showing the relationship between inputs and output of a logic OR gate is referred to as its truth table.
2-input Logic OR Gate
The truth table of a two-input OR gate is given below.
 Inputs | Output | |
| A | B | Y |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 1 |
3-input Logic OR Gate
The truth table of a three-input OR gate is given below:
 Inputs | Output | ||
| A | B | C | Y |
| 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 |
Multi-Input OR Gate
The logic OR gate can take any number of inputs. Although, in practice, the logic OR gate is mainly implemented for having two-inputs, three-inputs, and four-inputs.
However, we can implement a higher-order logic OR gate using multiple lower-order logic OR gates. For example, the following figure specifies a logic OR gate of 6 inputs using four 2-input OR gates.
The logical expression of the above circuit of 6-input OR gate is as,
Implementation of Two-Input OR Gate in Resistor-Transistor Logic
The following figure depicts the realization of a two-input OR gate in RTL (Resistor Transistor Logic).
- When input A and input B is zero, transistor T1 and T2 will remain in the off state and no collector current flow through the transistor, and emitter current IE is zero. Therefore, the drop across resistance R2 ( IER2)is zero and Y=0 Volt, thus we get output 0 when A=0 and B=0
- When A=0 and B=1, the transistor T2 conducts and IER2=5 Volts, Y=1
- When A=1 and B=0, the transistor T1 conducts and Y=1
- When A=1 and B=1, the transistor T1 and T2 conducts and Y=1
Electrical Equivalent Circuit of OR Gate
The electrical equivalent circuit of a two-input OR gate is shown in the following figure.
In this circuit, when both switches, i.e. A and B are open, the bulb will not glow. If any of the switches is closed or both switches are closed, the bulb will glow.
OR Gate Integrated Circuits
The commonly available OR gates in the form of integrated circuits are as follows:
74LS32 Quad 2-Input (TTL OR Gate)
The pinout diagram of 74LS32 is given below.
CD4071 Quad 2-Input (CMOS OR Gate)
The pinout diagram of CD4071 is given below.
CD4075 Triple 3-Input (CMOS OR Gate)
The pinout diagram of CD4075 is given below.
CD4072 Dual 4-Input (CMOS OR Gate)
The pinout diagram of CD4072 is given below.
Hence, this is all about logic OR gate in digital electronics.