In digital electronics, the OR Gate is one of the most fundamental logic gates. It produces a HIGH (1) output when any of its inputs are HIGH, and a LOW (0) output only when all inputs are LOW. The operation of an OR Gate is also represented using a standard OR Gate symbol, which helps visualize how inputs combine to produce the output in circuit diagrams.
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.
OR Gate Symbol
The logic symbols of two-input and three-input OR gates are depicted in the following figure.
OR Gate symbol helps quickly identify how inputs combine to produce an output in digital circuits.
Logical Expression of OR Gate
The operation of the OR gate is represented by the ‘+’ symbol.
2-Input OR gate
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.
3-Input 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
2-Input 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.
3-Input OR gate
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 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 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.
Applications of OR Gate
- Smart Irrigation Systems: Logic OR gates can be used to automate irrigation. If any sensor detects dryness in the soil, the output activates the watering system.
- Home Automation Controls: OR gates combine signals from multiple sensors or switches. For instance, if any room’s motion sensor is triggered, lights or HVAC systems can be turned on automatically.
- Emergency Systems: In buildings or facilities, OR gates can control emergency lighting or alarms. If any connected switch or sensor detects a fault, the system activates the necessary response.
- Industrial Monitoring: OR gates are employed in machinery to monitor multiple conditions. If any safety parameter is violated, the machine can automatically stop or signal an alert.
Advantages of OR Gate
- Simple Design: OR gates have a straightforward design, making them easy to understand and implement in circuits.
- Versatile Usage: They can handle multiple inputs, which makes them suitable for a wide variety of digital applications.
- Foundational Component: OR gates serve as the building blocks of complex integrated circuits, enabling the creation of other logic functions like NAND, NOR, and XOR.
- Efficient Logic Implementation: They allow designers to implement decision-making processes effectively, checking if at least one condition is met.
Disadvantages of OR Gate
- Limited Output Detail: OR gates only indicate if any input is high; they do not reveal which specific input triggered the output.
- Not Suitable for Precision Tasks: Applications that require exact input tracking may not benefit from OR gates, since the output is binary.
- Non-Invertible Nature: The output remains high if any input is high, which may limit certain logic designs that require inversion.
- Higher Power in Complex Circuits: Compared to simpler gates like NOT or AND, using OR gates in complex networks can increase power consumption.
Conclusion
OR gates are essential components in digital electronics. They simplify decision-making by providing a high output when any input condition is met. From smart home systems to industrial monitoring and emergency setups, OR gates are fundamental in designing both simple and complex digital circuits. Understanding their operation, symbol, and truth table is crucial for anyone working in electronics or digital systems design.
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