OR Gate

This article will explain the concept of OR gate operation in digital electronics along with its truth table, logic symbol, switching circuit diagram, etc. The OR gate is one of the main building blocks of Digital Logic Circuits.

Here, we will cover OR Gate using transistors with the working of the circuit and its applications, advantages, and disadvantages, accompanied by solved examples and answers to frequently asked questions.

What is OR Gate?

OR gate is the type of Logic Gate. Let’s suppose in binary numbers 0 and 1, 0 is low and 1 is high. So when at least one of the provided inputs is high then the output is high otherwise it’s low.

Operation of NOT Gate

1. If inputs are A= 0, B = 1, and C = 0 here Input B is high so the overall output will be high.

2. If Input B = 0 then all of the inputs are low so the output will also be low means 0.

Inputs and outputs in OR Gate –

1. Input: The input to an OR gate is the binary signal provided by the user to its input terminals on which it will perform logical operations and provide output.
2. Output: The output of an OR gate is the result that came after a logical operation performed on its inputs. It can be high means (1) or low (0). A single output is provided after an operation.

There are some simple OR gates that take only one input but there are also OR gates that take more than one input they are known as Multi-Input OR Gate.

In this article, we will focus on the working, functions, and truth table of OR gate.

Types of Logic OR Gates

There are two types of Logic Gates :

2-Input Logic Gate

It takes only two inputs and provides single output. It gives a high output (1) if at least one of the binary inputs is high (1)

X= (A+B)

Multi-Input or 3 Input OR Gate

It can take more than 2 inputs. Its working is the as same i.e.2 input OR gate i.e. takes multiple inputs and provides at least high output if at least one input is high (1).

Symbol of OR Gate

The logic symbol of OR gate is shown in the following figure. In this figure, the variables A and B represent the input lines and A + B is the output of the OR gate.

2 Input OR Gate

Truth Table of OR Gate

The truth table for the OR gate is given above. It shows the relationship between input applied and output generated where output followed by the addition of inputs given.

3 Input OR Gate

OR Gate Circuit Diagram using Transistors

Let’s construct a simple OR gate circuit using transistors to demonstrate the required components and the circuit’s functioning.

OR Gate using Transistor

Components Needed

Resistance, NPN transistor, power supply in voltage, wires, switches

Working of the Circuit

• When both switches (inputs A and B) are in the OFF state (LOW), there is no power supply to the bases of the transistors.
• As a result, both the base-to-emitter and base-to-collector junctions of the transistors have a voltage lower than the threshold voltage of a diode.
• This causes both transistors to go into their cutoff state, acting like open switches and blocking current flow.
• With no current flowing through the transistors, the output remains in a low voltage state (LOW), turning off the LED connected to the output.

Applications of OR Gate

1. Automatic Watering System: OR gate can be used to create a simple watering system. If any of the sensors demand water (high) then the output will become high means the watering system will turn on.
2. Home Automation: OR gates can be part of home automation systems. They can be used to combine signals from various sensors and switches to trigger specific actions, such as turning on the heating or cooling system when any room temperature exceeds or declines a specific number.
3. Emergency Lighting: In the event of a power outage, OR gates can be used to combine signals from multiple emergency switches if any of the switches is high means turned on then emergency lighting will turn on.
4. Smart Doorbell: OR gates can be used in smart doorbell systems to combine signals from various sources (e.g., doorbell button, motion sensor, camera) to activate the doorbell chime or trigger notifications to connected devices.

Advantages of OR Gate

1. Simplicity: OR gates are simple and easy-to-use logic gates. Their design is also simple and understandable.
2. Flexibility: OR gate works with also works with multiple inputs which increases their applications and digital use so it’s very flexible to use.
3. Logical Summation: OR Gate works on logical summation. This means it outputs a high (1) when at least one input is high (1), which is useful for combining signals or determining the occurrence of events.
4. High Importance: OR gates are said to be fundamental building blocks in digital electronics. They act as a base for complex ic(integrated circuits). Without OR Gate making circuits is not possible.
5. Implementation of Logical Functions: Creating Functionality of other logic gates such as NAND, NOR, and XOR gates, is possible using OR gate.

Disadvantages of OR Gate

1. Loss of Information: By OR Gate functionality we can only understand that one of the provided many inputs is high but we cannot know which of the input is high so it doesn’t provide complete information.
2. Lack of Precision: In certain applications that require precise decision-making based on individual input values, the OR gate might not be suitable as it only produces a binary output (1 or 0).
3. Non-Invertible: The OR gate is not invertible; its output is always high (1) when at least one input is high (1). It cannot produce a low output (0) for any specific input combination.
4. Increased Power Consumption: Since the OR gate requires more transistors compared to other basic gates like NOT and AND gates, it can consume more power in complex circuits.
5. Signal Interference: In multi-input OR gates, a high signal on one input can interfere with the desired output if another input is also high. This can lead to unexpected results in certain circuit configurations.

OR Gate Examples

Example 1: Suppose we have a simple alarm system with two sensors, A and B, installed on windows. The alarm should trigger if any of the sensors detect an intrusion.

Input A = 0 (No intrusion detected on window A) Input B = 1 (Intrusion detected on window B)

Using the truth table for the OR gate:

Input A	Input B	Output
0	0	0
0	1	1
1	0	1
1	1	1

Since Input A is 0 (no intrusion) and Input B is 1 (intrusion detected), the OR gate will produce an output of 1 (high), triggering the alarm as at least one sensor detected an intrusion.

Example 2: Let’s consider an automatic watering system for plants. A system is designed which have two sensors to sense moisture in the soil (A and B) placed in two different areas. The water pump should activate when any of the sensors indicate the need for watering.

Input A = 0 (Soil moisture level is sufficient in area A) Input B = 1 (Soil moisture level is low in area B)

Using the truth table for the OR gate:

Input A	Input B	Output
0	0	0
0	1	1
1	0	1
1	1	1

Since Input A is 0 (sufficient moisture) and Input B is 1 (low moisture), the OR gate will produce an output of 1 (high), activating the water pump as at least one sensor indicates the need for watering.

Example 3: Suppose we have a smart doorbell system with a doorbell button, a motion sensor, and a camera. The doorbell chime should activate when either the doorbell button is pressed or motion is detected by the sensor or the camera detects movement.

Input Doorbell Button = 1 (Button is pressed) Input Motion Sensor = 0 (No motion detected) Input Camera = 1 (Camera detects movement)

Using the truth table for the OR gate:

Input Doorbell Button	Input Motion Sensor	Input Camera	Output
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

Since the Doorbell Button is pressed (Input Doorbell Button = 1), the OR gate will produce an output of 1 (high), activating the doorbell chime. The motion sensor and camera inputs also contribute to the output, but the doorbell button takes precedence in this example.

OR Gate – FAQs

Can an OR gate have more than two inputs?

Yes, an OR gate can have more than two inputs, allowing for multiple inputs. It provides a logical OR operation for all inputs, resulting in a 1 output if at least one input is 1.

Differentiate between the AND gate and the OR gate?

There is a basic difference between logical operations of AND & OR Gate AND gate produces high output when both inputs are high means 1, whereas the OR gate produces output as binary 1 when either of the inputs is 1 or it will give 0 as output

How to implement an OR gate circuit using electrical components?

An OR gate circuit can be implemented using transistors, resistances, and other components, a simple example is shown above.

How to create a NOT gate using an OR gate?

Yes, it is possible to create a NOT gate using an OR gate. For creating a NOT gate first connect one of the input signals of the OR gate to a constant voltage level (either 0 or 1) and connect the other input to the signal that is to be inverted. When the input signal is 0, the OR gate produces a 0 output, and when the input is 1, the output becomes 0. In this way, a NOT operation can be achieved using an OR gate.