The OR gate is a logic gate that takes two or more inputs and produces an output that is high (ON or 1) if any of the inputs are high.
Computers are a series of ones and zeros. Ones and zeros are just another way to say a ON/OFF, or a high/low voltage. Computers combine these signals by using gates (such as this OR gate) to do interesting things.
A quick example of the OR Gate might be an alarm system. When any one or more doors are opened, then the output of the OR gate is high (1) and the alarm rings. The OR Gate ensures you’ll always know whether one or multiple doors are open.
The OR Operand
The logic or Boolean expression of an OR Gate uses the symbol "+".
In the context of Boolean logic and gates, the "+" symbol isn't the same as traditional addition when summing integers. But it is often described as a logical addition because it combines the inputs in a way that is similar to addition. (It's actually a one bit adder, without a carry bit).
When talking about digital logic, it's often helpful to create a truth table, which shows all the possible combinations of inputs and what each output would be.
*Fyi, you’ll see the following terms used interchangeably
1 | HIGH | ON
O | LOW | OFF
In the truth table ON = 1 and OFF = 0. Let’s show you what that looks like.
Truth Table for a two input OR Gate
Here we exhaust all the input combinations for a two input OR gate:
As you can see from the truth table, if any of the inputs are true (high), the output will also be true (high). Only when all the inputs are false (low) will the output be false (low).
How does an OR Gate work?
Or gates are built out of switches, this could be transistors, mosfets, relays, or any kind of switch. You just need to tie up the output line to turn on when either of the switches (or both) are turned on, and you've created an OR gate. Here's an example schematic using transistors:
Here are some other real world applications of the OR Gate.
Applications of the OR Gate
Here’s how we applied an OR Gate to our book, Computer Engineering for Babies.
*OR Gate image from Dennis Wheeler's YouTube review of CE for Babies
OR gates have many applications in digital circuits, including:
- Data processing: OR gates are used to combine multiple input signals to form a single output signal. For example, in a computer's memory unit, an OR gate can be used to combine the read and write signals to produce a control signal that determines the operation of the memory unit.
- Logic circuits: OR gates are used in combination with other logic gates to form complex logic circuits that can perform arithmetic, comparison, and other operations.
- Control systems: OR gates are used in control systems to implement logical decision-making. For example, in an automobile, an OR gate can be used to determine if either the brake pedal or the parking brake is engaged, which will turn on the brake warning light.
Within each of those applications are different types of OR Gates, here are a few.
Different types of OR Gates
There are several different types of OR gates that are used in electronic circuits and systems:
- 2, 3, 4, 8-input OR gate
- Open collector OR gate: This type of OR gate has an open collector output that can be used to interface with other devices or circuits. The output is either connected to ground or left open, depending on the state of the inputs.
- Tri-state OR gate: This type of OR gate has three states: high, low, and high impedance. The output is either high or low, or disconnected from the circuit altogether.
- Schmitt trigger OR gate: This type of OR gate has hysteresis, which means that the input threshold levels are different for rising and falling edges. This can help to reduce signal noise and improve circuit performance.
- Overall, the choice of OR gate type depends on the specific application and the number of inputs required. Open collector and tri-state OR gates are particularly useful for interfacing with other devices or circuits, while Schmitt trigger OR gates are useful for reducing signal noise.
Let’s get into the advantages and disadvantages of using OR Gates.
Advantages of using OR Gates
OR gates offer several advantages in electronic circuits and systems:
- Versatility: OR gates can be used in a wide range of applications, including data processing, control systems, and logic circuits.
- Flexibility: OR gates can have any number of inputs, making them very versatile in terms of system design.
- Cost-effective: OR gates are relatively inexpensive and widely available, making them a cost-effective solution for many applications.
- Simplicity: OR gates are straightforward to design and implement, making them an excellent choice for simple circuits.
- Speed: OR gates are fast and efficient, making them suitable for use in high-speed applications.
- Reliability: OR gates are typically very reliable, provided that they are used within their specified voltage and current ratings.
The use of OR gates can simplify circuit design, reduce costs, and improve system performance and reliability.
Disadvantages of using OR Gates
While OR gates offer several advantages, they also have some disadvantages that should be considered:
- Limited functionality: OR gates can only perform the logical OR operation, which may limit their usefulness in some applications.
- Output voltage limitations: The output voltage of an OR gate may be limited by the input voltage levels, which can be a problem in certain applications.
- Output signal noise: In some cases, OR gates may produce output signal noise or glitches, which can be a problem in sensitive circuits.
- Power consumption: OR gates can consume a significant amount of power, particularly when used in large numbers or in high-speed applications.
- Heat dissipation: OR gates can generate heat when operating, which can be a problem in high-temperature environments or when used in circuits with high power dissipation.
There are many ways to use a Gate in Computer Science and relating fields, here's how the OR Gate compares to the others.
Comparing OR with Other Logic Gates
OR gates are just one type of logic gate used in electronic circuits and systems. Here's a brief comparison of OR gates with other common logic gates:
- AND gates: AND gates produce a true output only if all inputs are true, while OR gates produce a true output if any of the inputs are true. In other words, AND gates are the opposite of OR gates. AND gates are often used in combination with OR gates in more complex circuits.
- NOT Gates: NOT gates produce a true output if the input is false, and vice versa. NOT gates are often used in combination with other gates to produce more complex logic functions.
- XOR gates: XOR gates produce a true output only if one, but not both, of the inputs is true. In other words, XOR gates are a combination of AND and NOT gates. XOR gates are often used in data processing and communications applications.
- NAND gates: NAND gates produce a false output only if all inputs are true. NAND gates are the opposite of AND gates and can be used to simplify complex logic functions.
- NOR gates: NOR gates produce a true output only if all inputs are false. NOR gates are the opposite of OR gates and can also be used to simplify complex logic functions.
In summary, different logic gates have different functions and can be used in combination to create more complex logic functions. We wrote an article that directly compares the AND OR NOT Gates if you'd like to check it out.
The choice of gate(s) to use in a circuit depends on the specific application and the desired function.
The OR Gate in Digital Logic | Final thoughts
In summary, the OR gate is a fundamental logic gate used in digital circuits to implement logical decision-making. It takes two or more inputs and produces an output that is true if any of the inputs are true. The OR gate is essential in many applications, including data processing, logic circuits, and control systems.
Did you notice the book images throughout this post? That’s our book, Computer Engineering for Babies, check it out!
OR Gate FAQs
- What happens if all the inputs of an OR gate are low?
If all the inputs of an OR gate are low, the output will also be low.
- Can an OR gate have more than two inputs?
Yes, an OR gate can have any number of inputs.
- What is the difference between an OR gate and an XOR gate?
An OR gate produces a true output if any of the inputs are true, while an XOR gate produces a true output if only one of the inputs is true.
- How do you represent an OR gate in a circuit diagram?
An OR gate is represented by the symbol "+", which represents the logical OR operation.
- What are some common applications of OR gates?
OR gates are used in many applications, including data processing, logic circuits, and control systems.