Introduction to Logic Gates

Introduction to Logic Gates
Introduction to Logic Gates

If you’re curious about computers’ fundamentals, logic gates are great. Understanding logic gates is simple, despite their appearance. This will clarify how digital logic builds complex circuits. This beginner’s guide covers active high vs. active low signals, TTL logic levels, and diode and transistor logic gates. We’ll explain these topics without electrical engineering lingo. You’ll learn enough about logic gates to comprehend how they work and how they’re employed in simple computers and complex microprocessors. We’re excited to explain logic gates!

An Introduction to Logic Gates

In digital devices and integrated circuits, logic gates keep them running. They form the foundation of Boolean thinking and electronics programs.

What is a Logic Gate?

Electronic logic gates create sensible decisions using one or more input signals. It receives Boolean inputs (1 or 0), meaning “true” or “false,” and outputs them. Gate type and sources determine output.

An Introduction to Logic Gates
An Introduction to Logic Gates

Active High vs Active Low

Logic gates can have active high and active low inputs and outputs. A logic 1 indicates high voltage when active high is on. Logic 1 indicates low voltage. Check the logic gates’ specs to see if the inputs and outputs are active high or low.

TTL Logic Levels

Transistor-transistor logic (TTL) logic gates are popular. TTL logic gates employ standardized voltages:

  • Lower logic 0: 0.8 V to 0 V
  • 1 (high): 2–5 VFrom 1.2 to 1.6 V

Gate output varies from 0 to 1 at the threshold voltage. Anything below the threshold is 0, and anything over it is 1.

Simple Logic Gates

The first three logic gates are:

  • The AND gate only outputs 1 if all inputs are 1. To determine if there are several messages.
  • If any inputs are 1, the OR gate outputs 1. Used to check for two indicators.
  • NOT gate: Flips the input, turning 1 into 0 and 0 into 1. This reverses a signal.
  • Logic gates and transistor-based integrated circuits are built on these fundamental gates. Combine logic gates to create circuits that add, subtract, and store data.

Understanding Active High and Active Low Signals

In digital logic devices, signals can be “active high” or “active low”. The voltage level of a binary “on” or logical “1” state.A voltage between 0V and 5V usually represents “1” and 3-5V means “0” during a high signal. You’ll see this mindset often.

For an active low signal, lower voltage implies “1” and higher voltage means “0”. Active low indicators can indicate “not” working or turn a device on or off.Imagine an LED that lights on when a live high signal arrives. To turn on the LED, add 3-5V. To turn it off, use 0–0.8V.

Live low LEDs work the other way. You power the LED less to turn it on. Increase its power to turn it off.Many digital logic devices and circuits can handle active high and low signals. This allows designers additional circuit and data management options.AND, OR, and NOT gates can be wired as active high or active low. Choose active high or active low to flip that gate’s thinking.

To work with digital logic devices, you must understand active high and active low signals. Pay attention to IC and part descriptions to see what input or output data they can handle. If you mess up, some circuits will behave strangely!

TTL Logic Levels Explained

You must understand logic levels to comprehend logic gates. TTL, or “Transistor-Transistor Logic,” is an integrated circuit that uses transistors to make logic gates. Message voltages in TTL circuits are termed logic levels.

TTL logic values indicate signal on/off. What you can do:

  1. An active or “on” state has a greater voltage, usually 5V. Devices with 0V or less are “off”
  2. The opposite of High is Active Low. 0V is active, 5V idle.
  3. Many TTL logic gates use Active High logic levels. Active 5V inputs are needed for a basic AND gate to output 5V. If either input is 0V, the outcome is 0V.

Consider Active Low instead for these reasons:

  • Compatible with other Active Low devices.
  • Noise immunity: Active Low is less susceptible to accidental gate activation.
  • Pulling a signal low requires less power than driving it high.
  • Below are simple TTL logic gates:

AND gate outputs are only active when all inputs are active.
OR gate output is on if at least one source is on.
NOT gate: Flips inputs so active inputs turn off active outputs and vice versa.
NAND gates reverse AND gate results.
OR gate output is reversed by NOR gate.
These gates can be combined to create more complex logic functions. TTL integrated circuits with numerous gates on a chip simplify ideas. To develop digital logic, understand logic levels and gates.

Building Simple Logic Gates With Diodes

Start with the simplest logic gates to learn about them. Diode logic gates are easy to design and demonstrate gate operation.

Electronic diodes allow electricity to flow one way. Diodes can be arranged to create gates that send high or low signals based on the input signals.

Place two diodes face-to-face. Making an AND gate. Two high input signals allow current to flow through both diodes and raise the output. The output is low if either input is low because current can’t flow.

An OR gate has two diodes in a row facing opposing directions. Current can flow through at least one diode, hence both signals must be high for a high output. Only when both sources are low will output be low.

NOT gates simply need one diode to invert incoming signals. When input is high, the diode cuts electricity, lowering output. Current flows through the diode when input is low, sending a high signal.

These diode logic gates demonstrate digital logic at its simplest. Multiple gates can be connected to create complex circuits that modify data. Early computers employed diode logic before transistors. However, all digital logic systems and integrated circuits use the same principles.

To build increasingly complex circuits and use integrated chips, you must understand how logic gate input and output signals interact. Diode logic gates help visualize these essential pieces.

Transistor-Based Logic Gates

Transistor-based logic gates convert binary data. Despite being more sophisticated than diode logic gates, they perform logic tasks better. TTL switches are usually utilized.

TTL gates perform Boolean logic with BJTs. They are faster and better for complex digital circuits than diode logic gates. An TTL gates use logic level restrictions to distinguish “1” (high) from “0” (low) signals.

TTL Logic Levels

TTL logic levels determine the voltage range for logic “1” or “high” signals (H) and “0” or “low” signals (L). A standard TTL gate receives an input signal.

H (high) at 2–5 volts
L (low) at 0–0.8 volts.
Avoid the “undefined” area between 0.8 and 2 volts.
Most TTL ICs derive their power from 5V, so high signals are near 5V and low signals are near 0V. TTL logic levels enable TTL gates to accurately interpret signals and select outputs.

Common TTL Gates

Most TTL gates look like this:

  • AND gates only output H if all inputs are H.
  • OR gates output H if any inputs are H.
  • NOT gate: Flips input signal. L becomes H and H into L.
  • NAND gates reverse AND gate results.

OR gate output is reversed by NOR gate.
Combining these fundamental gates creates more complex digital logic processes and functions. TTL technology dominated digital design for years and is still used in some contexts, but CMOS is increasingly common.


Finished! You now comprehend logic gates and levels. Now that you understand voltages, you can distinguish active high and active low indications. We discussed the common TTL logic levels that convert those signals into digital circuit ones and zeros. Diodes and transistors were used to produce logic gates, the building blocks of all digital systems. Don’t worry—you’ll be learning simple high- and low-sign words. Experiment with NOT, AND, and OR gates. You’ll soon make more sophisticated digital circuits!

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