What are logic gates?
All complex digital electronic circuits, such as microprocessors, random access memory (RAM), microcontrollers, and others, are composed of a large number of complex internal logical circuits. These, in turn, are made up of smaller and less complex logical circuits, until we finally reach the smallest logical electronic circuits that have simple functions and are called logic gates.
Logic gates are electronic circuits with simple functions. Their internal structure consists of a set of interconnected transistors. The way these transistors are connected determines the function of the logic gate. Logic gates are electronic circuits that have inputs ranging from a single input to, in some cases, up to sixteen inputs, and they have a single output.
The NOT logic gate
The "NOT" logic gate is the only one with a single input among all the logic gates we will learn about, and it has another name, which is the inverter. We assign conventional symbols to logic gates, agreed upon internationally, to facilitate the drawing of electronic circuits.
The symbol for the NOT logic gate
In the image, we have the symbol for the Inverter or “NOT” logic gate. As we can see in the image, the “NOT” logic gate has a single input labeled with the letter A and a single output labeled with the letter F. These letters are names for the inputs and outputs that will help us in the future to write logical functions algebraically.
The function of the NOT logic gate
The function of the logic gate is simple: when a logical state 0 is input, the gate inverts it to a logical state 1 and provides the result at the output F.
The truth table of the NOT logic gate
The function of the NOT logic gate in algebraic form
As we mentioned earlier, we can express logical functions in the form of algebraic equations by assigning each logical function a mathematical symbol. For the “NOT” logic gate, we express it with a bar above the letter representing the name of the input. So, we write:
The OR logic gate
The second logic gate is “OR,” and its function is that it produces a logical state 0 at the output in only one case, which is when all inputs are 0. Otherwise, it produces a logical state 1 at the output.
The symbol for the OR logic gate
The truth table of the OR logic gate
Algebraic equation of the OR logic gate
We express the function of the OR gate algebraically using the mathematical symbol +, but this does not represent the operation of addition. Instead, it symbolizes parallelism or choice, as it is a logical symbol rather than a mathematical one.
To further clarify the idea, it is possible to draw an electric circuit equivalent to the OR logic gate, where switches **A** and **B** represent the inputs, and the bulb **F** represents the output. Placing the two switches in parallel performs exactly the same function as the OR logic gate.
When the switch is open, it is in logic state 0, and when it is closed, it is in logic state 1. When the light bulb is off, it is in logic state 0, whereas when it is on, it is in logic state 1. "OR" logic gates exist with more than two inputs, and it is possible to combine two-input logic gates to create multi-input logic gates as needed.
The AND logic gate
symbole de la porte logique ET
Function of the AND logic gate
The AND logic gate gives a logic state 1 at the output in only one case: when all the inputs are at logic state 1.
Truth table of the AND logic gate
Algebraic expression of the AND logic gate
To express the function of the AND logic gate algebraically, we use the dot, the symbol for multiplication in mathematics, because the function of the AND gate in electronics is the same as that of multiplication in mathematics.
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