Basic Logic Gates

Basic Logic Gates

A logic gate is an electronic circuit/device which makes the logical decisions. To arrive at this decisions, the most common logic gates used are OR, AND, and NOT.

Logic gates have one or more inputs and only one output. The output is active only for certain input combinations. Logic gates are the building blocks of any digital circuit. Logic gates are also called switches. With the advent of integrated circuits, switches have been replaced by TTL (Transistor Transistor Logic) circuits and CMOS circuits.

The NOT Gate, or Inverter

The inverter is always has exactly one input as well as one output. Whatever logical state is applied to the input, the opposite state will appear at the output.

Truth table and symbol of the NOT gate is shown in the figure below.

The OR Gate

The OR gate performs logical addition, commonly known as OR function. The OR gate has two or more inputs and single output. The output of OR gate is HIGH only when any one of its inputs are HIGH (even if one input is HIGH, Output will be HIGH).

If X and Y are two inputs, then output F can be represented mathematically as Y = A+B. Here plus sign (+) denotes the OR operation. Truth table and symbol of the OR gate is shown in the figure below.

In the figure, A and B are two switches which have been connected in parallel, and this is connected in series with the load LED and source battery. When both switches are open, current does not flow to LED, but when any switch is closed then current flows.

If A = 0 and B = 0, then the LED turn OFF (0/LOW).

If A = 0 and B = 1, then the LED turn ON (1/HIGH).

If A = 1 and B = 0, then the LED turn ON (1/HIGH).

If A = 1 and B = 1, then the LED turn ON (1/HIGH).

The AND Gate

The AND gate performs logical multiplication, commonly known as AND function. The AND gate has two or more inputs and single output. The output of AND gate is HIGH only when all its inputs are HIGH (i.e. even if one input is LOW, Output will be LOW).

If X and Y are two inputs, then output F can be represented mathematically as Y = A.B, Here dot (.) denotes the AND operation. Truth table and symbol of the AND gate is shown in the figure below.

In the figure below, A and B are two switches which have been connected in series (or just cascaded) with the load LED and source battery. When both switches are closed, current flows to LED.

If A = 0 and B = 0, then the LED turn OFF (0/LOW).

If A = 0 and B = 1, then the LED turn OFF (0/LOW).

If A = 1 and B = 0, then the LED turn OFF (0/LOW).

If A = 1 and B = 1, then the LED turn ON (1/HIGH).