For this lecture let’s start with the very basics.

What is Digital Electronics?

It is the branch of physics which deals with logic signals and the electronic components that produce them. It includes usage of boolean algebra, logic circuits and ICs which have complex transistor logic implemented inside them. It is somewhat similar to Computer Engineering.

Truth Tables

A truth table shows every single possible input combination and the respective outputs for each. For example, an AND Gate has the following Truth Table:

Don’t worry I will be explaining what an AND gate is as well

In digital electronics, 1 denotes “ON” while 0 denotes “OFF”

The meaning of 1 and 0 will be described in more detail in Lecture #2: Logic States, pull-up & pull-down

The 6 Basic Logic gates

AND gate

The AND gate has 2 inputs. When both of the inputs are set to 1, only then will the output also result in 1.

The Truth Table for an AND gate is as follows:

NOT gate

The NOT gate has 1 input. In it the output is always the exact opposite of the input

The Truth Table for a NOT gate is as follows:

OR gate

The OR gate has 2 inputs. If any of the inputs is 1, then the output also result in 1.

The Truth Table for an OR gate is as follows:

NAND gate

The NAND gate has 2-inputs. It is basically a NOT gate connected to the output of an AND gate. When both of the inputs are set to 1 only then will the output result in 0.

The Truth Table for a NAND gate is as follows:

NOR gate

The NOR gate has 2-inputs. It is basically a NOT gate connected to the output of an OR gate. When both of the inputs are set to 0 only then will the output result in 1.

The Truth Table for a NOR gate is as follows:

XOR gate

The XOR gate is sometimes also referred to as the “Exclusive OR gate”. It has 2-inputs and when both of the inputs have different values. only then will the output also result in 1.

The Truth Table for an XOR gate is as follows:

Multi-Input Logic Gates

The Logic Gates AND, OR, NAND, NOR, XOR all have 2 inputs, but it is not uncommon to see these same logic gates with a different number of Inputs. This is because the circuit is arranged in a certain way that the form a “Tree-like” structure.

For example, a 4-input AND gate is basically an AND which has each of it’s 2 inputs connected to the outputs of two other AND gates, effectively emulating as a single 4-input AND gate instead of a 4-input Logic Circuit.

The Truth Table for such a circuit would be as follows: