4. Electrons and electronics

4.5 Digital electronics

Digital electronics

1. Analog signals: The electrical signals which are continuous and which can have any value over a given range are called analog signals. In an analog signal, the values of current or voltage vary continuously with time.

Digital electronics

2. Digital signal: A digital signal is a discontinuous function of time. It has two discrete values or voltage levels 0 and 1.The zero refers to some low value while 1 refers to some high value.

Digital electronics

Analog watch Digital watch

Digital electronics

3. What is logic? Logic is a form of reasoning which

tells us whether a statement is true or false when certain conditions are true or false.

Eg:1. AND operation2. OR operation3. NOT operation

Digital electronics Electrical analog

of AND operation

Digital electronics Electrical analog

of OR operation

Digital electronics Electrical analog

of NOT operation

Digital electronics

4. Logic circuit: A logic circuit is an electronic circuit, which represents a logic function.

Digital instruments use logic circuits to measure a quantity and then display it in a numerical reading form.

Digital electronics

Logic gate : A logic gate is an electronic circuit with one or more inputs and only one output. There are three basic logic gates

1. OR gate 2. AND gate and 3. NOT gate

Digital electronics

Truth table: A table which gives all the possible combinations of input variables and the corresponding output for a logic gate is called a Truth table.

Digital electronics The OR gate An OR gate has

two input signals and only one output signal. The logic symbol of a two input OR gate is shown in the figure

Output Y = A + B

Digital electronics

Digital electronics

The AND gate: The AND gate has two or more

inputs and only one output. It is a circuit whose output is one if and only if all the inputs are one.

Digital electronics

Output Y = A B

Digital electronics

The NOT gate: A NOT gate has only one input

and one output. It is a device whose output is always the complement of the input i.e. the output of a NOT gate is one when its input is zero and vice-versa.

Digital electronics

Digital electronics

UNIVERSAL GATES By combining the three basic

gates a large number of gates can be obtained. Hence these three gates are called Universal gates.

Digital electronics

NAND gate: If the output of an AND gate is

connected to the input of a NOT gate, the resultant gate so obtained is known as NAND gate. NAND is the abbreviation for NOT-AND

Digital electronics NAND gate

Digital electronics

NOR gate: When the output of an OR gate is

connected to the input of a NOT gate the resultant combination is known as a NOR gate. NOR is the abbreviation for NOT-OR.

Digital electronics

NOR gate

Digital electronics NOT gate using NAND gate

Digital electronics OR gate using

NAND gates

This is same as

Digital electronics The OR gate is the NAND gate with the

inputs inverted

Digital electronics

This same as

AND gate using NAND gate

Digital electronics

NAND-NOT = AND

Digital electronics

NOR gate using NAND gates

Digital electronics This completes the description of the

basic logic gates. These gates can be combined to make the larger circuits needed to make a computer processor. These larger circuits are multiplexors (data selectors), registers, the adder, and the logic that controls the computer.