In this guide, we’re going to focus on another important part of the IGCSE Physics syllabus: Logic gates.

Analogue and digital

There are two types of electronic systems, circuits, or devices:

  1. Analogue
  2. Digital

In analogue circuits, voltages can have any value- positive or negative. An example of an analogue device is a variable resistor; variable resistors can give any value of resistance.

In digital circuits, voltages can have only two fixed values: either ON or OFF. An example of a digital device is a switch; switches can only give two fixed values- ON or OFF.

Logic gates

Logic gates are electrical components whose output voltage depends on one or more input voltages.

Logic gates are digital devices as they only give two fixed values for output voltage- either ON, or OFF.

  • A high voltage is referred to as ON, and is represented by the symbol 1.
  • A low voltage is referred to as OFF, and is represented by the symbol 0.

NOT logic gates

A not gate is the simplest of all logic gates; it has one input and one output.

How does a NOT gate work?

When a HIGH (1) voltage enters a NOT gate, it produces an output in the form of a LOW voltage (0).

When a LOW (0) voltage enters a NOT gate, it produces an output in the form of a HIGH voltage (1).

Truth table of a NOT gate

Input voltage Output voltage
0 1
1 0

 

OR and NOR logic gates

OR and NOR logic gates have two inputs and one output unlike a NOT logic gate.

How do OR and NOR logic gates function?

OR logic gate The output of an OR gate is ON, if:

1.    Either input 1, OR input 2 is ON.

2.    Both input 1 and input 2 are ON.

The output of an OR gate is OFF if:

1.    Both input 1 and input 2 are OFF.

NOR logic gate The output of an NOR gate is ON, if:

1.    Both input 1 and input 2 are OFF.

The output of an NOR gate is OFF if:

1.    Either input 1, OR input 2 is ON.

2.    Both input 1 and input 2 are ON.

 

Truth table of an OR logic gate

Input voltage 1 Input voltage 2 Output voltage
0 0 0
0 1 1
1 0 1
1 1 1

Truth table of an NOR logic gate

Input voltage 1 Input voltage 2 Output voltage
0 0 1
0 1 0
1 0 0
1 1 0

Exam tip!

Remember that you can make a NOR logic gate by connecting a NOT gate to the output of an OR gate. So, OR+NOT = NOR.

AND and NAND logic gates

AND and NAND gates are another pair of logic gates which have two inputs and one output (just like OR and NOR logic gates).

How do AND and NAND logic gates function?

AND logic gate The output of an AND gate is ON if:

1.    Both input 1 and input 2 are ON

The output of an AND gate is OFF if:

1.    Either input 1, OR input 2 is ON.

2.    Both input 1 and input 2 are OFF.

NAND logic gate The output of an NAND gate is ON if:

1.    Both input 1 and input 2 are OFF.

2.    Either input 1, OR input 2 is ON.

The output of an AND gate is OFF if:

1.    Both input 1 and input 2 are ON

 

Truth table for an AND logic gate

Input voltage 1 Input voltage 2 Output voltage
1 1 1
1 0 0
0 1 0
0 0 0

 

Truth table for an NAND logic gate

Input voltage 1 Input voltage 2 Output voltage
1 1 0
1 0 1
0 1 1
0 0 1

 

Uses of logic gates

Logic gates are used in computers and electronics, as well as in processors in a control system.

 

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