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Design & Technology Resources
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A logic gate is a circuit that can make decisions. A number of logic gates are normally housed within one single chip.

 

Logic gates are digital circuits. The output of a logic gate can only be on or off and nowhere in between. When we are describing how logic gates work we use the words “logic 1” to describe when an input or output is on and “logic 0” when an input or output is off. These levels are also sometimes referred to as being “high” and “low”.

 

 

On = 1

Off = 0

 

 

The two graphs below show the difference between digital and analogue signals:

Logic Gates
Represented by a positive voltage - e.g 5
Represented by 0 Volts

Voltage (V)

Time (seconds)

Voltage (V)

Time (seconds)

An Analogue Signal

A Digital Signal

An analogue signal be any voltage with no distinct boundaries.

A digital signal can be of only two specific voltages. The signal can be either on or off but nowhere in between.

How Logic Gates Work

Logic gates are electronic circuits designed to perform logical functions and make decisions depending on their input levels. Their inputs and outputs can only be either on or off.

 

There are three types of logic gates that you need to know about: AND, OR and NOT. You need to be able to recognise the logic gates by their symbols, draw their truth tables and describe how they work. (I have also shown an XOR gate) but you don’t need this for the GCSE exam).

 

Al truth table is a chart that we can use to describe how a logic gate works. In an exam you could be asked to complete one of these for a given logic gate. The animations below show how each of the gates work. Remember, all that 0 and 1 refers to is whether the input or output is turned on or off.

AND

Gate Symbol

OR

Gate Symbol

NOT

Gate Symbol

Also know as an inverter

XOR

Gate Symbol

You do not need to know this for Electronic Products

Truth Table

NOT Gate

This is the most simple logic gate. It has one input and one output. The output level is just the opposite of the input.

 

• The output is high when the input is low

 

• The output is low when the input is high

AND Gate

The output of an AND gate is only high when the input level of input A AND input B are high.

Truth Table

OR Gate

The output of an OR gate is high when either input A OR input B OR both inputs are high.

Truth Table

XOR Gate

The output of an OR gate is high when either input A OR input B are high but not when both inputs are high.

Truth Table

Equivalent Circuits

It is possible to make logic gates using normal switches. In an exam you may be asked to draw the equivalent circuits for a logic gate or vice versa. The circuit below show how NOT, AND and OR gates can be made with switches.

 

AND

 

OR

 

NOT

NB: You don’t need to know about XOR gates for GCSE Electronic Products.