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:
Represented by a positive voltage - e.g 5
Represented by 0 Volts
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.
Also know as an inverter
You do not need to know this for Electronic Products
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
The output of an AND gate is only high when the input level of input A AND input
B are high.
The output of an OR gate is high when either input A OR input B OR both inputs are
The output of an OR gate is high when either input A OR input B are high but not
when both inputs are high.
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.
NB: You don’t need to know about XOR gates for GCSE Electronic Products.