An Introduction to logic gates

Electronics - Devices

Nov 2, 2013 (4 years and 6 months ago)

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An Introduction to logic gates

Fig

1
.

Fig 1 shows the simplest of circuits, comprising of a switch (SW1), connected in series with a battery
(B1) and a lamp (BL1), when the switch is closed the lamp comes on, when it is open the lamp

is off
because there are just two

states for this circuit, this is said to be a
digital circuit
.

If I use two binary numbers , 1 and 0, to represent these two states 1 can represent the switch
closed and the lamp on, whereas 0 can represent the switch op
en and lamp off
.

Fig 2.

Input A

Output X

Switch open A=0

Lamp off X=0

Switch closed A=1

Lamp on X=1

Fig 2 summarises the input and output information, it is commonly known as a truth table as it ‘tells
the truth’ about the function of the circuit.

Fig 3
.

Fig 3 shows

two switch
es SW1 and SW2 connected in series, the
lamp cannot light unless both of the switches are closed.

The switches provide input information
and again the lamp provides the output information. The truth table in Fig 4 summarise
s the
function of this basic digital circuit which is known as an
AND

gate.

Fig 4.

Input A

(SW1)

Input B (SW2)

Output X

0

0

0

1

0

0

0

1

0

1

1

1

It says that the output state has a binary value of 1 (lamp on) only if SW1 and SW2 each have a
binary value of 1 (both switches closed). If either or both switches are on binary 0 the output is
binary 0

(the lamp is off). This digital circuit is called a ‘gate’ because the switches open and close to
control the information reaching the output.

Fig

5
.

Fig 5 shows another basic digital circuit but this time the two switches are connected in parallel. In
this circuit the lamp lights if either SW1 or SW2 are closed, this
OR

gate’s truth table is shown below
in
Fig 6.

Fig 6

Input A (SW1)

Input B
(SW2)

Output X

0

0

0

1

0

1

0

1

1

1

1

1

The electronics

shown here are known as digital logic. The
AND

and
OR

gates are called
LOGIC
GATES
,
because the output states are the logical or predictable results of a certain combination of
input states.
Logic gates and many others are very important to electronics nowadays as they are
used in things like computers, calculators and watches. Thousands of gates are built from transistors
in the form of integrated circuits.

A logic gate has the intermediate
task of receiving signals from sensors such as

a keyboard or
temperature sensors and it will make a decision based on the information that is received and will
send an output signal to a circuit which then provides the action which could be something like
switching on a motor.

Digital circuits can be designed using individual diodes and transistors or even mechanical switches.
(Function
s of these components are

explained in S2 of the audit).

Most digital circuits

now make use of logic gates in integrated circuit packages.

‘’
Logic gates are ke
y components that are part of a
digital circuit
. Essentially, a logic gate helps to
define a logical sequence of actions that take place under given circumstances. The typic
al logic gate
will provide one output and two inputs as part of the logic sequence.

In addition, the digital circuits
will function at different voltage levels, using at least one of the two
binar
y

conditi
ons in any given
execution of
the sequence.

As of 2008, there are a total of seven different configurations for the logic gate. Each one is designed
to handle various combinations and sequences of logical response that help the digital circuit to
function at optimal efficiency. Because the logical command will vary according to circumstances, a
digital circuit often employs several if not all of these seven configurations.
’’ www.wisegeek.com

There are two alternative systems in use for showing the

symbols for logic gates in circuit diagrams,
the American ‘mil spec’ system and the British standards system.

These are the 7 American mil spec symbols for logic gates, the
AND, OR, NOT, NAND, NOR, EOR

and
X
OR