42Digital electronics
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Analogue and digital electronics
Logic gates
Logic gate control systems
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●● Analogue and digital
electronics
There are two main types of electronic circuits,
devices or systems – analogue and digital.
In analogue circuits, voltages (and currents) can
have any value within a certain range over which they
can be varied smoothly and continuously, as shown in
Figure 42.1a. They include amplifier-type circuits.
voltage
+
0
time
a
voltage
0
‘high’
‘low’
time
–
b
●● Logic gates
a) NOT gate or inverter
This is the simplest gate, with one input and
one output. It produces a ‘high’ output if the
input is ‘low’, i.e. the output is then NOT high,
and vice versa. Whatever the input, the gate
inverts it. The symbol and truth table are given in
Figure 42.2.
▲
▲
Figure 42.1
In digital circuits, voltages have only one of two
values, either ‘high’ (e.g. 5 V) or ‘low’ (e.g. near
0 V), as shown in Figure 42.1b. They include
switching-type circuits such as those we have
considered in Chapter 41.
A variable resistor is an analogue device
which, in a circuit with a lamp, allows the lamp
to have a wide range of light levels. A switch is a
digital device which allows a lamp to be either ‘on’
or ‘off’.
Analogue meters display their readings by the
deflection of a pointer over a continuous scale (see
Figure 47.4a, p. 220). Digital meters display their
readings as digits, i.e. numbers, which change by
one digit at a time (see Figure 47.4b, p. 220).
Logic gates are switching circuits used in
computers and other electronic systems. They
‘open’ and give a ‘high’ output voltage, i.e. a
signal (e.g. 5 V), depending on the combination
of voltages at their inputs, of which there is usually
more than one.
There are five basic types, all made from
transistors in integrated circuit form. The behaviour
of each is described by a truth table showing what
the output is for all possible inputs. ‘High’ (e.g.
5 V) and ‘low’ (e.g. near 0 V) outputs and inputs
are represented by 1 and 0, respectively, and are
referred to as logic levels 1 and 0.
–
+
Problems to solve
Electronics and society
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42Digital electronics
Input
Output
0
1
1
0
NOT gate
input
output
c) Testing logic gates
Figure 42.2 NOT gate symbol and truth table
b) OR, NOR, AND, NAND gates
All these have two or more inputs and one output.
The truth tables and symbols for 2-input gates are
shown in Figure 42.3. Try to remember the following.
OR: output is 1 if input A OR input B OR
both are 1
NOR: output is 1 if neither input A NOR
input B is 1
AND: output is 1 if input A AND input B
are 1
NAND: output is 1 if input A AND input B are
NOT both 1
OR gate
A
B
NOR gate
F
A
B
B
F
0
0
0
0
0
1
0
1
1
0
1
0
1
0
1
1
0
0
1
1
1
1
1
0
B
F
NAND gate
F
+5 V
+5 V
power
supply
A
0V
0V
LED
indicator
module
logic gate
module
B
F
IC
0V
F
A
A
B
The truth tables for the various gates can be
conveniently checked by having the logic gate
integrated circuit (IC) mounted on a small board
with sockets for the power supply, inputs A and
B and output F (Figure 42.4). A ‘high’ input
(i.e. logic level 1) is obtained by connecting the
input socket to the positive of the power supply,
e.g. +5 V and a ‘low’ input (i.e. logic level 0) by
connecting to 0 V.
Figure 42.4 Modules for testing logic gates
A
AND gate
Note from the truth tables that the outputs of the
NOR and NAND gates are the inverted outputs
of the OR and AND gates, respectively. They have
a small circle at the output end of their symbols to
show this inversion.
A
B
F
The output can be detected using an indicator
module containing an LED that lights up for a 1
and stays off for a 0.
●● Logic gate control
systems
Logic gates can be used as processors in electronic
control systems. Many of these can be demonstrated
by connecting together commercial modules like
those in Figure 42.8b
a) Security system
A
B
F
A
B
F
0
0
0
0
0
1
0
1
0
0
1
1
1
0
0
1
0
1
1
1
1
1
1
0
A simple system that might be used by a jeweller
to protect an expensive clock is shown in the
block diagram for Figure 42.5. The clock sits on
a push switch which sends a 1 to the NOT gate,
unless the clock is lifted when a 0 is sent. In that
case the output from the NOT gate is a 1 which
rings the bell.
Figure 42.3 Symbols and truth tables for 2-input gates
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Logic gate control systems
CLOCK
ON = 1
push
switch
CLOCK
OFF = 0
input
sensor
NOT
gate
0
bell
1
0
temperature
sensor
NOT
gate
1
output
transducer
processor
1
AND
gate
heater
control
1
light
sensor
Figure 42.5 Simple alarm system
b) S afety system for a machine
operator
1
AND
gate
switch
B
processor
Figure 42.7 Heater control system
A safety system could prevent a machine
(e.g. an electric motor) from being switched
on before another switch had been operated,
for example, by a protective safety guard being
in the correct position. In Figure 42.6, when
switches A and B are on, they supply a 1 to each
input of the AND gate which can then start the
motor.
switch
A
1
d) Street lights
A system is required that allows the street lights
either to be turned on manually by a switch at any
time, or automatically by a light sensor when it is
dark. The arrangement in Figure 42.8a achieves this
since the OR gate gives a 1 output when either or
both of its inputs are 1.
The system can be demonstrated using the
module shown in Figure 42.8b.
1
motor
1 or 0
switch
OR
gate
1
1
street
lights
Figure 42.6 Safety system for controlling a motor
c) Heater control system
The heater control has to switch on the heating
system when it is
light
sensor
0
NOT
gate
1
Figure 42.8a Control system with manual override
(i) cold, i.e. the temperature is below a certain
value and the output from the temperature
sensor is 0, and
(ii) daylight, i.e. the light sensor output is 1.
With these outputs from the sensors applied to
the processor in Figure 42.7, the AND gate has
two 1 inputs. The output from the AND gate
is then 1 and will turn on the heater control.
Any other combination of sensor outputs
produces a 0 output from the AND gate, as you
can check.
Figure 42.8b Module for demonstrating street lights
▲
▲
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