School Science Lessons
Appendix A: Bibliography
2008-12-20
Please send comments to: J.Elfick@uq.edu.au
1. TTL Cookbook Don Lancaster (Howard W Sams, 1974) A masterful
exposition of basic digital electronics and an authoritative and
comprehensive account of TTL devices and their application. Highly
recommended.
2. CMOS Cookbook Don Lancaster (Howard W Sams, 1977) As brilliant and
authoritative a guide as its TTL predecessor.
3. Microelectronics, practical approaches for schools and colleges (BP
Educational Service, 1981) Sections 3.4 to 3.6 of this are particularly
good. They
describe a modular approach to digital electronics which is much less
comprehensive than that of the present course, but which uses
electronic
systems in a similar way and progresses to memory as an end-point.
4. The European CMOS selection Low power Schottky TTL The standard
Motorola reference books containing information on their products and
complete sets of data sheets. Reprinted and up-dated regularly.
5. Adventures with digital electronics Tom Duncan (John Murray, 1982)
Describes plenty of advanced CMOS projects likely to appeal to
youngsters.
The initial chapters provide a very condensed but clear account of
selected aspects of basic digital electronics. 6 Electronics G E
Foxcroft, J L Lewis
and M K Summers (Longman, 1985) This is a text written to
accompany the work of this Electronics course. It contains a larger
number of
projects than this book does. Teachers may find these useful in the
earlier parts of this work. There is also a Teacher's Guide.
Appendix B: Technicals details
1. Modules for Part 1
The following diagrams are module "plans" for the modules required in
Part 1. In these diagrams R is a protective resistor chosen to limit
the
maximum current through the device to a safe value. In the cases where
R protects a switch, its value is given by R = 61 max. switch current.
In the
case of the LDR, (R = (36 / P) - X), where P is the maximum power
dissipation and X is the minimum resistance of the LDR.
For the motor and buzzer, R is only necessary if the devices are
designed to operate from a smaller voltage than 6 V. If these items are
designed for V
volts, then (R = (6 - V) / I) where I is the normal device current.
LEDs Module
LDR Module
Push-Button Switch Module (2)
Buzzer Module
Motor Module
Reed Switch Module
Reed Relay Module 2
2. Integrated circuit families
The electronic functions that the modules of Part 2 perform can be
implemented with a variety of different devices: relays, transistors,
integrated
circuits, for example. Integrated circuits are used in this course for
the obvious reasons that they are modern, cheap and provide the
necessary circuitry
for experimental work in a very compact and convenient form. In
addition, they provide the opportunity to show the real meaning
of
microelectronics, not some new type of electronics but
miniaturization, which has been known to be valuable for a long time.
Take the opportunity to view a
silicon chip with a microscope.
There are several different families of integrated circuits, which
perform the same basic tasks in different ways. For example, a NAND
gate from one
family will perform the same logical function as one from another
family, but the internal processes may be very different. In recent
years, two
families have gained particular prominence. One is the CMOS family
(Complementary Metal Oxide Semiconductor) while the other is the TTL
family (Transistor Transistor Logic). The latter has now been
superseded for many applications by a subfamily LSTTL (Low
power
Schottky TTL). LSTTL ICs can directly replace standard TTL chips in
most applications.
Since this course is concerned mainly with ways in which building
bricks such as NAND gates are used, the particular methods of
implementation
within an IC are of no great importance. The experimental work can
therefore be carried out using any IC family. The relative methods of
using CMOS,
TTL or LSTTL ICs for this work are discussed below.
Selecting an IC family for work at school level
3. CMOS
The features of CMOS integrated circuits are as follows.
3.1 They will operate from any supply voltage from 3 V to 18 V
(extremes best avoided).
3.2 The supply current for the ICs is minute (a few microamps in most
cases).
3.3 Noise immunity is high (relatively large fluctuations in the
voltage level at an input can be tolerated before the output voltage
level is affected).
3.4 They can be damaged by static electricity and need handling
carefully.
3.5 Inputs cannot be left floating, they all go somewhere
3.1 and 3.2 above are obviously useful for school work, because
ordinary batteries can be used and with reasonably long life. However,
batteries have
also to supply the current to operate LEDs and 7 segment displays, and
the current might then be as high as 0.2 A. High noise immunity is a
major
advantage for complex circuitry, especially if high speed switching is
necessary, but it is not important for work in this course.
With modules, the problem of static electricity is not important since
you do not handle the ICs directly. However, the need to
ensure that all
inputs go somewhere is a nuisance, since more wiring and additional
resistors are needed.
4. TTL
This family of ICs has the following features:
4.1 A 5 V smoothed power supply is essential (the supply voltage range
is 4.75 V to 5.25 V). A regulated supply is to be preferred.
4.2 The IC supply current is relatively large (milliamps, or sometimes,
tens of milliamps).
4.3 Noise immunity is not so high as for CMOS devices.
4.4 Floating inputs assume a high logic state and be left floating.
4.5 The ICs are not susceptible to electrostatic damage and are more
robust than CMOS ICs.
For work at school level, a regulated supply will almost certainly have
to be used, but this can be built quite cheaply (see below). In some of
the more
complex experiments in the course, the supply current may be as high as
1 / 2 A or more.
The fact that TTL inputs can be allowed to float is very useful when
constructing modules, since the necessary wiring is reduced to a
minimum (for
further information see later). If you handle chips directly,
TTL should be preferred to CMOS and especially as noise immunity
problems are
unlikely to arise in this experimental work.
5. LSTTL
These ICs are functionally identical to TTL. The pin connections are
the same and, in most cases, direct replacement is possible. With work
at school
level, the major advantage of LSTTL compared with TTL is that only
about 1 / 5 of the power supply current is needed. Otherwise, the
comments
above on TTL apply to LSTTL.
The CMOS family have been chosen for this course because of the less
stringent demands on power supply needs, and because you do not handle
ICs directly. However, you may wish to use
TTL / LSTTL, and so, details are provided in this Appendix for the
TTL / LSTTL
families as well as the CMOS family. Great care has been taken with
circuit design to ensure that modules behave in the same way whichever
family
is used.
6. Power needs
CMOS circuits are recommended because they operate over a wide supply
voltage range (3 V to 18 V) with higher noise immunity and lower power
needs than TTL / LSTTL---. A simple dry battery supply at 6 V can
be used, for rarely will the current required exceed 0.2 A.
For TTL / LSTTL ICs, power needs are more stringent. For
reliability, manufacturers recommend a stabilized supply of between
4.75 V and 5.25
V. (Maximum supply voltage is 7.0 V and maximum input voltage is 5.5
V.) Batteries can be used with the circuit shown. In use, the voltage
drop
across the IN5401.0 diode produces an output voltage which is just
acceptable for TTULSTTL operation. The INS401 is used for reasons given
on page 26.
However, it is very convenient to use a fully regulated, mains
driven supply. An effective and inexpensive 5 V 1 A supply is shown
below. The circuit
converts an a.c. or d.c. input in the range 8 V to 12 V to a regulated
5 V supply suitable for CMOS modules as well as TTULSTTL modules. Using
low voltage transformers, this circuit represents a very cheap way of
obtaining a mains operated supply. Note that a bridge rectifier, rated
at 3 A,
should be used.
7. PULL-UP AND PULL-DOWN RESISTORS
TTULSTTL
The high output voltage level for a TTULSTR logic gate is usually about
3.4 V. If the full supply voltage is required a 2.2kQ resistor can be
connected
between the output and the positive power supply rail. The measured
output voltage will be near 5 V in this case. The resistor is called a
pull-up
resistor. When connecting TTULSTR ICs to each other, pull-up resistors
are not required for a voltage input in excess of 2.7 V is recognized
as a high voltage. They are useful, however, when interfacing TTL to
other families or to non-TTL devices.
Unconnected inputs float high and for work in this course, this is
permissible. In the TTULSTTL module designs which follow, nearly all
inputs are
left floating so simplifying construction. in the case of a more
permanent electronic circuit, an unused input should be connected to
the positive rail to
increase noise immunity.
If the input of a TTL gate is to be normally low, but to go high when a
switch is closed, a pull-down resistor must be used to keep the input
normally
low, for otherwise switch closure would short circuit the 5 V battery.
For TTL gates, the pull-down resistor must be less than about 50M (see
reference
1, Appendix A). Thus, when using an LDR to switch a gate, switching
only occurs when the LDR is well enough illuminated for its resistance
to have
fallen below 500 £1. In the dark, the resistance is several
hundred thousand ohms and the input floats high. A pull-up resistor is
unnecessary, except
when "sensitivity" is to be increased.
8. CMOS
The case of CMOS gates is different because unconnected inputs have an
undefined, state. Each input acts as a small capacitor. Once the
capacitor is
charged, the p.d. across it remains constant because the input
resistance of the gate is virtually infinite. Thus, the input voltage
depends on the past
history of the gate and logic states cannot be predicted. All unused
inputs must be connected to one of the supply rails, not only because
of the above
fact but also to ensure small supply currents (see reference 2,
Appendix A).
In the case of inputs which are to be used, it is necessary to ensure
that they have a definite logic state when unconnected. This is done by
using a
pull-up or a pull-down resistor as required (typically 10M1). Thus CMOS
gates can be made to behave like TTL gates by using pull-up resistors
to
hold unconnected percentage inputs high. In the CMOS module designs
which follow
this has been done. For example, in the case of 76 the CMOS NAND
gate, the pull-up resistors are part of the CMOS NAND gate module and
have not been shown on the circuit diagrams for the experiments. The
advantage of ensuring that TTL and CMOS modules behave similarly is
that
the circuit diagrams for the experimental work are then the same for
both families.
9. The QUAD NAND gate module: The use of SCHMITT NANDS
The
Ouad NAND Gate module is constructed with a quad Schmitt NAND gate IC.
The input / output voltage characteristic of an ordinary
CMOS NAND gate and a Schmitt NAND gate are shown. The latter shows
hysteresis, switching from high to low when the input voltage reaches
V1, but not going back to Pot NpNAWP -9tt~-high until the input voltage
fails to V2. The V. 5,\4 output logic level of the Schmitt NAND
changes
VO 4 if very rapidly even if the input voltage changes fl 3
comparatively slowly. Clocked circuits need rapid changes at the clock
input if they are to VI.%V, function reliably, and the Schmitt gates
must beV. / V 0 0 1 1 -34 / V used between bistables and
components
such as LDRs which
respond relatively slowly and are somewhat noisy. The effect of using a
Schmitt circuit with a slowly changing and noisy input is illustrated
in
the diagram opposite. The logic level of the output of the Schmitt gate
changes rapidly and cleanly, despite the rather erratic and slowly
varying
input voltage. The hysteresis (V1-V2) amounts to about 0.5 V using a
5 V supply.
8. Circuit diagrams of modules for Part 2
The circuit diagrams on the following pages are drawn for CMOS and for
LSTTL ICS. TTL circuits are identical with those for LSTTL, except that
the
IC type numbers do not include the letter LS. Note that the 7-segment
decoder display for LSTTL ICs uses a common anode display, and has a
test
socket connected to the negative rail. This is because an active output
on the decoder is at a low voltage level.
QUAD NAND GATE
1. CMOS version
LED INDICATORS
CMOS and LSTTL versions
2. LSTTL version
DRIVER AMPLIFIER
CMOS and LSTTL versions
SEVEN SEGMENT DISPLAY WITH DECODER
3. CMOS version
NB Terminal pins are required at each of the decoder outputs.
4. LSTTL version
CLOCKED BISTABLES
5. CMOS version
6. LSTTL version
BINARY COUNTERS
7. CMOS version
PULSER and ASTABLE
CMOS and LSTTL versions
LSTTL version (two on one board)
The SPIDT press switch is shown in the "rest" position.
R1 and R2 should be chosen to give output frequencies of about 1 Hz and
100 Hz. The values are about R, = 1 OkQ and R 2 = 1 Mn. R 2 could be a
preset.