BASIC
ELECTRONICS COURSE
Page 26
INDEX
DIGITAL BUILDING BLOCKS "GATES"
Most complex circuits are
built around a microprocessor. For less than $10.00 you can get
a Microprocessor chip that will take the place of 5, 10 or even
20 simple chips, similar to the one's we are about to describe.
If you want to know more about the MICROPROCESSOR, see our other
section on PIC
PROGRAMMING. It covers designing and programming
microcontrollers.
Writing programs for microcontrollers is a completely different
field of electronics and is really not very difficult. But you
need to know basic electronics so you can correctly design the
stages that connect to the microcontroller. That's why designing
with microcontrollers will be introduced later.
In the meantime you need to know the basics of GATES. These are
classified as DIGITAL BUILDING BLOCKS. They are called DIGITAL
because they operate on digital principles. They require a
signal (also called a PULSE or INPUT) that is either 0v
(LOW) or rail volts (HIGH) and give out a signal that is
either LOW or HIGH.
Gates can also accept MORE THAN ONE INPUT but always have
ONLY ONE OUTPUT.
Gates do not amplify (we are talking about voltage
amplification). The amplitude of the input must be very close to
rail voltage for the gate to work and the output is very nearly
rail voltage.
Gates are DECISION MAKING BLOCKS.
They take 1, 2 or more inputs and make the output HIGH if a
certain condition is present. It may be all inputs HIGH. Or all
inputs LOW. Or only one input HIGH or LOW.
A "gate operation" can be programmed into a
Microcontroller, constructed with individual components such as
diodes and resistors or obtained as a CHIP. This discussion
describes that last two.
There are lots of times when a GATE is required in a circuit and
it's important to understand HOW THEY WORK so you can
make one if needed.
For example, an alarm circuit may require to be triggered
when a door is opened AND a pressure mat is activated. This
feature can be built into the microprocessor controlling the
whole circuit or you can use a separate chip. But if you know
how gates work, it may only require a few extra
components.
The situation above is
an AND decision. The digital block for this is drawn as:
The block has two
inputs and a single output. It can have 3 or more inputs but we
will describe only the two-input version of all gates. The
"conventional" symbol is the easiest to remember. It
is like the letter "D." The mechanical way of explaining how the gate
works is shown below. Two relays inside the block must be
activated for the globe to illuminate.
Note: When an input to a gate is not connected to a switch, it
is pulled-down internally. In other words it "sees" a
LOW.
The
electrical equivalent of the AND gate is:
There is also the OR gate, where ONE input will activate the
output. The digital block for this is:
The mechanical OR is shown below:
The electrical
equivalent of the OR gate is:
If you require a
globe to NOT WORK when an input is HIGH, the gate is called an
INVERTING DECISION MAKER or INVERTER. The block for this
is drawn as a triangle with a circle or "bubble" on
the output. The circle indicates inversion and the block is
called a NOT function (remember the circle as a bow or
KNOT).
The NOT gate is an
inverting gate. When the input is LOW the output is HIGH and
when the input is HIGH, the output is LOW. This is shown on the
diagram below:
We can combine the AND
gate with the INVERTER to obtain a gate with 2 (or more)
inputs and "inverts." The result is
a NAND function (comes from Not
+ AND).
For a NAND gate, the output is HIGH when any input is LOW. For
a project using NAND gates click HERE.
(Touch Switch - using two NAND gates of a 4011 IC)
A NOR gate can be produced by combining an OR gate
with an INVERTER.
For a NOR gate, the
output is LOW when any input is HIGH.
Another
very handy gate is the EXCLUSIVE OR. The output is HIGH
when ONLY ONE of the inputs is HIGH. You will
notice the OR gate above has this feature but the output is also
HIGH when BOTH inputs are HIGH. The standard OR gate is
sometimes called the INCLUSIVE-OR
for this reason.
There is one gate that really
doesn't perform like a gate. It is the BUFFER. The output is the
same as the input. It was needed for the original logic chips as
they required a small amount of current into the input lines and
if a chip had to drive a number of other chips, a BUFFER was
required.
Chip-design has now improved and BUFFER chips are no longer
required.
To complete this discussion on gates, we have included the
BUFFER:
Each gate produces a number of results and these can be
placed in a table so it is easy to see exactly what will happen
when each combination of HIGH's and LOW's is presented to the
gate. This table is called a TRUTH TABLE and the
truth table for each gate is shown below:
Study the following waveform diagrams to see how each gate operates:
Question 119: Name the gate that that has a single input and the output
is INVERTED.
Ans: The NOT gate.
Question 120: Name the gate that
is HIGH, only when both inputs are HIGH.
Ans: The AND gate.
Question 121: Name the gate that
is only HIGH when ONE INPUT is HIGH.
Ans: The XOR gate. This is the advantage of the exclusive-OR
gate.
WHAT
DO GATES DO?
We have seen how the output of a gate goes HIGH or LOW when
certain conditions are present on the input lines. But how does
a gate work and what does it do when it is placed in a circuit? This is the first
thing to be discussed on the next page.
MAKING
A GATE
Modern circuits require very few individual "logic
blocks" (digital gates) as any "decision making"
can be programmed into a microcontroller. If an individual gate
is required, it can sometimes be created with separate
components. On the next page we show how to create the "effect"
of some of the gates. It is called
"GATING."
We say "effect" of some of
the gates because the signal normally goes though a gate (as you will
see) but with a gating pulse, a signal is "blocked" or
"allowed to pass."
All the gates discussed above can be produced by using 1, 2,
3 or 4 gates of a QUAD NAND GATE chip, such as 74HC00. This is
fully discussed in the book "Electronics Notebook 1 with
STARTING IN TTL." A kit of components is also available for
you to make your own TTL trainer deck and by using jumper wires
(supplied in the kit) you can create all the LOGIC BLOCKS
described above. The total cost for the TTL Trainer deck (all
components - and batteries) and the book:
"Electronics Notebook 1 with STARTING IN TTL."
including pack and post $42.00
Click HERE
to order.
The Deck also allows you to create FLIP FLOPS, a RIPPLE COUNTER
and a SHIFT REGISTER. These are the basic building blocks of a
computer and you get "hands on" experience with seeing
the circuits working.
An Instructors TTL Trainer Deck is also available (50% larger)
($46 including all components and book). Click
HERE
to order.
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