BASIC
ELECTRONICS COURSE
Page 19
INDEX
So
far we have seen how the Flip Flop works. Each transistor takes
it in turn to deliver current to its LOAD. A LOAD is any device
placed between the collector and positive rail and may be a LED,
motor, speaker, relay, globe or solenoid (a solenoid is a coil
of wire wound on a former and when a current is applied, it
draws a soft-iron core into its centre - this is also called a
linear actuator).
The Flip Flop (also called a Multivibrator or Square-wave
oscillator) on the previous page operated at a very low
frequency. This is necessary so you can see the LEDs flashing.
But if you connect a speaker to the circuit, you will only get a
very soft "click" every time the LED is turned on. The
circuit is not designed to operate a speaker and we will show
you how to change the value of the capacitors (the 100u
electrolytics) to turn the Flip Flop into a MULTIVIBRATOR. But
first let's look at the circuit with a CRO.
This is a CATHODE RAY OSCILLOSCOPE and is a very handy piece of
test equipment that displays the signal at any location on a
circuit.
A CRO is capable of displaying very high as well as very low
frequencies signals, and when the signal has a very low
frequency, the TRACE (the line going across the screen) can be
slowed down to show the waveform. The animation
below shows the output of the Flip Flop on a CRO:
When
the CRO is not connected to a circuit, the trace moves across
the the X-axis. This indicates ZERO input voltage. When the CRO
is connected, the left-hand transistor is ON and the LED is
illuminated. This means the collector voltage is very low and
the trace moves across the screen slightly above the
X-axis.
When the circuit changes state, (the change is very RAPID) the
trace moves UP-SCREEN and continues across the screen. When the
circuit "flops" back to it original state, the trace
follows just above the X-axis. If the above animation is
displaying too fast, the lower animation has been slowed down
considerably so you can see the trace moving across the screen
and rising or falling very quickly when the circuit changes
state.
If
the electrolytics are replaced with much smaller capacitors the
circuit will operate at a higher frequency and a speaker can be
connected to one side to detect the output.
The transistor will be turning on and off very quickly and this
will put a pulse through the speaker to result in a tone being
heard. The tone will be a SQUARE WAVE.
When
the CRO is "locked-in" on the signal, the trace (the
picture) on the CRO will be stable and it does not move at all.
That's the beauty of a CRO. It stabilises a "picture"
of a waveform so you can see it clearly. This will only happen
when the settings on the CRO are correct such as:
1. The Sweep speed - the speed at which the trace moves across
the face of the screen.
2. The Trigger is set to external so the CRO
"locks-in" on each cycle of the waveform.
When the trace is stable you can read values from the screen
such as time (in milliseconds) for each cycle and the amplitude
of the waveform. You can also see it is perfectly square and if
any distortion is occurring.
A good quality square wave should rise and fall instantly and be
square at the top. Any rounding of the wave will be due to
"switching" problems and you can experiment on the
circuit to see where it is coming from.
CRO's are very easy to use. You just have to get it set up by
someone "in the know" and don't fiddle with the knobs.
You should be allowed to touch only two or three knobs at the
beginning.
1. The VOLTS/DIV knob attenuates (adjusts) the waveform so that
its height fits onto the screen.
2. The TIME/DIV or SWEEP control determines how fast the trace
moves across the screen so that the "picture" has the
required number of cycles on the screen.
3. The TRIGGER (also called the SOURCE switch) stabilises the
picture so that it is not jumping or flickering.
Question
78: What is the definition of a "LOAD"
Ans: Any device being driven by a
transistor. For the transistor in the multivibrator circuit
above, the device is placed between the collector and positive rail. There are other ways to connect a transistor to
the supply rails and the load can placed in the emitter line -
more about this later.
Question
79: What do the letters CRO stand for?
Ans: Cathode Ray Oscilloscope.
Question
80: Name the components that control the frequency of a
multivibrator:
Ans: The base resistor and
capacitor.
Question
81: When the "timing capacitor" in a multivibrator is
reduced, what happens to the frequency?
Ans: The frequency increases.
Question
82: Give the two other names for a flip flop:
Ans: Multivibrator, Square Wave
Oscillator.
Question
83: When a LED is illuminated in the flip flop above, is the
voltage on the collector of the driving transistor HIGH or
LOW?
Ans: LOW
Question
84: In the 1kHz circuit above, why is a 300R resistor in
the collector of the first transistor?
Ans: The collector of the transistor in
the 1kHz circuit above must not be connected directly to the
positive rail as it will create a SHORT-CIRCUIT when it is
turned on and it will be damaged. The resistor limits the
current and allows the voltage on the collector to vary between
rail voltage and near-zero voltage.
NEXT
. . .
The
next page will cover another type of "flasher" circuit
that flashes a LED with a completely different circuit to a Flip
Flop. It's actually a very high gain amplifier and it's very
interesting, once you know how it works.
Before
we go to the next page, click the button below for a memory
test. The picture shows 15 components. Study the picture and
start the test. You should recall about 8 items on your first attempt,
11 on your second and about 14 or 15 on your third
attempt.
This proves that you absorb about 50% of a document on the first
reading, about 75% on second reading and nearly 100% on the third
reading. That's why you should come back to these pages at
a later data and go through them again. You will gain a
further 15% to 25% on each additional reading. So, let's start
the memory test . . .
After you do the test, you will see how right I am. That's because
you have not been taught HOW TO REMEMBER.
On each additional attempt, you made a concerted attempt to
remember exactly where each of the components resided. This is
called "PUTTING YOUR MIND INTO GEAR". In other
words, you said "I'll remember this!"
All the test taught you is how to put your mind into FUNCTIONING
MODE - it didn't help you identify any of the components
with their names. But it's a start.
It did show you exactly how to put your mind into "ABSORPTION
MODE." All you have to do is tell yourself
that you will remember a particular fact and then reference the
fact to something you already know. It's called a
"hitch" or "crutch." This way you are
telling yourself that you don't have to remember anything
complex. It's simply a matter of recalling something you already
know and then bringing in the "new, companion fact."
It's called learning by association. If you tell yourself three
things, your absorption rate will increase:
1. Tell yourself it is important to learn these facts,
2. Keep your mind from wandering, and
3. Use the tool of ASSOCIATION.
More on this later . . .
NEXT
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