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BASIC
ELECTRONICS COURSE
Page 20
INDEX
The next circuit we will cover is a LED FLASHER. It
flashes a single LED and produces a higher illumination from a
LED than the Flip Flop project. You will see how it does this in
a moment.
The LED Flasher and FLIP FLOP have been presented as "beginners
circuits" because they
are simple, with a great effect and demonstrate the
principle of transistor operation.
They are ideal for animating and you can "see" how
they work because this is the way electronics designers see a
circuit in operation. By seeing a circuit working in their mind, they can
"see" if it will work.
These two "flasher" circuits are also very reliable in operation and will operate every
time they are switched on. The value of the components are not critical and the supply
voltage can drop considerably without affecting the frequency or
reliability. You may think it's easy designing new circuits but lots of
flasher circuits have been designed but many were very
fiddly to get going - only a few survived the "test of
time."
THE LED
FLASHER CIRCUIT
The
circuit looks completely different to the Flip Flop circuit
described in the previous section and
if I said it was basically a very high gain amplifier, you would
be surprised. But that's what it is and in the "5-PROJECTS"
section, you will be able to
make up this circuit without the 10u electrolytic and
create a very high gain amplifier. In place of the
470k resistor a TOUCH PLATE has been added and by placing your
finger on the plate you will be able to regulate the resistance
and control the operation of the circuit.
To understand this circuit fully you need to go to the
HIGH
GAIN AMPLIFIER project and carry out the interactive
experiments, then return to here.
The LED FLASHER circuit consists of an NPN and PNP transistor. We have shown
how an NPN transistor "turns on" when the voltage on
the base rises above 0.7v and current is allowed to flow into
the base. This causes the resistance between the collector and
emitter leads to reduce and effectively reduce the voltage
between these two leads.
The PNP transistor is a "mirror" of the NPN
transistor. It works in exactly the same way except the voltages
on the leads are to opposite polarity. When the base voltage is
lower than the emitter (for the BC 557), the transistor turns on
and current will flow in the collector-emitter circuit. But
collector-emitter current does not flow yet. We already know
that the "characteristic voltage" across a red
LED is about 1.7v and the base voltage has to drop 1.7v plus
0.7v for the transistor "characteristic base
voltage."
"When the base is 2.4v lower than the positive rail,
current will begin to flow through the LED. In fact the voltage
on the base drops much more than 2.4v because the first
transistor "turns on" (as you will see) and this
causes the PNP transistor to turn on.
Now we can go to the start. When the power is switched on, the
470k base-bias resistor charges the 10u electrolytic (in the
reverse direction - but they can be charged slightly in reverse)
and when the base rises to 0.65v to 0.7v, the transistor turns
on and causes the voltage on the base of the PNP transistor to
drop to only a few volts.
This allows the PNP transistor to turn on and the LED
illuminates. At the same time, current flows through the 22R
resistor and when current flows through a resistor, a voltage
drop is produced across it. This voltages drop raises the
voltage on the positive side of the 10u electrolytic and this
makes the negative side rise ALSO.
The effect of this is to turn on the NPN transistor VERY HARD
and the whole circuit becomes FULLY TURNED ON. The illumination
from the LED is very HIGH because the current limiting resistor
is very low.
We have already mentioned that the current limiting resistor for
a LED and 9v supply should be about 470 ohms and if a lower
value is fitted, the LED will be damaged. So, why put a 22R in
the circuit for the dropper resistor?
The reason is to produce a very bright flash from the LED. One
point we failed to mention in the previous pages is the LED will
accept a very high current for a very short period of time
without being damaged.
This circuit does this and nothing is damaged.
The circuit is now fully turned on and the 10u electrolytic
continues to be charged due to the voltage drop across the 22R.
While the electrolytic is charging up, the charging current is
high but as the voltage across the electro increases, the
charging current reduces and the NPN begins to turn off. This
makes the PNP transistor begin to turn off and the current
through the LED and 22R reduces. The voltage-drop across the 22R
reduces and this brings the positive side of the 10u down
towards the 0v rail.
The voltage on the base of the NPN is reduced by this action and
the NPN turns OFF. The whole circuit is now fully turned off.
The energy stored in the electrolytic creates a voltage across
it and this is gradually removed by the 470k discharging the
electro. When the electrolytic is fully discharged, it can begin
to charge again (in the reverse direction) by the
470k.
These
actions are best presented in an animation (below), where you
can see the effect of the electrolytic charging and
discharging.
The
action of the capacitor (electrolytic) is quite complex but you
can see it charges slightly in the reverse direction at the
beginning of the cycle to turn the circuit ON then it is
charged in the forward direction due to the voltage developed
across the 22R resistor. This charging current keeps the first
transistor ON and after a short period of time the charging
current drops to a level that cannot keep the transistor on. The
first transistor turns OFF slightly and the voltage across the
22R drops. This makes the right-hand side of the capacitor fall
and the left-side falls too. The voltage across the capacitor is
gradually removed by the discharging effect of the 470k resistor
and during this time the circuit is OFF. When the
voltage across the capacitor rises to 0.65 to 0.7v, the circuit
turns on again.
MAKING
A PROJECT
Now
we come to the stage where you can build some of the circuits we have discussed in
the course. Talking Electronics has produced a PC board
containing projects to give you "hands on" experience
with the circuits we have covered. It is called "5
PROJECTS" and contains five separate projects on a small PC
board. The first section of the PC board is a
HIGH
GAIN AMPLIFIER and shows how a transistor
"turns-on" by placing a finger on a TOUCH PAD.
The second section of the board is a
LED
FLASHER as shown in the page above.
The third is a
FLIP
FLOP as shown in the previous page of this course.
The fourth is a
FIBRE
OPTIC display. It uses the LED Flasher project or Flip
Flop project to produce a miniature display suitable for model
train layouts.
The fifth project is a
SIMPLE
SIREN. It changes tone according to the pressure of
your finger on a touch plate.
The total cost of the kit, PC board, batteries, and
including pack and post: us$15.00
It is called "5 PROJECTS" and will allow you to
build 4 separate circuits on one PC board and an extra PC
board is included for the Fibre Optics project, making 5
separate experiments.
The "5 PROJECTS" kit can be ordered by clicking
HERE.
When
you have your kit, go to the chapter:
"5
PROJECTS" and start on the projects.
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