okay in the last video we saw how a
transistor is going to try to or it's
going to allow a collector current
called I sub C going to allow a
collector current to flow that is equal
to the base current that that may happen
to be flowing I sub B times some large
number like 100 so it's going to allow
this collector current is large
collector current to a flow that is 100
times whatever the base current that's
flowing and so let's we want to use that
in this circuit that we have here so the
circuit we have that switching we have
we have some sort of signal coming in
here that's switching from 0 volts to 5
volts but it's not enough current here
to to power an LED adequately so if I go
and I look at the current if I switch my
level my multimeter over here to look at
current and I look at the current here
where the LED would go I need to be on
DC milliamps you can see it's switching
from 0 to 0.3 milliamps not not a whole
lot so I have this point 3 milli amp
current that's switching on and off and
I want to use that to drive an LED but
of course if I put the LED directly in
there as we saw before the LED is not
very bright at all because we just don't
have enough current to drive it so but
we can take that current and we can run
that current through this transistor
from the base to the emitter and the
transistor will then allow a current to
flow from the collector to the emitter
that is 100 times that which you know a
point I guess this is this is a in our
case this would be zero point three
milliamps and so it would allow a
collector current that is up to 30
milliamps to flow when this base current
is flowing and of course an LED requires
you know typically 20 milliamps so this
collector current you know the
transistor would allow plenty of current
to flow more than the LED needs so the
transistor would go into saturation and
basically just allow current to flow
through it without without impeding it
at all so
if we come back to our circuit here
let's look at just the basic circuit we
had which was a down here we have a five
volt supply that is able to provide
plenty of current with the five volts
going through the led through the
resistor and then back to to ground in
the conventional flow so if I want to
put a transistor into this circuit that
will switch this circuit on and off
based on the current that may or may not
be flowing on these signal wires let's
see how we would do that so I come back
to look at my schematic here we've got
the transistor and the transistor is
going to switch the current flow on or
off depending on this base current and
so I just need to put it in series with
the the LED circuit so I can have my
five volt supply up here and actually
let me do this let me just do this in
yellow I can do my five volt supply up
here and then I'm going through my LED
and then I have my resistor of course
because I'm I always normally want a
resistor with an LED and this resistor
happens to be 220 ohms and then I will
take this and go through the transistor
before I go to ground and so right now
if I look at the circuit that I have
hooked up I have this part and it hooks
directly to ground so the transistor is
not there so what I what I want to do is
insert this transistor so the transistor
can then switch this circuit on and off
so when the base current is flowing the
transistor will allow current to flow
through the circuit and when the base
current is not there when this voltage
isn't there then the transistor will not
allow this to flow and the LED will turn
off and so the LED will then give us an
indication of whether the base current
is flowing or not and because the
transistor allows lots of current to
flow through the collector to emitter
the LED should be nice and bright so let
me just hook this up and then we'll come
back to figure out how we get how we get
this base current flowing so over here I
have my circuit and basically what I
want to do is I want to hook the
transistor up
somewhere in this circuit and so if I
look at my transistor the left lead is
the emitter the middle is the base the
right is the collector and so in this
case I want the emitter which is on the
left to be connected to my ground the
base will will leave unconnected we'll
deal with that in a moment
and then the collector I want connected
to the resistor which goes to the led
which goes to the positive voltage so
really what I'm what I'm doing is I just
want to break my circuit here put the
transistor in and then connect the
ground over here and so now what we have
is we have if we start start at the top
here we start at the positive here it's
we have positive voltage coming in going
through the led going through the
resistor going into the collector of the
transistor the base of course is
unconnected right now and then the
emitter of the transistor then goes back
to ground and so if we come back we look
at our circuit this is exactly what we
have hooked up right now and of course
the led you saw was off right now and
that's because there's no current
flowing from the base to the emitter so
how do we get a current flowing from the
base to the emitter well we have that
that five volt signal that we're that
we've been playing with that is able to
give us a small amount of current so
let's just hook that up so we have our
little mystery circuit over here and it
has that five volt signal coming to us
and then it also has its own ground
which will just go and connect to our
ground over there so basically we'll
just connect this mystery signal between
the base and the emitter like this and
it should when it's when it is on
current will flow from the base to the
emitter in this direction and when that
current is flowing it will cause more
current to flow up to 100 times more
current to flow from the collector to
the emitter and that should allow
current to flow through our led and
resistor and we should see the LED come
on so let's give it a try
so again we'll hook the ground part of
our little signal up to our ground and
also up to the emitter and then we want
the signal to flow from the bay
to the emitter so we'll just hook this
up to the base and you can already see
the LED is flashing and it's flashing
quite brightly and again that's because
the current flow through the LED is
coming from our nice 5 volt power supply
through the LED through the resistor
through the transistor but only when the
transistor is also seeing a small amount
of current flowing from its base to its
emitter and so hopefully you can see all
of that I'll zoom in just a little bit
in case you can't just to take a closer
look at the circuit when you slide
things over here so you can see and of
course I'll get rid of I'll actually get
rid of all the stuff we had up on the
top here just so we aren't confused with
what we're looking at and zoom in a
little bit more and so what's going on
just to just to be just to clarify here
we've got the voltage coming in it's
going through the LED through the
resistor into the collector of the
transistor and then when the transistor
is on it will switch it'll allow that
current to flow from the collector which
is the right pin to the emitter which is
the left pin which is then connected to
our ground so that completes the circuit
and then the LED is on when current is
flowing from the base to the emitter so
when there's current from going through
this signal then it turns the LED on and
so here we're basically allowing our
signal to turn this LED on and off even
though this signal originally was not
giving us enough current or still not
giving us enough current to actually
turn the LED on and hopefully this makes
sense why transistors are often called
current amplifiers because we're in some
sense amplifying the current that we're
getting from here to a much larger value
of current that's going through the LED
now and also why transistors might be
referred to as switches because we're
using or more specifically transistors
are often referred to as current
controlled switches and so we have a
small current here that's controlling
the transistor which is switching on and
off our LED