okay so what I have here is I've taken
this transistor circuit that we just
built and built another copy of it just
up here and and just sort of connected
them in parallel here so so basically I
have two independent copies of this
transistor circuit so just just to
clarify what's going on if we look at
the bottom circuit we have our power
coming in going through our led through
the resistor into the the emitter of the
reading this is the collector of the
transistor and then of course our base
is connected to our signal that's coming
in from our little mystery device and
then the current is flowing out of the
emitter back to back to ground over here
and then on top I have the exact same
thing I've just connected the positive
power supply up to the top here and so
that's going through this led through
this resistor and then this transistor
is is going from the collector to the
emitter and then back down I've just
connected the ground from from the top
here to the bottom so the positive
voltage and the ground are disconnected
so that these are both getting power and
then I've also connected the signal to
both bases that's what this yield of a
yellow jumper is is I've connected the
signal that's coming in from our little
mystery device to both bases and so now
we see that this transistor is switching
this led this transistor is now
switching this LED and the whole reason
I did that is just so that I can make
some modifications to one of these
circuits and keep the other circuit just
as it was because I hopefully we
understand what's going on with with
this circuit down here and I can make
some modifications to this and just sort
of compare what's going on between the
two so I'm going to make a modification
and basically the modification I'm going
to make is I'm just going to remove the
LED from the top circuit so I'm going to
take my power and instead of going to
the LED I'm going to go to the resistor
and I'm going to remove the LED from
from the circuit altogether so now the
power for the top one is coming in here
the voltage the positive voltage is
connected to the resistor the resistor
through the transistor and then to
ground and there's no LED so this is not
maybe not as quite as interesting
because there's nothing to look at but
the transistor is should be still
switching because it's still connected
to
our signals so let's take a look at a
schematic of what we've got so this is
the schematic and basically I have the
positive 5 volt supply here going
through the resistor through the
transistor to ground and there's no LED
now and then I still have our little
mystery device sending its signal and
creating the the current that's
switching on and off from the base to
the emitter like this and so that base
current is still switching on and off
and so as a result the the collector
current should still be switching on and
off it's just going through the resistor
to ground and there's no LED so it's
maybe not as interesting to look at but
let's think about what are some
different voltages that are that are
along here so if we look at the very top
here if we look right here at the
voltage that's there and and when I say
voltage at a point of course you can't
you can't talk about voltage at a point
voltage has to be between two points so
I'm always going to be referencing to
ground so the voltage between here and
ground is is five volts it's always five
volts right this is just this point here
if we look at that particular point it's
always connected to five volts so that's
always five volts
conversely if I come down here and I
look at the voltage at this point
reference to ground well it's connected
directly to ground so there's zero
voltage there but what about this point
right here what is the voltage here both
when this current is on when the
transistor is on and when the transistor
is off so let's let's consider the
transistor off right now so if the
transistor is off then the voltage here
is going to be five volts because again
we can kind of imagine that the
transistor is not there all together so
there's no there's nothing there's no
conductivity through this transistor and
so this point has a resistor but it's
connected to five volts so there's five
volts at this point with reference to
ground because the sort of if you think
of the path of least resistance for
electrons they're going to go through
this resistor to five volts
there's no other place for them to go
because the transistor is off
now when current is flowing here the
transistor is on then this point here is
going to have what kind of voltage well
right now if the transistor is on the
path of least resistance for electrons
that come in here is going to be to
ground and so the voltage if we measure
the voltage here will actually be zero
because the path of least resistance is
to ground not through this resistor to
five volts so this is kind of
interesting because just just to clarify
here when the transistor is off this
point let's call it you know point I
don't know
it's call it point a right here a is
equal to five volts right the transistor
is off so this is the potential here is
is connected through this resistor to
five volts but when the transistor is on
and in saturation which means that the
transistor is basically completely on
it's not it's not restricting current at
all which means we have enough current
here to create that saturation so when
the transistor is on which means we have
current flowing here then the voltage at
a is equal to zero volts because again
that the path of least resistance is
going to be from a to ground so in
effect a is connected directly to ground
and so the voltage at a is is going to
be zero with reference to ground and so
this seems maybe a little bit backwards
then then what you might think when the
transistor is off we have 5 volts here
when the transistor is on we have zero
volts here so maybe we could connect an
LED to this point a maybe we could
connect an LED here and actually kind of
see this sort of backwards if you will
voltage so let's let's let's look up an
LED right here and we'll connect it to
point a there and then of course we'll
connect this side down to ground so
essentially we're connecting the LED
from from the collector
to the emitter of of the transistor
we're just connecting the LED across so
let's do that in the circuit here so I
have my LED and I'm just going to
connect it from just across the
collector to the emitter like that and
sure enough we see the LED is on when
our signal when our when our base
current is off and it's off when our
base current is on and we can see that
of course because this bottom circuit we
haven't modified at all and so you know
this LED is on when the base current is
on and this current ability is off when
the base current is on and so
essentially we're seeing the the
opposite of what of what our signal is
so our signal remember is turning on and
off the same time as this LED but this
led up here is giving us the opposite of
that so that's very interesting because
maybe there's maybe there's some
application to that but just to clarify
before before I end this video just to
clarify kind of to give a little bit
more intuition about what's going on you
can imagine a circuit here with a
battery so we'll have a battery here and
we'll connect the battery through a
resistor to a switch actually we won't
even put a switch here we'll put an LED
we'll have an LED and then back to the
battery and so this is our positive
terminal of course this is our negative
terminal so in this case the LED would
be on no surprises there but if I put a
switch over here in parallel with our
LED and so I have a switch right here
like this when the switch is off in
other words the switch is open then the
LED is on right the circuit is going to
flow this way but if I turn the switch
on that is closing the switch then I
actually create a short circuit around
the LED and the LED will go off the
current will flow through the resistor
through the switch which offers less
resistance than the LED and then and
then back around and so in this case you
can see the switch being on turns the
LED off and the switch being off turns
the LED on and that's exactly what's
happening with the transistor because
look at the transistor we have the
transistor now in
parallel with with the LED and so when
the transistor is off current flows
through the LED when the transistor is
on current flows through the transistor
basically a short circuit through the
transistor bypassing the LED and the LED
turns off so hopefully that that was
interesting I think it's interesting
because it gives us the ability now to
basically take a signal like this and
and basically invert it and get the
opposite of that signal by using a
transistor