so oftentimes in electronics we find
ourselves interfacing with with other
circuits and I have an example of
another circuit here and this is this is
some other circuit that I built that I'm
actually not going to talk too much
about what it does so I'm gonna I'm
gonna kind of push it aside over here so
that we're not looking at it because
like I said oftentimes electronics we
find ourselves interfacing with other
circuits like that that we don't know
all the details of exactly how they work
and we don't really want it we don't
necessarily even need to know all the
details in order to interface with them
you know like if I want to build
something that plugs into a computer I
don't need to know all the details of
the circuitry inside that computer I
really just need to know what the signal
is that it's sending me and how to
interpret that signal and so I have this
circuit here it it is sending me some
sort of signal across these across these
two wires and I want to look at what
that signal is and see if we can do
something interesting so I have a
voltmeter here so we can start by just
looking at the voltage across these
these two wires to see what's going on
and if we take a look at the voltmeter
we we see something kind of interesting
here it seems like this is switching
between zero volts and a little over
five volts so zero volts 5 volts zero
volts 5 volts and so it looks like the
signal that I'm being sent is is is
either 0 volts or 5 volts and and I
happen to know since I built that
circuit that that's exactly what it's
supposed to be doing it's supposed to be
just switching between those two those
two voltages so that's interesting so
though I have I have an LED here so
maybe we can maybe we can use this LED
and hook it up in some way to the signal
in so that the LED turns on and off when
the signal switches between 0 and 5
volts so I'll get this out of the way
and of course when it said when it's at
0 volts that that's pretty easy the LED
is going to be off when 2 5 volts we've
we kind of already looked at how to hook
an LED up to a 5 volt source and so we
we found that we needed a resistor of
some sort and so I have an LED and the
resistor so we should be able to hook
the resistor and the led up to this and
when this signal is at zero volts the
LED should be off
if we hook it up right when the signals
at five volts the LED will turn on and
so we should actually be able to get the
LED to flash on and off by by using this
signal so I have the resistor here this
is a 220 ohm resistor which is the same
one we used before when we when we
calculated how big of a resistor we
needed to hook an LED up to five volts
and I have of course the LED so we'll
hook will hook all this up and so we
have the the negative the electrons of
course are flowing in the negative side
here through the resistor through the
LED and then out the the positive side
here and we see the LED flashing on and
off and in case you you don't see that
I'll turn the lights off and you should
see the LED is flashing on and off so
when the circuit is sending us five or
when this signal is sending us five
volts we see the LED come on when it's
at zero volts we see the LED go off and
so it's flashing on and off like that
but one thing that seems kind of a
little bit mysterious here is that for
some reason it looks like the LED is not
as bright as it was when we just had it
hooked up to five volts before so I
don't know if I'm just imagining that or
what's going on but just to just to
double check that we have this this five
volt this 5 volt little USB adapter
thing that we modified so that we could
we could get 5 volts out of it and so
I'm gonna go ahead and just actually
hook up an almost identical circuit down
here with another resistor and another
LED so I'll hook my 5 volt source into
just right below here into the end of
the breadboard and we'll go ahead and
get another resistor this is the exact
same sort of resistor this is a 220 ohm
resistor and we'll get the exact same
sort of LED this is the same yellow LED
that we've been using so we'll plug that
in right here so this is the same
circuit that we have up here it's just
that now instead of this sort of mystery
5 volt signal that's just turning on and
off coming in here we have the 5 volts
from from this from this power adapter
from this little USB adapter that gives
us 5 volts so we'll go ahead and
plug this in and the LED comes on and
definitely right away you notice that
this bottom LED is much brighter and
especially if I turn the lights off you
can see the bottom LED is just you know
really a lot brighter than the top one
when when the top one is on of course so
so what's going on here let's let's see
if we can do a little a little detective
work here and find out what might be
what might be the difference so let me
get my voltage my meter probe here so
I'm gonna take the LEDs out for a moment
and just measure things so this bottom
we think of is five volts so let me just
double check and sure enough it's five
point one a little of our five point one
volts and I'm gonna go to the top and
just compare and see what's going on up
here and up here it's either zero or a
little more than five point one so when
it's on its it's the same voltage and of
course we've got the same the same
resistors here too and just a double
check I mean who knows maybe these
resistors are kinda weird we can
actually measure the resistors as well
so you know the bottom one here is two
hundred seventeen point six ohms and
it's a 220 ohm resistor so I guess
that's because that's close enough and
the top one we can measure this resistor
two hundred seventeen point seven ohms
so the resistors are the same basically
the same the voltage is the same the
LEDs these are the same LEDs as each
other so so what's going on you know
down here let me just double check this
again go back to our voltage reading
five point one volts we connect our LED
in here that comes on nice and bright we
can just double check all of our other
voltages here so the resistor is
dropping three point one volts the LED
here is taking almost two volts which is
which is right what it should be taking
so of course this bottom one looks okay
let's see if we see those same things on
the top one so I'll plug in here and
just to see
yep we're still getting the five point
one volts when when it's on so I'll
connect the LED and the LED starts
flashing but something interesting is
going on here you notice now the voltage
is only going up to one point eight
volts and when I had the LED out of
there let me take the LED out it goes up
to five point one volts when I put the
LED back in the voltage only goes up to
one point eight volts now I'm not
changing anything with with the circuit
that's that's sending us this signal
it's only just when I add the LED for
some reason we're not getting the the
same amount of voltage on this signal
when I put that LED in the circuit so
that's that's kind of curious so that
might be a clue as to what's what's
going on here but I'm actually gonna end
the video here because I want you to
think about what might be the difference
between the five volt power source at
the bottom and this five volt signal at
the top you know both of these you know
we measured are giving us five volts but
what might be different about the power
that we're getting from this five volts
versus the power we're getting from this
five volts and and you should think
about this before you watch the next
video see if you can come up with any
any hypotheses of your own and then
we'll we'll dive into this a little bit
more in the next video