so in the last video we built this
circuit with an LED and a resistor and
the reason we were using this resistor
to limit the current through the LED so
that so that the LED would only draw you
know the 20 milliamps of current and it
would and the resistor would drop any
excess voltage so the LED would only
would only operate at the at the 1.9 or
2 volts that it requires and we were
connected up to this you know laboratory
power supply that allowed us to adjust
the voltage with with quite a bit of
precision but now that we have this
resistor in here we don't need that that
precise voltage source anymore and we
can we can look at some other options
which which is great if you want to
build this at home and you don't happen
to have a very expensive power supply so
let me make a little bit more room here
and one of the other options are the
most convenient option maybe is is a
battery so this is just a simple 9-volt
battery and again because we saw with a
resistor particularly this this is a
1000 ohm or 1 kilo ohm resistor we can
we can put quite a bit more voltage in
here and the resistor will drop the
excess voltage and so the easy way to do
this is with one of these little one of
these little battery snaps here and so
the battery just snaps into here and
then you can you can connect this to the
circuit and so will connect just the
same way the negative side here and the
positive side here and that corresponds
to the the way that the LED is connected
and then if we connect the battery the
LED lights up so pretty simple and you
and you can certainly try this at home
and if we're interested so it looks like
the LED is working fine it's a you know
reasonable reasonably bright it doesn't
appear to be you know hot or drawing too
much current nothing nothing smells like
it's burning which is always always a
good sign but if you're interested to
see just exactly how much voltage the
LED is drawing we can measure the
voltage across it if you have a if you
have a voltmeter and any any voltmeter
will do I'll use I'll use this one but
you can use any any voltmeter you have
and we'll just set it to
DC voltage and it says that it's
measuring volts DC and first what I'll
do is I'll measure the voltage across
the entire circuit so from this side
where then where the negative terminal
is coming in from the battery to the
positive terminal and you see it says
it's about nine volts which makes sense
because we have a nine volt battery now
the resistor is supposedly dropping some
voltage so the LED operates with the
appropriate voltage and if we look at
what the manufacturer says manufacturer
member said that this LED should have
between 1.9 and 2 volts and it seems to
be operating fine so hopefully that's
the voltage that it's getting if not
then we need to kind of maybe look at
and maybe a different value for that
resistor but we'll see what we'll see
what we have so if I look at the voltage
across this LED I see 1.95 you know or
so volts which is right right in that
range from 1.9 to 2 volts which is which
is perfect and explains why the LED is
working just fine and just one more
thing I'll measure is look at the
voltage on the resistor so from this
side of the resistor to the other side
I see seven point one two volts which
makes sense so the resistor is dropping
seven point one volts the LED is
dropping one point nine five volts and
the battery is providing nine volts
which all makes sense so if you have the
battery is plus nine the resistor is
dropping seven point one the LED is
dropping 1.9 all of that together
cancels out and we are obeying
Kirchhoff's voltage law which is that
the all of the voltage sources add to
zero and so in this case the battery is
a positive voltage source and then the
drops across each component is a
negative voltage source and all of that
adds to zero which which works out
nicely so this is a great way that you
can build this circuit at home another
way that I actually really like is
instead of using a battery one of the
things about batteries of course is that
you know they they get used up and you
have to have to buy new ones or recharge
them
so what I what I find is convenient and
actually let me make sure I disconnect
this here so that we don't short this
and damage the battery what I find is
convenient is these little USB chargers
so this plugs into the wall of course
and it gives you USB and USB has a 5
volt a 5 volt DC voltage available on it
and so you can use these things which
you probably might have around the house
or you can get relatively inexpensively
and you can get 5 volts out of this and
just to double check if we if we take a
close look at this and you should check
if you're using one of these you should
check the one you're using but let me
just take see if I can zoom in here if
we look very closely right here it'll
say input is 100 to 240 volts 50 60
Hertz at 0.15 amps and that and that
matches what what you would get from
from the power source from the wall in
in the US or overseas really any country
100 to 240 volts the output is 5 volts
and this little symbol here means means
DC Direct Current which is what we want
1 amp which is great so 5 volts at up to
1 amp is what we get out of this so in
order to in order to use this we need to
find some way to kind of plug in to the
other side of here and get the 5 volts
out and so a way to do that is to get an
old USB cable or one that you don't you
don't need or don't aren't using for
anything else and let me let me just
zoom out here just a little bit more a
little bit more so this is just a USB
cable and in this end of course plugs
into the wall adapter and this other end
is this is just like a mini USB but what
we want to do is plug into the
breadboard so this isn't going to plug
in in here so what I can do is just and
again this is obviously one that I'm not
using for anything else is I can just
cut this off
and of course don't do this while it's
plugged in cut this off and if I if I
kind of dig into this sheath here let me
just kind of nip away at the outside
chief and I'm being very careful not to
cut the wires inside so I'm just taking
this little plastic sheath off and let
me let me zoom in a little bit so you
can kind of see what I'm doing but what
I've done is I'm taking the the outside
plastic off and inside there's some
shielding it looks like and you may find
something different when you take apart
your USB cable but inside there's some
shielding and inside of that there's
some other little wires that we want to
get at so let me try to cut some of the
shielding apart
it looks like there might be a little
bit of plastic coating here let me get
the plastic coating out of the way again
I'm not really sure what to expect here
each each cable is gonna be made a
little bit differently this one seems to
have a little plastic coating and
there's some some stranded wire
shielding that I'm just gonna pull back
for now and then there appears to be
some foil here as well I'll pull the
foil back and then inside we find we
find a few conductors we find a few a
few wires here and your USB cable might
be constructed a little bit differently
you might have some different layers in
here but you should find you should find
four wires inside just as I have here
and there's a there's a black a red and
then in this case a green and a white
but what we're interested in is the
black and the red because the black and
the red are the positive and negative
five volt source and then the the green
and the white are used for data if this
USB were plugged into a computer then
you might want to send data across it
and that's what these are used for I
would definitely not use your computer
to power our experiment here because you
could short something out and you know
the worst case you could you could
damage your computer and you don't want
to do that so definitely only if you're
gonna do this only power through one of
these little wall adapters because the
worst case is you might damage the wall
adapter but this is relatively
inexpensive you don't want to risk
damaging your computer so with that out
of the way basically the next thing to
do is just kind of get all this
shielding out of the way and I'm just
going to kind of get it all together
here and cut it off because you don't
want any of this
this is they just put this in here to
prevent to prevent interference with the
data signals but we don't we don't have
any we're not using this for data and
like I said the you're interested in the
in the red and the black because that's
where the the five volt power is the
other conductors that are in here we can
actually just cut them off because we're
not using them and so I'm I'm left with
the red in the black and I could I could
clean this up a little bit more but now
what I want to do is strip the
insulation off of these then I'll use a
little wire stripper here and strip the
insulation off of the black strip the
insulation off of the red and here I
have here I have the red and the black
which should give me five volts and what
I'll do is I'll go ahead and connect
these into the breadboard so these are
stranded so they're a little bit finicky
to get into the breadboard but I'll
twist it together the black again is the
negative side of the of our power source
so I'll put that in here actually this
is a little bit finicky because it is
stranded I want to make sure all the
little strands are twisted together so
that when I put it in the breadboard
they all they all go in so that's that
and then the red side which is the
positive side of our voltage goes over
here just like it did when we were using
the laboratory power supply and just
like we did when we were using the
battery and so that completes our
circuit so I'll zoom out here just to
see what we have and again
we've got we've got our circuit here and
now it's connected to this USB cable
that I'm connecting I'm going to go
ahead and plug into the wall and again
you could well you shouldn't plug this
into your computer your computer will
put out five volts but I wouldn't
definitely not do it because
if if anything here is shorting or
anything you know there's some other
voltage that's coming in here you you
could damage your computer or you
certainly risk doing that and it's just
not worth that risk Peters are kind of
expensive so I'm just gonna plug this
into the wall and they just reach down
here and it's plugged into the wall and
the LED came on as you can see there and
this is a great way to to power a
circuit like this and I'll just zoom
back in so we can take a look at what
we're doing this is a great way to power
a circuit like this and again we can
kind of explore what's going on with the
voltmeter so like before we can measure
the voltage across the entire circuit or
really across the power source in this
case we have five volts first with a
nine volt battery we had nine volts but
this is a five volt USB power source so
we're getting five volts but that's
still fine because if we look across the
LED it's 1.91 it's a little bit less you
know which which is kind of because we
have a little bit less voltage going in
but still like perfectly what that LED
wants and then again if we look across
the resistor we see the resistor is
dropping the rest of the voltage and so
this is another great way to power a you
know an experiment like this and I
actually prefer this because when we
start getting into digital logic you'll
see that five volts is is typically the
voltage we want to use for a lot of
digital logic and so setting up
something like this if you can is a
great way if you're going to start doing
a bunch of projects with digital logic