so the last video we had connected up
these two different circuits one that
had a five volt signal that's switching
on and off coming from some other
circuit and the other that had five
volts coming from just a regular power
supply and for some reason the five
volts coming from this power supply is
is making this led much brighter than
the five volts that's coming from this
signal and we were a little bit puzzled
as to why that might be because we we
measured both of these and when we
measured the voltage coming into this
top circuit we we saw we had five volts
when when it when it was on we'd had
five volts and the bottom one we have
five volts and we looked at the
resistors and the top resistor is 220
ohms the bottom resistor is 220 ohms so
the voltages are the same the resistors
are the same but for some reason the
LEDs are not the same brightness when
when they're both on and so if voters is
the same and resistance is the same the
next thing to kind of check out might be
might be the current how many amps are
actually flowing through this circuit
and so we can do that with our
multimeter we can we can go ahead and
set it to measure amps and I need to
move the power lead over here and so
we'll go ahead and measure we want to
measure milliamps we want DC and so what
we have to do to measure amps is
actually put the ammeter in series with
the circuit so what I'll actually do is
take the LED out of the circuit now to
both of these circuits and I'll put the
ammeter in place of where the LED was
just to see how many amps were flowing
through that part of the circuit where
the LED was so the LED was here and what
we see is that we either get about zero
amps or it looks like almost point three
milliamps so when this is on we get
point three milliamps when it's off we
get zero and it just seems to be
switching back and forth between those
two which is which is what we expect
we'd expect it to switch back and forth
between zero and some number of
milliamps so it switching back for
between zero and point three milliamps
now let's come down here and see how
many amps are flow
through this part of the circuit where
this brighter LED was and here we see 23
milliamps this is a this is almost a
hundred times the amount of current that
was flowing through the top circuit when
it was on so we have 23 milliamps here
and the top circuit we had 0.3 milliamps
and so that that right there is
basically the explanation of why the LED
on the top circuit was so much less
bright than the LED in the bottom
circuit the voltage is the same the
resistance is the same but the current
is almost a hundred times less and so
the LED is much less bright and so the
question is what can we do about this so
of course the bottom we have plenty of
current because this is a power supply
and power supplies are generally
designed to be able to deliver a lot of
current at and maintain their voltage
whereas this signal is is really just
trying to give us I mean it's hard to
say what what the signal is trying to do
but in this case oftentimes a signal
will will will indicate you know whether
it's off or on with a voltage but it may
or may not allow you to draw a ton of
current from it so a power supply is
designed to let you draw a ton of
current you know a signal like this
might not necessarily so again if we go
back and look at the voltage here when
this is on we're definitely getting you
know over 5 volts so the signal is is
using that 5 volt potential to indicate
whether it's on or off so if I see 5
volts then that's telling me this signal
is saying on if I see 0 volts the signal
saying off but when I try to connect any
kind of load like this led to the signal
the LED tries to draw you know you needs
to draw some current in order to light
up if it wants to draw 20 milliamps
ideally but there's there isn't 20
milliamps available you know this this
signal just can't provide 20 milliamps
it can only provide you know almost uh
you know hundreds of that and so the LED
ends up not being very bright and when
the LED does draw that much current
relative to what this can provide this
can't even maintain the 5 volts anymore
and that's why we see the voltage is so
much less
so the question is how can we use this
voltage differential somehow detect that
there's a voltage change there and still
get current through the LED and the
answer to that is an ingenious little
component called the transistor which
you've probably heard of and I have some
transistors here these are transistors
and hopefully you can see them down
there so these are transistors and
basically a transistor can be thought of
in a couple ways it can be thought of as
a switch or a current amplifier and
don't worry too much if that doesn't
make sense in the next video we'll go
through how we can use a transistor
basically to take this five volt signal
at least if we get the LED out of the
way it'll be 5 volts how we can take
this 5 volt signal that's turning on and
off and use that to switch an LED on and
off and have the LED be bright and have
this 5 volt signal not be not being
strained not be dropping so much
by putting the LED right right in the
circuit so we're going to use
transistors to do that and in the next
video we'll talk about how how the
transistor is going to help us do that
and then we'll we'll build it and see if
we can get it to work