Welcome back. 
This is Dr Ferri and we're continuing in 
our course on Linear Circuits. 
We're almost finished with this module. 
We wanted to finish it out looking at 
some applications. 
In this particular module, we'll look at 
a lab demo, applications of capacitors. 
So far in this module, we have covered a 
lot of theoretical background on how to 
analyze circuits with capacitors and 
inductors in them. 
So at this point, we want to look at some 
physical applications of these quantities 
of capacitors and inductors. 
Let's start out with capacitors. 
Now we use capacitors all the time, in 
the field of electrical and computer 
engineering. 
We use them in circuits because we like 
the behavior that they give to circuits. 
We like the reactive behavior. 
We like the transient that they, that 
they cause in circuits. 
Now we also use them as miniature batters 
to hold charges. 
For example in charge pumps they're used 
this like little miniature batteries 
they're also used in memory in computers. 
The main memory or DRAM in a computer has 
capacitors in them. 
Every bit is determined. 
Every bit of information is determined by 
whether a capacitor is charged or 
discharged. 
You've got a one if it's charged. 
And a zero if it's discharged. 
But those are all ECE applications. 
Let's look outside the field of ECE. 
In that case, the most common application 
is as sensors, capacitive sensors. 
Capacitive sensors work by varying the 
capacitance. 
If you look at this image here of a 
capacitor, this is what we've looked at 
before when we first started talking 
about capacitance. 
We showed that the capacitance is 
determined by this formula here. 
So, depends on the permittivity, the 
cross-sectional area of the plates, and 
the distance here between the plates. 
If you vary any one of those, you vary 
the capacitance. 
Another effect is by looking at the 
fringe effect, which is where the 
electric field bends around the outside 
or the edges of the capacitor. 
So as a common application of varying 
capacitance is a fluid level sense. 
Fluid level sensor displaces the material 
between the plates from air to fluid. 
So as a fluid level grows, then the 
material between these plates changes 
which means that it's going to change 
this, this permittivity capacity. 
Or this permittivity constant. 
So in this lab demo, we will look at some 
physical examples and applications of 
capacitance. 
Let's take a look at a capacitive keypad. 
Some capacitive touch screens and keypads 
have a hard surface, like this one. 
A finger close to the surface may act as 
a ground to reduce the local charge in 
the capacitors. 
Or your finger may change the fringing 
effects by near the edge of the plates as 
a result the effective capacitance 
changes. 
Now some, some keypads have a squishy 
surface when you press down, and in these 
cases pressing down changes the distance 
between the plates. 
A similar phenomena happens in capacitive 
microphones. 
Let me show you one here. 
This is a capacitive microphone. 
It's a very cheap microphone. 
It's omnidirectional. 
That means it's sensitive to sound that 
hits it in along this surface right here. 
Let me take one apart so you can see the 
inside. 
So I took off the outer covering there. 
And the next that goes on is one of the 
plates that capicator. 
So that's a capacitor plate. 
Then you've got a spacer. 
It's a red ring, o-ring, is a space in 
there. 
And then to top it off, you've got the 
second plate right here. 
Now inside this plate, inside the round 
part. 
The outer edge of this round part is kind 
of hard. 
It keeps it, the spacer. 
But the inside is a very thin, metallic 
flexible membrane. 
So, when the sound waves hit this, that 
flexible membrane on the inside here. 
On the inside there goes in and out. 
And that means that you're going to have 
the distance between the plates change. 
And as the distance gets smaller, the 
capacitance increases. 
It's hard to measure capacitance 
directly. 
So instead, we measure how a circuit 
response changes when we change the 
capacitance. 
To demonstrate that, let's look at this 
circuit. 
This is an RC circuit, and let me show 
you what's going on here. 
We've got an RC, a resistor and capacitor 
In series with one another. 
Along this rail here is the ground. 
And this is the the voltage source right 
here. 
So the voltage source goes into the 
resister which is in series with the 
capacitor, and that's connected to 
ground. 
I want to look at the voltage source, so 
my oscilloscope is in these green wires 
is showing the voltage source. 
And that's between, you know it's 
connected to the voltage source and to 
ground. 
And I also want to look at the voltage 
across the capacitor. 
So it's on this side of the capacitor and 
then to ground. 
[SOUND]. 
Now I'm showing this capacitor here. 
I don't have it hooked up right now. 
I wanted to be kind of flexible with it. 
It's, it's connected to this other 
capacitor right there, because it's in 
the same hole. 
This hole right here, so this, the two 
capacitors are connected right here, and 
then on the other side [SOUND]. 
I'm going to touch it to the other leg of 
the other capacitor. 
So when I have it loose, it's open loop, 
and it's, it has no affect on the 
circuit. 
When I touch the capacitor's legs like 
this, that means I'm increasing the 
capacitance. 
Because remember, capacitors in parallel 
add. 
So I increase the capacitance, decrease 
it, increase, decrease it. 
So it's as if I'm touching something, I'm 
touching a touch pad. 
I'm increasing the capacitance 
momentarily while I touch it, then I let 
go. 
So let's look at the response, let's look 
at the screen. 
The screen shows the oscilloscope, 
response. 
In green is shown the voltage source. 
So I'm showing this with a square wave, 
because what I'm really interested in 
doing is triggering the transient 
response. 
So I have to have a change in the voltage 
to this circuit to be able to see that 
transient response. 
Now if I increase the capacitance by 
touching that other capacitor to it, you 
can see, suddenly the change. 
The time constant increases, while the 
time constant is RC, So increasing the 
capacitance increases that time constant. 
The way I can detect the change in 
capacitance, is I can put a separate 
circuit in there with a comparator. 
A comparator is a circuit I can build 
that, that checks when this voltage of 
the blue. 
So checks when this blue voltage, or blue 
signal, the voltage crosses capacitor, 
reaches a certain level. 
For example, I might want to see when it 
reaches this level that's indicated by 
the red line. 
And as soon as it reaches that level, 
then I trigger a pulse. 
So, if I look at the time difference 
between when the voltage source goes 
high, that's the green line going high. 
And the blue line crossing that, that 
threshold line, it's longer time delay 
than in this case. 
And by that shifted time delay, I can 
tell When this capacitance is increased. 
In other words I can tell when the keypad 
has been touched. 
And touchscreens work, capacitive 
touchscreens work in the same way, but 
they also have spacial configurations. 
So you have to figure out where you have 
touched it on the touch pad, not just 
when you touched it, but where you 
touched it. 
Another device that uses varying 
capacitance is a tuner. 
This tuner has these plates on it, and 
these plates actually are the, the 
capacitor plates. 
By rotating the device, you change the 
distance between the plate, or the 
overlap area of the plates. 
You can calibrate this to know the di, 
the capacitance as a function of angle, 
so let me turn it. 
Right here, there's no overlap between 
the plates. 
And here, I start to get overlap. 
And about half overlap, so I'm increasing 
the area of the capacitance plates. 
And that means I'm going to be increasing 
the capacitance. 
Here, I've got 100% overlap. 
Not only that, but if you look at the 
difference between these plates, they're 
now smaller because of, I'm half the 
distance. 
And then as I keep moving it around I'm 
changing that overlap and that distance, 
I'm changing the capacitance. 
This is used as in an old-fashioned 
radio. 
So a dial was attached to this as you 
tuned. 
You'd change the dial, you were tuning 
the radio, you were tuning the frequency, 
by changing that capacitance. 
Another device that works on the same 
principle Is an antenna tuner. 
This is a case where it has two sets of 
plates. 
These up here and these down here. 
And it's got a rotary dial here. 
So as I turn this, I'm making these 
plates in the center, rotate around. 
At this point they're half overlap the 
top ones, and half overlap the bottom 
ones. 
If I keep rotating, at this point, they 
completely overlap with the top points 
and don't overlap with the bottom points 
at all. 
So by changing the way I hook this up, I 
can change the capacitance a great deal, 
and this is used to tune antenna. 
So, in summary, we showed capacity 
sensors; such as, touch pads and 
capacitive microphones and antenna 
tuners. 
And, there are a number of people that 
helped to make this demo, possible and in 
the next lesson, we will go on to look at 
examples of inductive. 
Inductance in, physical applications. 
Thank you.