Welcome back. 
We're, we're going to start a couple of 
lessons on real applications, and we're 
going to show some lab demos. 
And these lab demos fit in a kind of 
nicely end off this whole module because 
we're looking at particular applications. 
And the applications are going to be 
sensors, physical sensors in other 
devices, in other applications. 
And how resistive resistors are used in 
those sensors. 
Let's define a sensor. 
A sensor is a device that converts a 
physical quantity to an electrical 
signal. 
Number one, it convert physical 
quantities to electrical signals because 
we want to be able to record them we want 
to be able to data log them. 
Or perhaps, we want to be able to use 
that information to control something for 
example the engine speed of your car 
based on an oxygen sensor. 
Or maybe the, the temperature in a room 
based on the thermostat reading. 
There are some very typical variable 
resistors that are used in sensors. 
And they're ones that resistance varies, 
again, is the, since their physical 
quantity varies. 
So, piezoelectric resist, piezoelectric 
resisters, when that, where the 
resistance goes down as pressure goes up. 
And in, and this case, in a lot of these 
cases the relationship is not linear. 
So, the resistance goes down in a 
non-linear fashion as the pressure goes 
up. 
A thermister is a resistor that the 
resistance goes down as temperature goes 
up. 
In a strain gauge, the resistance goes up 
as a strain gauge elongates. 
And a strain gauge is just a simple 
device that can stretch. 
A potentiometer is a varying resistor 
that varies with position. 
So then, the lab demo is variable 
resistors used in sensors. 
Looks like it's physical resistance 
resistors that vary their resistance 
based on some physical quantities. 
This particular resistor varies it's 
resistance based on pressure. 
At the end right here is a circle. 
Inside that circle, there are, there's 
some pressure sensitive ink. 
And you can see a, in the outline here, 
there's a circuit that goes to that 
pressure ink, through it and back. 
Now I want to hookup my leads, my ohm 
meter to these leads. 
These leads coming out of the sensor out, 
out of this device, okay? 
Now let me try that. 
I'm going to start pressing on this. 
And I'm going to be increasing my 
pressure. 
You see it it's like 10 megohms. 
As I increase the pressure down to 5, 4 
megohms, 3, really pressing hard now. 
I'm down to 1.8. 
It's about as hard as I can press very 
easily on this. 
And you saw, so as I increase the 
pressure, the resistance went down. 
Let me go in the other direction. 
I'm going to start loose, letting up on 
the pressure here. 
And it's going 5, 6, 8, 10, 11, 13. 
So as I decrease the pressure, the 
resistance went up. 
So, that's an example of a physical 
resistor that changes its resistance 
based on a physical property. 
Another very common one that you see is 
called a Potentiometer. 
It's a Potentiometer here, this is a 
round one, a rotary one. 
Let me go ahead and hook up. 
I put alligator leads on here. 
This is a resistor that varies its 
resistant based on angle. 
So, it starts with about about 0.2 ohms. 
And as I change the angle, you can see 
that the resistance is increasing. 
The larger the angle, the larger the 
resistance. 
All the way up to maximum 9.6 kilo ohms. 
So from zero all the way to 9.6 kilo ohms 
based on the angle that I rotated 
through. 
So we see that these resistors, we, we 
can vary resistance with respect to these 
physical quantities. 
But the problem is, I can't use 
resistance in a measurement system. 
I'm using resistance here. 
I'm measuring resistance with my ohm 
meter. 
But what most measurement systems need is 
a voltage. 
So, I would like to convert this change 
in resistance into a change in voltage. 
Because voltage is something that I can, 
I can use. 
I can input to my system. 
So, let's convert this, try to convert 
this Potentiometer into a sensor. 
Looking at the schematic here, I'm 
building this circuit. 
Here's my potentiometer just by itself. 
And I built the circuit around it. 
So, I put this what we call biased 
voltage across it. 
And usually, we'll pick, like 5 volts or 
10 volts constant voltage around it. 
High on one side, low on the other. 
And we have a pick off point and this is, 
in this particular case, it's, it's the 
additional lead that I had. 
I can move this pick off point up or 
down, and the way I move it up or, up or 
down is by changing the position. 
Let's, let's look at this schematic over 
here. 
It's the same schematic. 
Oftentimes we do that, as long as I've 
defined a ground, then this point is a 
potential of this many volts with respect 
to ground. 
The reason I wanted to show it this way 
is it's a little bit more clear that this 
is a linear Potentiometer, a slider. 
A slider meaning that I take the pick-off 
point, and I slide it up or down. 
So I'm, could be picking off down here, 
or here, anywhere along this point. 
If I pick off down here, I should be 
getting zero voltage because I'm 
grounded. 
As I start moving up higher, then the 
percent of the total resistance that is 
below my pick off point is larger. 
This is the voltage divider law. 
The larger the percent of resistance that 
I'm, I'm measuring, the larger the 
voltage will be. 
Now over here, I have distance marked. 
And this distance can be meters, 
millimeters. 
That's measured y at being some distance. 
If my pick off point is here, my distance 
will be zero. 
And the larger the voltage is and the 
more I move that slider up, the larger 
the distance is, the larger the voltage 
is. 
So in other words, my voltage that I read 
is proportional to this distance. 
Let's look at a, an angular type of 
potentiometer. 
Sometimes we call it a heliopot. 
The one that we had, that I showed you 
before, actually went around more than 
360 degree turns. 
But you can see, and say, I put it at the 
lowest mount, I get zero voltage. 
And as I increase this angle, then I get 
a larger, and larger voltage until I 
reach the maximum voltage. 
And if I want to look at this sensor, 
going back to this, I want to convert 
this, it, this Potentiometer into a 
sensor. 
So I need to hook it up to a power 
supply. 
This lead is for a five volt power 
supply. 
That's the ground on it. 
And I'm going to match that with the 
ground on digital voltmeter. 
And, and this is the other end of the 
lead. 
So, I've given it a plus 5 voltage power 
supply. 
And this is my pick off point. 
We convert that to volts. 
[NOISE] So, now I'm measuring voltage. 
If I start out with the smallest angle, 
and I start increasing angle. 
I now increase the voltage. 
So when I get about half way, this is 
about halfway. 
The halfway angle I get half the voltage. 
And as I keep increasing this, then I get 
up to the maximum voltage. 
Now, you can imagine instead of this 
being a screwdriver, maybe this is a 
lever, maybe there's a customer turning a 
lever. 
And you want to see well where did they, 
how far did they turn the lever? 
You can use a Potentiometer to measure 
that, or somebody's turning a dial. 
And you want to see how far they've 
turned the dial. 
Well, use a Potentiometer. 
You can even use this on a motor shaft to 
see what the angle of the motor is. 
And that's how we can convert a variable 
resistor into a sensor. 
To summarize, resistance often varies 
with physical properties. 
Sensor utilize this property to convert 
physical quantities to voltage. 
And then, we use those voltage signals to 
record the data or to be able to use it 
to control other systems. 
In the next lesson, we will use a 
Wheatstone Bridge as an application to 
build a particular type of sensor. 
Nathan and I will see you online in the 
forum.