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.