Hello again. 
We're going to be finishing up our lab 
demos on resistors used as sensors. 
Today we're going to be talking about a 
Wheatstone Bridge. 
And again, this kind of is, the capstone 
of this whole section, this whole module. 
You've already seen a schematic of a 
Wheatstone Bridge, and you've seen how to 
analyze it. 
And what we've found just to remind you 
is that you set up the Wheatstone Bridge 
to balance and you balance R2, which is 
right here. 
And R3 show that this voltage across here 
is 0. 
And you can often do that by trying to 
put a, ammeter across here, between, a 
and b. 
And make sure that there's no current 
flow here. 
And once you do that, once your 
Wheatstone Bridge is balanced. 
You have this relationship here between 
the, the resistors. 
Now when it's used as a, in a sensor, we 
often times take Rx, and we make that the 
variable resistor. 
So, Rx is a variable resistance, and then 
we measure the voltage between a, between 
points a, and b. 
Now why do we use a Wheatstone Bridge? 
Why don't we just measure this directly? 
Well it's because of relative values. 
Sometimes this resistance varies very 
very small. 
It and so, the voltage difference might 
be very, very small. 
So, for example, we can measure an 
absolute terms what v sub b is. 
And v sub b maybe it's two volts. 
But maybe the difference between this a 
and b is on the millivolt range. 
So, if we measure v sub b, it would be 
two volts, and if we are measuring 
everything relative, using a, like the 
voltage divider law directly along one 
leg of this like we did with the 
potentiometer. 
Then we'd be measuring maybe 2.0,0,15 
volts. 
And it's hard to get a reading like that. 
It's hard to scale, a meter like that. 
So, instead we get rid of, the basis, we 
get rid of the, we zero it out so we're 
only measuring this. 
So, once we put in the Wheatstone Bridge 
configuration we're only measuring the 
difference. 
So, we can scale up our meter, so that 
we're measuring things on the millivolt 
range. 
So, let's start our lab demo on the 
Wheatstone Bridge. 
Lets look at a very common application of 
a Wheatstone Bridge as a sensor. 
Look at this, is a flexible beam. 
It's kind of thin and long. 
So it, it can be bent, see how long it 
is. 
Very thin, and it, it can be bent. 
It's a flexible beam. 
I've mounted two strain gauges on each 
side. 
Two on the top and two on the bottom. 
Strain gauges are variable resistors. 
They vary based on the length of this 
strain gauge. 
As the strain gauge elongates, then the 
resistance goes up. 
And if it shortens, the resistance goes 
down. 
So, this is mounted, you see, these wires 
here? 
This is mounted and, and wired as a 
Wheatstone Bridge, with R1 and R4 on the 
top. 
Now let's look at the schematic of this. 
It's just a, a sketch of, a cutout of the 
beam, with R1 and R4 strain gauges on the 
top, R2 and R3 on the bottom. 
You can see that some of these wires are 
connected to ground, and some of them to 
a power supply. 
This is a 5 volt power supply. 
And the other two are connected back and 
forth so the, the leads from the strain 
gauges are connected together. 
With pick-off points A and B. 
If I put that, that's back into the, the 
standard configuration of a Wheatstone 
Bridge. 
I've got R1 and R2 mounted on the top. 
Are rather R1 and R4 mount down the top, 
R2 and R3 on the bottom and I've got 
these pick off points. 
So, I'm going to be measuring the 
potential at point A and the potential at 
point B. 
Now if this bends backward, so R1 and R4 
elongate, and R2 and R3 shorten. 
So, R1 and R4 elongate, R2 and R3 
shorten. 
Then R1 goes up, R4 goes up. 
And the other two go down. 
So, remember how Wheatstone Bridge works. 
It's based in a voltage divider law. 
So, if this decreases and this increases, 
the potential at A will go down. 
Similarly, if this increases and R2 
decreases, the potential in B goes up. 
That means the voltage difference V sub A 
minus V sub B becomes negative. 
So, if I bend this backward, the 
potential, V sub A minus V sub B, becomes 
negative. 
Now, if I bend this up, and these two, R2 
and R3 elongate, so they become larger, 
larger resistors. 
These two become larger resistors. 
While R1 and R4 decrease in resistance. 
That means a potential A goes up, and a 
potential B goes down. 
So that V sub B minus V sub A, becomes 
more positive. 
So bending it, bending it back so these 
elongate. 
Means that the potential becomes 
negative, then the difference between VA 
and VB becomes negative. 
And then if I bend it the other 
direction, so that these elongate then 
that voltage difference becomes positive. 
Let's look back at our physical device 
here. 
So there's our one R4, and you can see 
the little wires back and forth, 
connecting the Wheatstone Bridge. 
Let me show how this bends, so right now 
I'll be bending it, so that R1 and R4 
elongate. 
If you look at the resistance, or the 
voltage on the digital multi meter, you 
can see it becomes negative, like we 
predicted. 
And the more I bend it, the more it 
becomes negative. 
And that reading is in millivolts. 
Now let me bend it in the other 
direction. 
And notice as we predicted it becomes 
positive. 
So, bending in this direction I get 
positive voltage. 
The more I bend it the higher the 
voltage. 
And if I go back to, the balance 
condition, it's close to 0 millivolts. 
You may ask why do we bother with four 
strain gauges? 
We looked at a Wheatstone Bridge before. 
We analyzed it with one flexible, one 
reed the variable resister. 
Why do we now use four? 
Well the reason isn't anything to do with 
electrical properties. 
The reason we use four variable resisters 
here we mount two strain guages on top 
and two on the bottom. 
Is because of thermo properties, we 
wanted to gate the thermo expansion of 
this beam. 
As it gets warm the beam will elongate, 
well if the beam elongates, so will those 
strain gauges, and that will change the 
resistance. 
But the thing is if these resistors, the 
top and the bottom, both elongate by the 
same amount, the ratios don't change. 
So, it still stays in a balanced 
Wheatstone configuration. 
And that way, we get rid of the, we, we 
negate all effects of thermal expansion, 
and the only effects that we will be 
measuring with this is the bending 
effects, and that's what we were trying 
to measure. 
Looking again, as I bend it this 
direction, it becomes positive voltage, 
bending it this direction becomes 
negative voltage. 
It's a very common application of a 
Wheatstone Bridge. 
And Wheatstone bridges with strain gauges 
are often times used in not only flexible 
beams, but also in bridges looking at 
loads on, on bridges. 
In summary, a Wheatstone Bridge is used 
to detect small changes in resistance. 
And we used it in a strain gauge, but it 
can be used in other examples as well 
with other types of sensors. 
Now, in the, with the particular strain 
gauge configuration, strain gauge 
application, we used four strain gauges 
in a Wheatstone configuration, and that 
was to remove the thermal effect. 
In the next lesson, we will be doing a 
wrap up for module 2. 
Nathan and I would like to encourage you 
to go back to the form, now that we're 
wrapping up this whole module. 
Make sure that you ask any questions you 
need, make sure that you finish all the 
homework problems, because those homework 
problems will prepare you for the quiz on 
this section.