Hello, my name's Dr.

George.

This lesson is called "LDRs and thermistors" and it's part of the unit "Circuit components." The outcome for the lesson is I can describe how an LDR or a thermistor can be used in a sensing circuit.

Here are the keywords I'll be using.

I'm not going to go through them now because I'll explain them as they come up, but you can come back to this slide anytime to remind yourself of the meanings.

The lesson has three parts called "Light-dependent resistors," "Thermistors" and "Sensing circuits." So let's take a look at light-dependent resistors.

A light-dependent resistor, commonly known as an LDR, is a type of resistor made from a semiconductor.

A semiconductor is a material that has some metallic properties and some non-metallic properties, and it's made from a semiconductor with a resistance that varies with the light level.

Here's what a light-dependent resistor looks like.

And when light shines on the top of that, its resistance changes.

And here's the circuit symbol for an LDR.

It has the symbol for a resistor, a rectangle at the centre of it, and the two parallel arrows represent the idea of incoming light.

The brighter the light, the lower the resistance of an LDR.

Now, which of the following circuit symbols are for resistors? And each time I ask a question, I'll wait five seconds.

But if you need longer, just press pause, and press play when you have your answer ready.

And there are two types of resistor here, a light-dependent resistor and a normal fixed resistor.

You're going to investigate how the resistance of an LDR varies with the brightness of light shining on it.

And when you do that, you can use an ohmmeter to measure resistance directly.

You may be more used to investigating the resistance of a component using an ammeter and a voltmeter.

You put the component in a circuit and measure the current through it and the p.

d.

across it.

But you can measure resistance directly if you have an ohmmeter.

And you can vary the light level by placing sheets of greaseproof paper on top of the LDR, blocking out some of the light.

Greaseproof paper is a kind of translucent paper.

Alternatively, you can use tracing paper for this.

And of course, you have to keep other variables constant, so control variables such as distance to the lamp.

You'll fix that throughout the investigation.

So which of the following are control variables when investigating the resistance of an LDR? The brightness of the light.

You don't want to change that because you're changing it by adding sheets of greaseproof paper.

And the distance of the light from the LDR.

The number of sheets of paper is something you will deliberately change, that's your independent variable.

And the materials of the electrical leads won't make a difference to the resistance of the LDR, and you wouldn't change that partway through the experiment anyway.

So now I'd like you to do the investigation.

You'll need to set up a table like this ready to record your results.

You'll probably want to record resistance in kilos because these resistances will be quite large.

And you're going to plot a graph of the resistance of the LDR against the number of sheets of paper.

And you'll need to make sure you take enough measurements so that you can see what kind of relationship there is between the two variables.

You don't know yet if it's going to be a straight line relationship or some sort of relationship represented by a curve.

So think about how many data points you'll need so that you'll be able to see how to draw your best fit line.

And then when you're finished, you should look at your graph and use it to state the relationship between the resistance of the LDR and the brightness of the light.

So pause the video while you do the investigation and come back when you've finished.

I hope your investigation went well and here are some sample results.

We have a line of best fit that is going upwards, it's slightly curved.

Although if there is a bit of experimental error in the measurements, then it could have looked like a straight line.

And we can summarise the results by saying the resistance of the LDR increases as the number of sheets of paper increases.

But what we're really interested in is the relationship between the resistance and the light level.

If we read the graph from right to left, decreasing number of sheets of paper, that's increasing light level, and as the light level increases, the resistance decreases.

So the resistance of an LDR decreases as the brightness of the light increases.

I hope you saw that in your results too.

Now let's take a look at another type of component, the thermistor.

A thermistor is also made of semiconductor, and it's also a type of resistor, but this has a resistance that varies with temperature.

Here's a picture of a thermistor.

And the circuit symbol, which again has the symbol for a resistor within it.

And the higher the temperature of a thermistor, the lower its resistance is.

And this is opposite to the behaviour of a normal piece of wire or filament.

For a normal piece of wire, as it gets hotter, its resistance increases.

But for a thermistor, as it gets hotter, its resistance decreases.

That's because a wire is made of metal while a thermistor is made of a kind of semiconductor which behaves differently.

Now, which of the following circuit symbols is for a component that has a resistance that varies with temperature? And the answer is C.

A is an LDR and B is a fixed resistor.

Now you can investigate how the resistance of a thermistor varies with the temperature, again using an ohmmeter to measure the resistance.

But we need to vary the temperature this time.

What you can do is place the thermistor into a beaker of hot water which you've boiled in a kettle and then simply change the temperature of the thermistor by waiting for the water to cool.

And as the water cools, you can take measurements of the resistance at different temperatures using a thermometer to measure the temperature.

Before you take each reading, you should stir the water either with the thermometer or with a separate stirrer, because when a liquid is changing temperature, it can have some regions that are warmer or cooler than others.

So you want to stir the water so that the temperature where the thermometer is is the same as the temperature where the thermistor is.

Let's check you were paying attention.

Why is hot water in a beaker stirred before taking its temperature? And the answer is to even out its temperature.

Now you're going to do this second investigation into how the resistance of a thermistor varies with temperature.

You'll need to clamp the ohmmeter to make sure it stays out of the water.

Just holding it with your hand isn't really a secure way.

And set up a table before you start taking results.

Take care with hot water.

And when you've collected enough results, plot a graph of resistance of the thermistor against temperature.

It won't really be practical to take repeat measurements at the same temperature since the temperature of the water will be falling all the time.

So just take plenty of measurements at different temperatures so that you'll be able to see where the best fit line should go.

And then finally state the relationship between the resistance of the thermistor and the temperature.

Press pause while you go away and do the investigation and come back to the video when you've finished.

So here are some sample results.

We have a curved line of best fit, and it shows that the resistance of the thermistor decreases as the temperature increases.

And now let's look at sensing circuits.

These are circuits that react to something that's happening in the outside world.

And first, a reminder of what a potential divider is.

So this is a potential divider circuit, it has a power supply and two components.

And the components are in series, and they share the p.

d.

that's across the power supply.

And the way they share it is in the ratio of their resistances.

So if we change the resistance of the variable resistor, that will change the p.

d.

s across these two components.

If we increase the resistance of the variable resistor, then that will cause the p.

d.

across the variable resistor to increase as well, and the p.

d.

across the lamp will decrease and the lamp will become dimmer.

Decreasing the resistance of the variable resistor will cause the p.

d.

across the variable resistor to decrease and the p.

d.

across the lamp to increase and the light will become brighter.

Now in this potential divider circuit there's an LDR and a variable resistor, and the p.

d.

across the LDR and variable resistor again add up to the p.

d.

across the supply.

If light levels fall, the resistance of the LDR increases, we've seen that's how LDRs behave, and that causes the p.

d.

across it to increase.

If the light levels rise, the resistance of the LDR decreases, so the p.

d.

across it decreases.

And now a question.

What are the effects of increasing light levels around the LDR in the circuit shown? And there are two correct statements here.

Firstly, the resistance of the LDR will decrease.

And that will cause the p.

d.

across the variable resistor to increase.

Because we have the supply p.

d.

shared between these two components, lower resistance LDR will have less of that p.

d.

across it, and so the variable resistor will have more of the p.

d.

across it.

Now streetlights use a potential divider circuit so that they switch on automatically when needed.

Look at this circuit.

As it gets darker, the resistance of the LDR increases, and so the p.

d.

across it increases.

And it's connected to a kind of electronic switch so that if the p.

d.

across the LDR reaches a certain level, that causes the switch to turn on the streetlamp.

So we have a circuit that is responding to the light levels to turn on the lamp at the right time.

But there's a variable resistor here and that could be adjusted by an operator so that the street light turns on at different light levels.

Increasing the resistance of the variable resistor reduces the p.

d.

across the LDR.

So it now needs to be darker for the p.

d.

across the LDR to be large enough to cause the electronic switch to turn on the street light.

So we've changed the setting so that the street lights come on a little later when it's a bit darker.

This potential divider circuit includes a thermistor and it can be used in a heating control unit to turn heating on and off automatically.

The user can adjust the resistance of the variable resistor and that controls the temperature at which the heating will turn on or off.

So what are the effects of reducing the temperature around the thermistor in the circuit shown? Well, one of the effects is the resistance of the thermistor increases.

We've seen that's how thermistor behave.

And so the p.

d.

across the thermistor increases.

And when the p.

d.

reaches a certain set value, it will cause the electronic switch to switch on the heating.

Now I'd like you to draw and label a potential divider circuit that can turn on a security light when it gets too dark, and then state what you need to do to make the security light come on sooner when it's not so dark.

Press pause while you're writing down your answers and press play when you're ready to check them.

So here are the answers.

Your circuit should look like this.

What's important here is that there's a power supply, there's an LDR in series with a variable resistor, and there's an electronic switch that's connected in parallel across the LDR.

You don't need to know how to represent the electronic switch in symbols.

And the security light will come on more quickly, as in when it's not so dark, if the variable resistor is adjusted, so it has a lower resistance.

Then for any given light level, there's now a higher p.

d.

across the LDR, and so it will reach that threshold p.

d.

which is enough to turn on the switch at a higher light level than before.

Well done if you worked that out.

And now we've reached the end of the lesson, so I'll finish with a summary.

A light-dependent resistor, or LDR, is made of a semiconductor with a resistance that decreases as light levels increase.

A thermistor is made of a semiconductor with a resistance that decreases as temperature increases.

In a potential divider circuit, a thermistor can be used to automatically turn on or off an electronic switch if it gets too hot or too dark.

So well done for working through this lesson.

I hope you learned some things about these components that you didn't know before, and I hope your investigations went well.

Bye for now.