11th Class Physics Chapter 9 | Light Dependent Resistor | 11th Class Physics New Book 2025

[00:00:06]Assalam awaikum du student this is Kashi Majeed your physics teacher hope you all are well student today our topic is LDR or Light Dependent Resistor now as the name suggests what a light dependent resistor would be that resistor on which as soon as light falls its resistance decreases and its conductivity increases because you know that resistance and conductance are reciprocal of each other. So as soon as you make light fall on it, what will be its resistance? It will decrease and its conductivity will increase. Ok? And these are basically made of semiconductor materials. And we will discuss all these things in today's topic and also see its applications.

[00:00:50]What is LDR? So wherever LDR is used, what does it mean? Light dependent resistor. So what will happen? Hus resistance decreases. For MCQs also, you have to remember what is resistance?

[00:01:01]Decrease with increase in light intensity. Ok? What will you have as the increasing light intensity increases? Resistance will decrease and conductivity will increase. It is also non- edge photoresistor. For this reason, for these MCQs also you should know what do you call LDR? It is also called photo resistor because as the light increases, its conductivity increases. The resistance decreases. So the LDR will come forward as to who makes it? LDR is topically made form semiconductor materials. What are they made of?

[00:01:35]Consists of semiconductor materials containing cadmium sulfide. Ok? MCL This is also your MCQs that of which semiconductor material are LDRs made?

[00:01:44]In which cadmium sulfide will come and this is basically an insulating sheet. On top of that, it is deposited in a special pattern.

[00:01:53]So you would say that cadmium sulfide is the material that is deposited in a special pattern on the insulating plate.

[00:02:01]Meaning what will be on top of the insulating plate?

[00:02:03]What is cadmium sulfide special pattern made of? This is deposited.

[00:02:08]Next comes what is its working principle? Why does this happen That as soon as the light falls, its resistance decreases. Its conductivity increases. Why does this happen? Now students, these are basically semiconductor materials. And in semiconductor materials, do you know what charge carriers are? There are free electrons and holes. And what do you do as soon as you get over it? If we make light fall, what will happen is that the energy of the charge carriers will increase. If their energy increases then basically what will happen is that they will move from that shell or orbit.

[00:02:43]Ok? If you get out of that shell or orbit, what will happen to the quantity of free electrons you have? It will increase.

[00:02:52]What will happen to the conductivity of the material because of this? It will increase. So you can say that the principle used in an LDR is the increase in conductivity of a material on exposing it to light.

[00:03:08]As you expose it to light, its conductivity increases.

[00:03:12]In the Darkness the Semiconductor Materials Have a Few Free Electrons. As soon as you keep it in the dark or where there is no light, basically what will happen is that there are only a few free electrons. Meaning the charge carriers are less. When the charge carriers are less, then obviously the conductivity will also be less. But as soon as you expose them to light, these free electrons or the charge carriers that you have increase.

[00:03:38]So you can say that ah in darkness the semiconductor material has a few free electrons or charge carriers you can say resulting in high resistance due to which the resistance is high. Ok? When the resistance is high, then as soon as the light photon heats the material, it transfers energy to the electron in the outer orbit.

[00:04:04]Similarly, the energy of that electron will increase and it will leave that orbit and the charge carriers will increase. Similarly, the conductivity of the resistor you have will increase as soon as you take it to light. You should remember its basic principle, after that its applications will come as to where we can use LDR in daily life.

[00:04:25]So first of all you must have seen most of the street lights that as soon as the day time comes, they automatically turn on? They get switched off and as it becomes night time, those street lights get switched on.

[00:04:37]What this means is that mostly it is automatic. So we will read that these are working as light sensors.

[00:04:46]Ok? And this is its use as a light sensor. Basically LDR used in automatic lighting system. Ok? Whatever automatic lighting system you have, which is used in homes as well as streetlights, is also used costly.

[00:05:00]What happens during the day?

[00:05:02]What basically happens during daytime? There will be low resistance.

[00:05:04]Because during day time light will be falling on the LDR. Because those lights have LDR installed in them and as soon as the light falls during day time, what it means is that the charge carriers will increase.

[00:05:16]If the charge carriers increase, the resistance will decrease. If there is low resistance then that is why it will keep the lights off. Ok? What lights will you have? Will remain off. But as soon as you talk about day time and night time, the resistance will increase.

[00:05:32]What will happen to the lights when the resistance increases?

[00:05:33]Basically it will be working as a switch with the circuit.

[00:05:35]Ok? So you can also say that it works as a light sensor. The second camera is also used for exposure control.

[00:05:41]Ok? That if the light intensity is high then you see that this is basically used in LTR camera and is mounted on the front.

[00:05:48]So what it has to do is produce an electrical signal. Ok? And the speed of the shutter and aperture is controlled based on the same electrical signal.

[00:05:59]Ok? So you can say that LDRs help in adjusting the exposure time in the camera based on the amount of available light that falls on it.

[00:06:11]What needs to be done according to this?

[00:06:13]Adjusts the exposure time in LDR cameras. So you can also take an example here as to what happens in bright light? The camera speeds up the shutter and narrows the aperture. The aperture becomes narrow and what happens to the shutter speed? Increases to prevent the exposure. To avoid exposure.

[00:06:33]So similarly, I asked what will the LDR used here do? It produces an electrical signal and based on that electrical signal the camera makes all these changes like speeding up the shutter and narrowing the pressure. Because of which what can you do that you are? You can control the exposure.

[00:06:50]Ok? And the third application of this, which the book discusses here, is that it acts as a voltage divider.

[00:06:58]As we saw in the previous topic also.

[00:07:00]Ok? In which the thermistor was being used as a voltage divider.

[00:07:05]So similarly, what does the photodiode, sorry, light dependent register (LDR) that you have, do? It acts as a voltage divider.

[00:07:17]Ok? So you can say that in these typical circuits LDR becomes a part of voltage divider. This is also a part of the voltage divider, basically that converts the resistance changes into measurable voltage changes or resistance changes because what happens when light falls on it? There has to be a change in resistance, that is the voltage change into which it has to be converted.

[00:07:35]So this is how the arrangement of a system looks like.

[00:07:38]This is used in the LDR you have. And here you have the voltmeter at this point B. And this second point is its ground.

[00:07:47]You have grounded the negative terminal of the battery here. And they use a resistor which is non-resistance of several kilo ohms like 100 kilo ohms.

[00:07:57]Ok? Ok? So this way it is available in this range.

[00:07:58]Now [nasal sound] as soon as the light falls on the LDR. Ok? What should be above this? The light will fall.

[00:08:06]So in this case you can see what its resistance will be when light falls on it? Now, mostly the resistance of this LDR is greater. With whom? Here you have used a resistance of a few kilo ohms. For example, the resistance of this LDR is more than the resistance of 100 kilo ohm or 700 ohm used.

[00:08:31]But as soon as light falls on it, what will be its resistance? It has to be reduced.

[00:08:35]When the resistance is low, what will happen to the voltage drop you have? Will reduce. So what will you have above this point B? The voltage will become less. It will reduce. When the voltage decreases at this point, what we basically do here is connect an external circuit above point B.

[00:08:55]In the external circuit, as you say transistor, there is transistor and it is implemented with NOT gate.

[00:09:01]Ok? Now as soon as the NOT gate gets low input. So what will the NOT gate do to the low input? This will convert it to high. Ok? What do you have with this not gate? That you can say it is a functioning in which if it gets low input then it converts low into high.

[00:09:19]So in the same way, you can connect any next system alarm system here.

[00:09:23]And in this way you can say that the potential is being divided in this circuit. Ok? And why is the potential divided?

[00:09:32]Because the resistance will drop. When the resistance drops, the potential you have will also decrease. The potential difference will drop. So in this way, in this topic, you basically saw what is LDR, what is LDR? What material is it made of? And secondly, you talked about its working principle that when light falls on it, your resistance decreases and conductance increases. Ok? And the reason for increase in conductance is that when the charge carrier is dark then there will be less free charge carriers. Ok?

[00:10:03]Laces will be available. So as soon as they are bound and as soon as the light falls on them, they will move from their orbit and the charge carriers will become free and in this way the conductivity will increase. So hopefully you have understood this topic well. See you in the next lecture, Inshallah. Allah Hafaz.

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