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# How an LDR (Light Dependent Resistor) Works

## What is an LDR (Light Dependent Resistor)?

An LDR is a component that has a (variable) resistance that changes with the light intensity that falls upon it. This allows them to be used in light sensing circuits.

A typical LDR
LDR Circuit Symbol

## Variation in resistance with changing light intensity

Typical LDR resistance vs light intensity graph

The most common type of LDR has a resistance that falls with an increase in the light intensity falling upon the device (as shown in the image above).Â The resistance of an LDR may typically have the following resistances:

Daylight
= 5000â„¦
Dark
= 20000000â„¦

You can therefore see that there is a large variation between these figures. If you plotted this variation on a graph you would get something similar to that shown by the graph shown above.

## Applications of LDRs

There are many applications for Light Dependent Resistors. These include:

### Lighting switch

The most obvious application for an LDR is to automatically turn on a light at a certain light level. An example of this could be a street light or a garden light.

### Camera shutter control

LDRs can be used to control the shutter speed on a camera. The LDR would be used to measure the light intensity which then adjusts the camera shutter speed to the appropriate level.

### Example -Â LDR controlled Transistor circuit

LDR controlled transistor circuit

The circuit shown above shows a simple way of constructing a circuit that turns on when it goes dark. In this circuit the LDR and the other Resistor form a simple 'Potential Divider' circuit, where the centre point of the Potential Divider is fed to the Base of the NPN Transistor.

When the light level decreases, the resistance of the LDR increases. As this resistance increases in relation to the other Resistor, which has a fixed resistance, it causes the voltage dropped across the LDR to also increase. When this voltage is large enough (0.7V for a typical NPN Transistor), it will cause the Transistor to turn on.

The value of the fixed resistor will depend on the LDR used, the transistor used and the supply voltage.

## Project kits and components

We have an electronic kit which utilises an LDR to detect lowering light levels and light a colour changing LED once it gets dark. This is a great example of an LDR in action. We also sell two different sizes of light dependant resistor, see below for more details.

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## 85 thoughts on “How an LDR (Light Dependent Resistor) Works”

• thanks a lot guys.....I am doing a little project on ldr.....thanks a lot for the info... It was specific and clear

• Thanks for the lesson, I'm fabricating a project using LDR as an automatic switch.

• Can u suggest me LDR SUITABLE FOR 45 W LED STREET LIGHT?

• i am doing a projct on home automation using LDR..So can u plz suggest me which LDR is suitable for dis and its cost??

• Hi Priyanka,

We really need a little more information in order to help you. What exactly does the project require and involve so far?

Rob

• My niece is planning to make a lampshade as her school project. And I found this, so is it possible to used LDR? Is this applicable? Can you send me a simple circuit & components to be use to come out with this project? Hoe long will it last? Just curious if so I'll be the one making it for her. Thank you! ^_^

• Hi Shuggs,

For a lamp which uses an LDR we have a night light kit, kitronik.co.uk/2120 which turns a light on when it gets dark. It is an Easy Build Kit which means anyone who can solder can build it, and it is very popular within school.

• second paragraph says "When the light level falls, the resistance of the LDR increase"
????????????????????????/

• Hi Fred,

That is correct, the relationship between light levels and resistance is inversely proportional.

Rob

• So, how are they used in night lights exactly if the current flow increases with high light intensity? :)

• Hi,

We use a normal resistor and a transistor along with an LDR (See LDR controlled transistor circuit above).

Rob

• which LDR is suitable for automatic intensity control circuit????

• Hi Rupali,

We'd really need a little more information about the circuit you plan to use. If you have a circuit in mind drop our support department an email ([email protected]) and they will be glad to assist.

Rob

• […] advantage of this factor of the LDR and use it in our Arduino LDR sensor DIY project. Check this link to know more about […]

• Alexander Mackingtosh May 19, 2015 at 3:37 pm

Hi! Sorry, was just wondering if you could clear something up for me:
If he resistance of a regular LDR (for example used in night lights) decreases in bright light, this allows more current to flow - so does this not mean that when their is a high light intensity more current flows and the light can be turned on?
Sorry, a little confused.
Thank you in advance!

• Hi,

You're right in that resistance decreases in bright light, but this means when there is high light intensity less current flows. We add the use of a transistor as in the final example to achieve turning a light on even though less current flows.

Rob

• hi
am still not clear on how LDR can be use as a switch in turning light on in the dark knowing full well that the resistace during dark period is high base on low light intensity but if is at the day time is very clear that as the light intensity increases the resistance drop and at a certain light intensity current flows and switching can now be archieved that is in the day time. so how can that be archieve in the night? so am confuse. need some clerification

thanks

• Hi,

We use a normal resistor and a transistor along with an LDR (See LDR controlled transistor circuit above).

Rob

• HI,
I'm doing my Physics Isa soon, and I have to come up with a hypothesis, iv been given a problem; Investigate a factor that affects the resistance of an LDR. I was thinking my factor would be smoke and context a smoke detector, do you think this would be suitable and my hypothesis something like the light there is the lower the resistance of an LDR and when clouded by smoke the resistance gets so low that it triggers an alarm. thanks
Venus

• Hi Venus,

In the case of smoke obscuring the LDR this would cause resistance to increase so you would have to have a transistor circuit as in the example given above.

Rob

• Can you please add equations involved?

• Mark Donnison July 4, 2016 at 5:23 pm

Hi, what in particular were you looking for? Here is some general information; An LDR (non-RoHS) light dependent resistor housed in a sealed epoxy case with clear lens window.

Resistance decreases as light falling on the window increases.

On resistance: 6.5K - 13.5K.

Off resistance: 0.5M

If this doesn't answer your question, if not let us know and we will try to help you.

• is it possible to install a LDR in a complicated circuit as the controller of the whole circuit

• Mark Donnison July 4, 2016 at 5:02 pm

In principal yes but we would need to know more about the circuit in question to give a definitive answer.

• Hi, Wondering if you could help me with a project I have going. The project uses an LDR to adjust the volume of the sounds output through an amplifier.

In channel one I want to volume to go up when there is more light, I have achieved this by using an LDR in the circuit with a potentiometer (to smooth the transition).

In channel two I want the opposite: the volume to go up when the light lowers (the difference between the light levels is probably not going to be a lot so it needs to be quite sensitive). I cannot for the life of me work out how to get it to do this. I know that I will need to use a switch with transistor etc as in your night light one but I just can't think how the components would go together.

My speaker output is 8ohms, and the amplifier has a 12V DC current. Music is supplied through Mp3 to the system.

Any help at all would be appreciated!
Bailee

• Hi Bailee, I've emailed you with an example circuit diagram that should answer your question. I hope this helps.

• I'm wondering if an LDR can be used as a night light then the light will go off at night since there would be more resistance in darkness but the opposite of that is what I hope to achieve. Any help would be appreciated.
Adam

• Hi Adam, we have a number of kits that make use of LDRs including those that turn on and off LEDs. Have a look at this kit, I believe it does what you've described: https://www.kitronik.co.uk/2120-dark-activated-colour-changing-night-light-kit.html

• hello,
I would like to know in which particular medical instrument LDR can be found

• Hi John, this is a bit out of our area of expertise but I believe there are pulse monitors that make use of LDR's.

• Hi
I would like to know what are three uses of LDR and how to describe them?

• Hi Janison, there are lots of possible uses for LDR's and a quick google search should turn up more information than you could use.

• How can an ldr circuit produce an increasing output voltage when the light intensity increases?

• Hi,

As the light intensity increases the resistance decreases, this would then increase the output voltage.

Rob

• Hi, I'm looking to create an alarm when a box is lifted up. I thought an LDR in a potential divider circuit would be perfect. As it would be dark under the box, and by lifting the box light would hit the ldr causing the a speaker and light to come on.

my problem is that I can see how to switch on the circuit when it is dark but not how to switch on the circuit when it is light.

• Hi Marc, you can get the effect you want by changing where the LDR is in the circuit. In the diagram on the resource, the LDR will switch on the Transistor when the light level decreases. To make the opposite happen just swap the positions of the resistor and the LDR so that the LDR is at the top of the circuit and the resistor is at the bottom. I hope this helps.

• Hi,
Are we using the transistor in the above given circuit as a switch??

• The lesson and comments I learnt from above found so helpful, that would guide me through to perform my practical on a relationship between light of the day and resistance of light dependent resistor [LDR]

• henrietta daniel March 15, 2017 at 9:30 am

Hi
Thanks a lot! This lesson and aaaaaall the comments really helped me out!
#AwesomeStuff

• Hi

we make automatic night lamp with using LDR. so we can use the LDR to turn on lamp in night ??..

• Is there any alternative device instead of using Ldr? forgetting cost and disadvantage parts of other devices

• Hi, we would need more information on a specific application to be able ti give any advice.

• 1.can the LDR works at 36 dc
2.can it works opposite ex.sensoring the dark

• Hi,

1. I'm not sure what you are asking.
2. You can get the effect you want by changing where the LDR is in the circuit. In the diagram on the resource, the LDR will switch on the Transistor when the light level decreases. To make the opposite happen just swap the positions of the resistor and the LDR so that the LDR is at the top of the circuit and the resistor is at the bottom. I hope this helps.

• Can i use the LDR to count the light that passes by on it?

• Hi Jomar, I'm not sure what you mean by count. The LDR reacts to the level of light that it is exposed to and it's resistance changes accordingly, you could monitor this using something like a microcontroller (BBC microbit, Arduino etc.) and deduce certain things, such as how many times it was completely dark or how many times it went from dark to light etc.

• Where would an ldr be useful.

• Hi Jude,

Any application where something needs to be switched on at a certain light level, a garden light coming on at dusk is a good example.

Rob

• So hope you can help. I want to use an ldr to light up a 7seg display when it is covered. What type of transistor do I need and does it just go in to the circuit after the ldr. I know this is probably super basic but just getting in to electronics!!

• Mark Donnison May 15, 2017 at 5:06 pm

Hi Kathryn, without knowing more about your circuit and the power requirements of your seven segment display it's not possible for us to advise you properly. If you were to email [email protected] directly with a little more information we should be able to give you some assistance.

• Shridhar Shirodkar June 1, 2017 at 1:02 pm

Does an LDR store energy like a solar panel so dat it could be utilized later?

• Mark Donnison June 2, 2017 at 10:15 am

Hi, no it doesn't it is just sensitive to light and changes resistance in line with how much light there is.

• from different resistance can we find color spectrum? if any please share...

• Mark Donnison July 20, 2017 at 1:04 pm

Hi, these LDRs react to the intensity of the light shining on them and not the wavelength of the light.

• I need to configure an ldr to activate the circuit when the light intensity decreases I.e to work at night......

PLS HELP!!!!

• Hi, In the diagram on the resource, the LDR will switch on the Transistor when the light level decreases. To make the opposite happen just swap the positions of the resistor and the LDR so that the LDR is at the top of the circuit and the resistor is at the bottom. I hope this helps.

• Hi,
Can someone help us to understand the lifetime of LDR?

• Hi, They tend to not wear out or fail in applications that we have experience of so we would expect them to last 15 years +. How many years the plus equates to we couldn't say but it is probably not a component you have come up with a replacement plan for.

• Hey, can an LDR be activated by artificial light sources? Like an LED? -or is it specific to UV light?

• Hi Chris, An LDR's sensitivity will vary across the spectrum and is largely determined by the materials used to make it. They react to certain light sources better than others and although LEDs will work you may find that some colours perform better than others. Hook your LDR up to a meter and run a couple of tests and you will know instantly if the LED you are using is suitable or not.

• Hi.
Which LDR is suitable for light sensor circuit ? pls answer my question...

• Hi, at a guess I would say any but without knowing more about your circuit and what you are trying to achieve it is difficult to give you an answer.

• Can u tell me what the load here means

• Hi Preetham, The term load in the circuit diagram means an 'active' component that is being controlled by the LDR. So, the load could be a light bulb or anything that consumes electrical power when the transistor is turned on. I hope this helps.

• Hi Team,

I would like to know that , can we use LDR ( alone ) as a switch in a circuit. Will that close the connection if light fall on it or else should I need to prepare any circuit to use LDR as a switch,

Note : My load will be DC motor or buzzer.

• Hi Syed,

The LDR can be used as an on/off switch, you would have to ensure that the power requirements of the components you are using are met and compatible.

• in the automatic light control unit I have a Question: we have to give power to the automatic light control using 220 volts supply. what I don't under stand is if there is power off, how can the system work amazes me. Could you kindly enlighten me.

• Hi, the LDR is an automatic switch that regulates the supply of power to the light, via changes in its resistance value caused by changes in the ambient light levels. Its switching does not turn off the circuit as a whole, it just regulates the power to the light. I hope this answers your question.

• my question was: IN CASE THE POWER GOES OFF, WILL THE AUTOMATIC LIGHT DEPENDENT PROJECT WORK.IF POWER OF 220 VOLTS IS NOT THERE, I DONT THINK THE SYSTEM WILL WORK.PL ENLIGHTEN ME EARLY PLEASE.

• actually i m doing project on Attendance Recorder so kindly guide me which LDR is Use in my project???

• Hi Hira, without knowing more about your circuit and the requirements of your project etc it is quite difficult to advise you on the best components to use. Also, we only do two different LDRs so options in this regard are limited. I would design my circuit and produce a prototype to test different light sources and LDRs until I found a configuration that works for my application. Below are the links for the two LDRs that we carry;
1 - https://www.kitronik.co.uk/c3515-standard-ldr.html
2 - https://www.kitronik.co.uk/c3514-miniature-ldr.html
I hope this helps.

• Thank u i have small doubt in LDR

• Can LDR be used to detect different colors of light?

• Hi Rale, Our LDRs detect light intensity and changes thereof. Also, the ones we stock are fairly generic which means they are not very precise.

• Imad Uddin Muhammad May 12, 2018 at 10:32 pm

Why my LDR is not working properly in both conditions that is in darkness and brightness? The LED lights remain turned on.

• Mark Donnison May 15, 2018 at 12:36 pm

Hi, I would guess that there is a problem with the circuit. Either an issue with the soldering, possibly a short somewhere, or that a component may be incorrectly placed. Check your circuit carefully against the diagram and check each of your solder joints.

• nwajagu chukwuebuka valentine May 9, 2019 at 1:25 am

please also encountering the same problem with my ldr still functioning the same both in light and darkness check severally but to no avail

• Mark Donnison May 9, 2019 at 11:41 am

Hi, Sorry you are having difficulties with the kit. It is most likely that there is a problem with how the circuit has been put together. Please double check that all components are in place and the right way around where applicable and that each solder joint is good. If you have done that and are still experiencing difficulties then please contact support who should be able to help you get to the bottom of the problem. Please include photos of both sides of your board that clearly show the whole side: [email protected]

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