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 (Light Dependent Resistor)

A typical LDR

 

 

Light Dependent Resistor LDR Circuit Symbol 
 LDR Circuit Symbol

 

Variation in resistance with changing light intensity

Typical LDR (Light Dependent Resistor) resistance vs light intensity graph
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). The resistance of an LDR may typically have the following resistances:

Daylight
= 5000Ω
Dark
= 20000000Ω
  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 - LDR controlled Transistor circuit

LDR (Light Dependent Resistor) 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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Project shopping list

For this project you will need the following items, you can adjust the quantities (including setting them to zero if you don't need the item), before the "Buy all items" button.

Standard LDR
£1.27
Qty

84 comments

Fred

Fred

second paragraph says “When the light level falls, the resistance of the LDR increase” ????????/

Mark Donnison

Mark Donnison

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: support@kitronik.co.uk

nwajagu chukwuebuka valentine

nwajagu chukwuebuka valentine

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

dennis ndungu

dennis ndungu

thanks for the online learning …

Mark Donnison

Mark Donnison

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.

Imad Uddin Muhammad

Imad Uddin Muhammad

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

Mark Donnison

Mark Donnison

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.

Sridevi

Sridevi

Thank u i have small doubt in LDR

Mark Donnison

Mark Donnison

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.

Arisya

Arisya

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

Mark Donnison

Mark Donnison

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.

Mark Donnison

Mark Donnison

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.

chris

chris

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

SLK

SLK

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

Mark Donnison

Mark Donnison

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.

Jaynam

Jaynam

I need to configure an ldr to activate the circuit when the light intensity decreases I.e to work at night…… PLS HELP!!!!

Mark Donnison

Mark Donnison

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

HIMA ZAFAR

HIMA ZAFAR

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

Mark Donnison

Mark Donnison

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

Shridhar Shirodkar

Shridhar Shirodkar

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

Mark Donnison

Mark Donnison

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 sales@kitronik.co.uk directly with a little more information we should be able to give you some assistance.

Kathryn

Kathryn

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!!

Rob Haywood

Rob Haywood

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

Jude

Jude

Where would an ldr be useful.

Mark Donnison

Mark Donnison

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.

Jomar

Jomar

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

Mark Donnison

Mark Donnison

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.

Mark Donnison

Mark Donnison

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

asesh

asesh

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

Vinodh

Vinodh

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

Mark Donnison

Mark Donnison

Hi, Yes you can use an LDR for this.

Rob Haywood

Rob Haywood

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

AHMED HAMISU

AHMED HAMISU

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

Rob Haywood

Rob Haywood

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

Rob Haywood

Rob Haywood

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

Benjy

Benjy

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

Alexander Mackingtosh

Alexander Mackingtosh

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!

Rob Haywood

Rob Haywood

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 (support@kitronik.co.uk) and they will be glad to assist. Rob

Rupali

Rupali

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

Rob Haywood

Rob Haywood

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

Fred

Fred

second paragraph says “When the light level falls, the resistance of the LDR increase” ????????/

Bob June

Bob June

Thanks, im doing a research project on Light Dependant Resistors aswell….very detailed. Helped a lot. :)

Rob Haywood

Rob Haywood

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.

shuggs

shuggs

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! _

Rob Haywood

Rob Haywood

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

priyanka singh

priyanka singh

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??

Rob Haywood

Rob Haywood

Any LDR may be suitable, it depends on the rest of the circuit.

prasad

prasad

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

Elijah

Elijah

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

Bharath

Bharath

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

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