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

  • Bharath June 19, 2014 at 7:08 pm

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

    Reply
    • Bob June September 20, 2014 at 9:10 am

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

      Reply
  • Elijah July 17, 2014 at 5:55 pm

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

    Reply
  • prasad August 11, 2014 at 7:17 pm

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

    Reply
    • Rob Haywood August 14, 2014 at 9:26 am

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

      Reply
  • priyanka singh September 1, 2014 at 8:35 pm

    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??

    Reply
    • Rob Haywood September 4, 2014 at 10:47 am

      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

      Reply
  • shuggs September 18, 2014 at 3:27 pm

    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! ^_^

    Reply
    • Rob Haywood September 18, 2014 at 5:02 pm

      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.

      Reply
  • Fred September 27, 2014 at 4:26 am

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

    Reply
    • Rob Haywood September 29, 2014 at 10:06 am

      Hi Fred,

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

      Rob

      Reply
      • Benjy May 19, 2015 at 3:38 pm

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

        Reply
        • Rob Haywood May 20, 2015 at 12:37 pm

          Hi,

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

          Rob

          Reply
  • Rupali September 30, 2014 at 2:24 pm

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

    Reply
    • Rob Haywood October 1, 2014 at 9:49 am

      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

      Reply
  • Arduino LDR Sensor Tutorial for Beginner's - DIY Hacking January 26, 2015 at 6:18 am

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

    Reply
  • 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!

    Reply
    • Rob Haywood May 20, 2015 at 12:36 pm

      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

      Reply
  • AHMED HAMISU January 2, 2016 at 1:10 pm

    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

    Reply
    • Rob Haywood January 6, 2016 at 10:03 am

      Hi,

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

      Rob

      Reply
  • venus February 19, 2016 at 11:42 am

    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

    Reply
    • Rob Haywood February 29, 2016 at 10:15 am

      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

      Reply
  • Dax June 30, 2016 at 7:28 pm

    Can you please add equations involved?

    Reply
    • 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.

      Reply
  • Roger David July 4, 2016 at 1:05 pm

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

    Reply
    • 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.

      Reply
  • Bailee July 9, 2016 at 1:15 am

    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

    Reply
    • Mark Donnison July 11, 2016 at 11:25 am

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

      Reply
  • Adam October 4, 2016 at 6:24 pm

    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

    Reply
    • Mark Donnison October 6, 2016 at 10:33 am

      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

      Reply
  • John Mayer October 14, 2016 at 7:43 pm

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

    Reply
    • Mark Donnison October 25, 2016 at 11:32 am

      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.

      Reply
  • Janison January 5, 2017 at 8:11 pm

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

    Reply
    • Mark Donnison January 10, 2017 at 2:57 pm

      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.

      Reply
  • Idowu I February 1, 2017 at 3:16 am

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

    Reply
    • Rob Haywood February 1, 2017 at 11:34 am

      Hi,

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

      Rob

      Reply
  • Marc Hill February 18, 2017 at 9:22 am

    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.

    Reply
    • Mark Donnison February 20, 2017 at 9:05 am

      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.

      Reply
  • Archanaa February 26, 2017 at 3:19 am

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

    Reply
    • Mark Donnison February 28, 2017 at 10:10 am

      Hi, yes it is acting as a switch in this circuit.

      Reply
  • Adamu China March 3, 2017 at 5:15 pm

    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]

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

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

    Reply
  • hard April 3, 2017 at 2:36 am

    Hi

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

    Reply
    • Mark Donnison April 3, 2017 at 11:28 am

      Hi, Yes you can use an LDR for this.

      Reply
  • Vinodh April 4, 2017 at 5:26 am

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

    Reply
    • Mark Donnison April 19, 2017 at 10:30 am

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

      Reply
  • asesh April 7, 2017 at 8:00 am

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

    Reply
    • Mark Donnison April 19, 2017 at 10:33 am

      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.

      Reply
  • Jomar April 21, 2017 at 12:42 am

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

    Reply
    • Mark Donnison April 21, 2017 at 3:44 pm

      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.

      Reply
  • Jude May 7, 2017 at 8:50 pm

    Where would an ldr be useful.

    Reply
    • Rob Haywood May 8, 2017 at 9:16 am

      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

      Reply
  • Kathryn May 11, 2017 at 1:32 pm

    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!!

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

      Reply
  • 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?

    Reply
    • 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.

      Reply
  • HIMA ZAFAR July 19, 2017 at 10:24 pm

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

    Reply
    • 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.

      Reply
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