## 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). 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 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.

## Phototransistors

In more recent years phototransistors have become more and more popular as unlike LDRs they don't use any nasty materials in the production process. They do exactly the same job as an LDR in that they detect changing light levels however they do electronically work slightly differently.

### What Is Ohm’s Law

Ohm’s law is used to describe the relationship between Voltage (V), Current (I) and Resistance (R). In this tutorial you will learn about the formula that connects these three values, see some examples and answer some questions to test your understanding.

Rale

Can LDR be used to detect different colors of light?

Mark Donnison

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.

Hira

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

Mark Donnison

Hi, in order to work your circuit requires power.

sreenivasan

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.

Mark Donnison

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.

sreenivasan

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.

Mark Donnison

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.

Syed

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.

Mark Donnison

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.

Preetham

Can u tell me what the load here means

hard

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

henrietta daniel

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

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]

Mark Donnison

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

Archanaa

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

Mark Donnison

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.

Marc Hill

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.

Rob Haywood

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

Idowu I

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

Mark Donnison

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.

Janison

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

Mark Donnison

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.

John Mayer

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

Mark Donnison

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

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

Mark Donnison

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

Bailee

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

Mark Donnison

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.

Mark Donnison

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

Roger David

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

Dax