As the name suggests LDRs or Light Dependent Resistors are resistors that exhibit a wide range of resistance values depending on the intensity of light incident on its surface. The variation in the resistance range can be anywhere from few hundred ohms to many megaohms.
The resistance value in an LDR is inversely proportional to the intensity of the light falling on it. Meaning when the light is less, the resistance is more and vice versa.
LDR Internal Construction
The following figure shows the internal dissected view of an LDR device wherein we can see the photoconductive substance applied within the zig zag or coiled pattern, embedded over a ceramic insulating base, and with the end points terminated as leads of the device.
The pattern ensures maximum contact and interaction between the crystalline photoconductive material and the electrodes separating them.
The photoconductive material generally consists of cadmium sulfide (CdS) or cadmium selenide (CdSe).
The type and thickness of the material and width of its deposited layer specify the range of the LDR resistance value and also the amount of watts it can handle.
The two leads of the device is embedded within an an opaque non-conductive base with an insulated transparent coating over the photo-conductive layer.
The schematic symbol of an LDR is shown below:
The diameter of photocells or LDRs may range from 1/8 inch (3 mm) to above one inch (25 mm). Commonly these are available with diameters of 3/8 inch (10 mm).
LDRs smaller than this are generally used where space may be a concern or in SMD based boards. The smaller variants exhibit lower dissipation. You may also find a few variants that are hermetically sealed to ensure reliable working even under harsh and undesirable environments.
Comparing LDR Characteristics with Human Eye
The graph above provides the comparison between the characteristics of photosensitive devices and our eye. The graph shows the plotting of Relative spectral response against wavelength from 300 to 1200 nanometers (nm).
The human eye characteristic waveform indicated by the dotted bell shaped curve reveals the fact that our eye has enhanced sensitiveness to a relatively narrower band of the electromagnetic spectrum, approximately between 400 and 750 nm.
The peak of the curve has a maximum value in the green light spectrum within the range of 550 nm. This stretches down into the violet spectrum having a range between 400 to 450 nm on one side. On the other side this extends into the dark red light region having a range between 700 to 780 nm.
The figure above also reveals exactly why cadmium sulfide (CdS) photocells tend to be the favorites in light-controlled circuit application: the spectral response curve peaks for Cds is near 600 nm, and this specification is quite identical to the range of human eye.
In fact, the cadmium selenide (CdSe) response curve peaks may even extend beyond 720 nm.
LDR Resistance Vs Light Graph
That said CdSe may exhibit higher sensitivity to almost the whole range of the visible-light spectrum. In general the characteristic curve of a CdS photocell may be as given in the following figure.
Its resistance in the absence of light can be around 5 megohms, which may drop to around 400 ohms in the presence of light intensity of of 100 lux or a level of light equivalent to an optimally illuminated room, and around 50 ohms when the light intensity is as high as 8000 lux. typically as obtained from a direct bright sunlight.
The lux is the SI unit for illuminance generated by a luminous flux of 1 lumen evenly spread over a surface of 1 square meter. The modern photocells or LDRs are adequately rated for power and voltage, at par with normal fixed type resistors.
The power dissipation capacity for a standard LDR could be around 50 and 500 milliwatts, which may depend on the quality of the material used for the detector.
Perhaps the only thing not so good about LDRs or photoresistors is their slow response specification to light changes. Photocells built with Cadmium-selenide typically exhibit have shorter time constants than cadmium-sulfide photocells (approximately 10 milliseconds in contrast to 100 milliseconds).
You may also find these devices having lower resistances, increased sensitivity, and elevated temperature resistance coefficient.
The main applications in which Photocells are normally implemented are in photographic exposure meters, light and dark activated switches for controlling street lights, and burglar alarms. In some light activated alarms applications the system is triggered through a light beam interruption.
You may also come across reflection based smoke alarms using photocells.
The following images shows a few of the interesting practical photocell application circuits.
Light Activated Relay
The straightforward circuit indicated in the above figure is built to respond whenever light falls on the LDR installed into a normally dark cavity for example the inside a box or housing.
The photocell R1 and resistor R2 create a potential divider that fixes the base bias of Q1. When it is dark, the photocell exhibits an increased resistance, leading to a zero bias on the base of Q1, due to which, Q1 and the relay RY1 remain switched off.
In case an adequate level of light is detected on the photocell LDR, its resistance level quickly falls to some lower magnitudes. and a biasing potential is allowed to reach the base of Q1. This switches ON relay RY1, whose contacts is used to control an external circuit or load.
Darkness Activated Relay
The next figure shows how the first circuit can be transformed into a darkness activated relay circuit.
In this example the relay activates in the absence of light on the LDR. R1 is used for adjusting setting up the sensitivity of the circuit. Resistor R2 and the photocell R3 work like a voltage divider.
The voltage at the junction of R2 and R3 rises when light falls on the R3, which is buffered by emitter follower Q1. The emitter output of Q1 drives common emitter amplifier Q2 via R4, and correspondingly controls the relay.
Precision LDR Light Detector
Although simple, the above circuits are vulnerable to supply voltage changes and also ambient temperature changes.
The next diagram shows how the drawback could be tackled through a sensitive-precision light activated circuit that would work without getting affected from voltage or temperature variations.
In this circuit the LDR R5, pot R6, and resistors R1 and R2 are configured with each other in the form of a Wheatstone bridge network.
The op amp ICI along with the transistor Q1 and relay RY1 work like a very sensitive balance detecting switch.
The balancing point of the bridge does not get affected, regardless of variations in the supply voltage or the atmospheric temperature.
It is only effected by the changes in the relative values of the components associated with the bridge network.
In this example the LDR R5 and pot R6 constitute one arm of the Wheatstone bridge. R1 and R2 form the second arm of the bridge. These two arms act like voltage dividers. The R1/R2 arm establishes a constant 50% supply voltage to the non-inverting input of the op-amp.
The potential divider formed by the pot and the LDR generates a light dependent variable voltage to the inverting input of the op amp.
The setting up of the circuit, pot R6 is adjusted so that the the potential at the junction of R5 and R6 goes higher than the potential at pin3 when the desired amount of ambient light falls on the LDR.
When this happens the output of the op amp instantly changes state from positive to 0V, switching ON Q1 and the attached relay. The relay activates and switches OFF the load which could be a lamp.
This op amp based LDR circuit is very precise and will respond even to minute changes in light intensities, which cannot be detected by human eye.
The above op amp design can be easily transformed into a darkness activated relay either by swapping the pin2 and pin3 connections, or by swapping the R5 and R6 positions, as demonstrated below:
Adding Hysteresis Feature
If required this circuit can be upgraded with a hysteresis feature as shown in the next diagram. This is done by introducing a feedback resistor R5 across the output pin and the pin3 of the IC.
In this design the relay actuates normally when the light intensity goes above the preset level. However when the light on the LDR drops and decreases than the preset value, it does not switch OFF the relay due to the hysteresis effect.
The relay switches OFF only when the light has dropped to a significantly lower level, which is determined by the value of R5. Lower values will introduce more delay lag (hysteresis), and vice versa.
Combining Light and Dark Activation Features in One
This design is a precision light/dark relay is designed by combining the previously explained dark and light switch circuits. Basically it's a window comparator circuit.
The relay RY1 is switch ON when either the light level on the LDR surpasses one of the pot setting or drops below the other pot setting value.
Pot R1 determines the darkness activation level, while the pot R3 sets the threshold for the light level activation of the relay. The pot R2 is used for adjusting the supply voltage to the circuit.
The setting up procedure includes adjusting the first preset pot R2 such that approximately half supply voltage is introduced at the LDR R6 and pot R2 junction, when the LDR receives light at some normal intensity level.
Potentiometer R1 is subsequently adjusted such that relay RY1 switches ON as soon as the LDR detects a light below the preferred darkness level.
Likewise, pot R3 can be set up so that the relay RY1 is switched ON at the intended brightness level.
Light Triggered Alarm Circuit
Now let's see how an LDR can be applied as a light activated alarm circuit.
The alarm bell or buzzer should be intermittent type meaning sounding with an continuous ON/OFF repetitions, and rated to work with current less than 2 amp. LDR R3 and resistor R2 make a voltage divider network.
Under low light conditions, the photocell or LDR resistance is high which causes the voltage at R3 and R2 junction insufficient to trigger the attached SCR1 gate.
When the incident light is brighter, the LDR resistance drops to a level sufficient to trigger the SCR, which turns on and activates the alarm.
Oppositely when it gets darker, the LDR resistance increases, switching OFF the SCR and the alarm.
It is important to note that the SCR here turns OFF only because the alarm is an intermittent type which helps to break the latch of the SCR in the absence of a gate current, shutting off the SCR.
Adding a Sensitivity Control
The above SCR LDR alarm circuit is quite crude and feature very low sensitivity, and also lacks a sensitivity control. The next figure below reveals how the design could be enhanced with the mentioned features.
Here, the fixed resistor in the previous diagram is replaced with a pot R6, and a buffer BJT stage introduced through Q1 between the gate of the SCR and the LDR output.
Additionally, we can see a push to off switch A1 and R4 parallel to the bell or the alarm device. This stage allows the user to convert the system into a latching alarm regardless of the intermittent nature of the bell device.
The resistor R4 ensures that even while the bell rings in an self interrupting sound, the latching anode current never breaks and the SCR remains latched once triggered ON.
S1 is used to break the latch manually and shut down the SCR and the alarm.
In order to further enhance the above explained SCR light activated alarm with improved precision, an op amp based triggering can be added as shown below. The working of the circuit is similar to the previously discussed LDR light activated designs.
LDR Alarm Circuit with Pulsed Tone Output
This is yet another dark activated alarm circuit featuring an integrated a low power 800 Hz pulse generator for driving a loud speaker.
Two NOR gates IC1-c and ICI-d are configured as an astable multivibrator for generating a frequency of 800 Hz. This frequency is fed into the speaker via a small signal amplifier using the BJT Q1.
The above NOR gate stage is activated only as long as the output of IC 1-b becomes low or 0V. The other two NOR gates IC 1-a and IC1-b are similarly hooked up as astable multivibrator for producing a 6 Hz pulse output and is also enabled only when the gate pin 1 is pulled low or at 0V.
Pin1 can be seen rigged with the potential divider junction formed by the LDR R4 and pot R5.
It works like this: When the light on the LDR is sufficiently bright the junction potential is high, which keeps both the astable multivibrators disabled, which means no sound output from the loudspeaker.
However when the light level drops below the preset level, the R4/R5 junction gets sufficiently lower which activates the 6 Hz astable. This astable now begins gating or switching the 800 Hz astable at 6 Hz rate. This results in a multiplexed 800 Hz tone on the speaker, pulsed at 6 Hz.
To add a latching facility to the above design, just add the switch S1, and the resistor R1 as given below:
For getting a loud, boosted sound from the speaker, the same circuit can be upgraded with an enhanced output transistor stage as shown below:
In our earlier discussion we learned how an op amp can be used for enhancing the LDR light detection precision. The same can be applied in the above design to create a super precision pulse tone light detector circuit
LDR Burglar Alarm Circuit
A simple LDR light beam interruption burglar alarm circuit can be seen below.
Normally, the photocell or the LDR receives the required amount of light through the installed light beam source. This can be from a laser beam source also.
This keeps its resistance is low and this also produces insufficiently low potential at the pot R4 and photocell R5 junction. Due to this the SCR along with bell remain deactivated.
However, in an event the light beam gets interrupted causes the LDR resistance to increase, significantly raising the junction potential of R4 and R5.
This immediately triggers the SCR1 switching ON the alarm bell. Resistor R3 in series with switch S1 are introduced to enable permanent latching of the alarm.
Summarizing LDR Specifications
There are many different names by which the LDR (Light Dependent Resistors) are known, which include names such as photoresistor, photocell, photoconductive cell, and photoconductor.
Normally the term which is most prevalent and used most popularly in instructions and the datasheets is the name “photocell”.
There are a variety of uses to which the LDR or photoresistor can be applied since these devices are good with their photosensitive property and are also available at low cost as well.
Thus, LDR could remain popular for a long period of time and extensively used in applications such as photographic light meters, burglar and smoke detectors, in street lamps to control the lighting, flame detectors, and card readers.
The generic term of “photocell” is used for the Light Dependent Resistors within the general literature.
Discovery of LDR
As discussed above, the LDR has remained the favorite among the photocells for a long period of time. The early forms of the photoresistors were manufactured and introduced in the market in the early nineteenth century.
This was manufactured through the discovery of the “selenium’s photoconductivity” in 1873 by the scientist named Smith.
A good range of different photoconductive devices have been manufactured since then. An important progress in this field was made in the early twentieth century, especially in 1920 by the renowned scientist T.W. Case who worked on the phenomenon of photoconductivity and his paper, “Thalofide Cell- a new photoelectric cell” was published in 1920.
During the next two decades in the 1940s and 1930s, a range of other relevant substances were studied for developing photocells which included PbTe, PbS, and PbSe. Further in 1952, the photoconductors the semiconductor version of these devices were developed by Simmons and Rollin using germanium and silicon.
Symbol of the Light Dependent Resistors
The circuit symbol which is used for the photoresistor or the light dependent resistor is a combination of the resistor animated to indicate that the photoresistor is light sensitive in nature.
The basic symbol of the light dependent resistor consists of a rectangle which symbolizes the resistor's function of the LDR. The symbol additionally consists of two arrows in the incoming direction.
The same symbol is used to symbolize the sensitivity towards light in the phototransistors and photodiodes.
The symbol of the “resistor and arrows” as described above is used by the light dependent resistors in majority of their applications.
But there are few cases where the symbol used by the light dependent resistors depicts the resistor encased within a circle. This is evident in the case when circuit diagrams are drawn.
But the symbol where there is absence of circle around the resistor is a more common symbol used by the photoresistors.
The surface of LDR is built with two cadmium sulphide(cds) photoconductive cells having spectral responses comparable to that of the human eye. The resistance of the cells drop linearly as light intensity is increased on its surface.
The photoconductor which is placed between the two contacts is used as a main responsive component by the photocell or the photoresistor. The resistance of the photoresistors undergoes a change when there is an exposure of the photoresistor to the light.
Photoconductivity: The electron carriers are generated when the photoconductor’s semiconductor materials used absorb the photons, and this results in the mechanism which works behind the light dependent resistors.
Although you may find that the materials which are used by the photoresistors are different, they are mostly all semiconductors.
When they are used in the form of photoresistors, then these materials act as resistive elements only where there is absence of PN junctions. This results in the device to become entirely passive in nature.
The photoresistors or the photoconductors are basically of two types:
Intrinsic Photoresistor: The photoconductive material which is used by a specific photoresistor type enables the charge carriers to get excited and jump to the conduction bands from their initial valence bonds respectively.
Extrinsic Photoresistor: The photoconductive material which is used by a specific photoresistor type enables the charge carriers to get excited and jump to the conduction bands from their initial valence bonds or impurity respectively.
This process requires non-ionized impurity dopants which are also shallow and requires this to take place when light is present.
The design of the photocells or extrinsic photoresistors is done specifically considering the long wavelength radiations such as the infra-red radiations in most of the cases.
But the designing also considers the fact that any type of thermal generation needs to be avoided since they are required to operate at temperatures which are very relatively low.
Basic Structure of LDR
The number of natural methods which are commonly observed for the manufacturing of the photoresistors or the light dependent resistors is very few in number.
A resistive material sensitive to light is employed by the light dependent resistors for constant exposure to light. As discussed above, there is a specific section which is processed by the light sensitive resistive material which is required to be in contact with both or one of the ends of the terminals.
A semiconductor layer which is active in nature is used in a general structure of a photoresistor or a light dependent resistor and an insulating substrate is further used for depositing the semiconductor layer.
In order to provide the semiconductor layer with the conductivity of the required level, the former is doped lightly. Thereafter, terminals are connected appropriately across the two ends.
One of the key issues in the basic structure of the light dependent resistor or photocell is its material’s resistance.
The contact area of the resistive material is minimized to ensure that when the device is exposed to the light, it undergoes a change in its resistance efficiently. In order to achieve this state, it is ensured that surrounding area of the contacts is doped heavily which results in the reduction of the resistance in the given area.
The shape of the surrounding area of the contact is designed to be mostly in the interdigital pattern or the zig zag form.
This enables the maximization of the exposed area along with the reduction in the levels of the spurious resistance which in turn results in the enhancement of the gain by contracting the distance between the two contacts of the photoresistors and making it small.
There is also a possibility of the usage of the semiconductor material such as polycrystalline semiconductor depositing it on a substrate. One of the substrates which can be used for this is ceramic. This enables the light dependent resistor to be of low cost.
Where Photoresistors are Used
The most attractive point of the light dependent resistor or a photoresistor is that it is of low cost and thus is widely used in a variety of electronic circuit designs.
Apart from this their rugged features and simple structure also provides them with an advantage.
Although the photoresistor lacks various features which are found in a phototransistor and a photodiode, it is still an ideal choice for a variety of applications.
Thus, LDR has been continuously used for a long period of time in a range of applications such as photographic light meters, burglar and smoke detectors, in street lamps to control the lighting, flame detectors, and card readers.
The factor which determines the photoresistor properties is the material type which is used and thus the properties can vary accordingly. Some of the materials used by the photoresistors possess constants of very long time.
Thus, it is quintessential that the photoresistor type si selected carefully for specific applications or circuits.
Light dependent resistor or LDR is one of the very useful sensing devices which can be implemented in many different ways for processing light intensity. The device is cheaper compared to other light sensors, yet it is able to provide the required services with utmost efficiency.
The above discussed LDR application circuits are just a few examples which explains the basic mode of using an LDR in practical circuits. The discussed data can be studied and customized in several ways for many interesting applications. Have questions? Feel free to express through the comment box.