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.
Technical Specifications of 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.
This a simple LDR based light activated relay circuit. The LDR is NORP12. In the presence of light or during day time light falling on the LDR keeps its resistance lower. This allows the transistor to remain biased, and the relay stays toggled ON. Any load configured across the N/O contact of the relay remains switched ON.
In the absence of light, or during darkness, the LDR resistance increases which removes the positive bias from the transistor, and the relay is switched OFF, which in turn switches OFF the connected load