Who invented 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. 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 by the renowned scientist T. During the next two decades in the s and s, a range of other relevant substances were studied for developing photocells which included PbTe, PbS, and PbSe.

Further in , the photoconductors the semiconductor version of these devices were developed by Simmons and Rollin using germanium and silicon. 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. 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. 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. 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.

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. 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. 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.

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 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. If you have any circuit related query, you may interact through comments, I'll be most happy to help! Your email:. Hi Swagatam, I am trying to design a circuit that will trigger a CK alarm chip. I want the circuit to trigger based on light. Say the alarm module is placed in a drawer and when someone opens the drawer, the alarm sounds.

I am using 3v battery supply. This works good. I have tried several power on delays with mixed success. I have tried your delay on timer using transistors, resistors and capacitor. I have tried delay timer. They all work, but I still get the hum from the CK which gets louder just before the chip fully triggers. Hi Norman, did you confirm the leakage by measuring it with a multi-meter? High Swagatam, I found a circuit on the internet that uses a timer as follows: Pin 1 is grounded.

Pin 4 and Pin 8 are 5v. Pin 2 and Pin 6 are shorted together. It works great. I breadboarded it with a uF electrolytic capacitor which give me a delay of about 17 seconds.

I would like a little longer delay, but the largest SMD capacitor I have is a tantalum capacitor of uF. Is there a way to increase the delay, say to 25 seconds using this circuit with my capacitor limitations. Hi Swagatam, The latest power on delay is not causing any hum on the CK The takes up a little more room on the PCB but for this type circuit, it seems to be a superior design. The timing and sound are very clear. My second post describes the design I am using.

I may try to fit to caps in parallel to increase the delay time. You make me so happy with your quick responses. When we apply light energy to the intrinsic photo resistor, only a small number of valence electrons gain enough energy and becomes free from the parent atom. Hence, a small number of charge carriers are generated. As a result, only a small electric current flows through the intrinsic photo resistor. We already have known that increase in electric current means decrease in resistance.

In intrinsic photoresistors, the resistance decreases slightly with the increase in light energy. Hence, intrinsic photoresistors are less sensitive to the light. Therefore, they are not reliable for the practical applications. Extrinsic photoresistors are made from the extrinsic semiconductor materials.

Let us consider an example of extrinsic photoresistor, which is made from the combination of silicon and impurity phosphorus atoms. Each silicon atom consists of four valence electrons and each phosphorus atom consists of five valence electrons.

The four valence electrons of the phosphorus atom form four covalent bonds with the neighboring four silicon atoms. However, the fifth valence electron of the phosphorus atom cannot able to form the covalent bond with the silicon atom because the silicon atom has only four valence electrons.

Hence, the fifth valence electron of each phosphorus atom becomes free from the atom. Thus, each phosphorus atom generates a free electron.

The free electron, which is generated will collides with the valence electrons of other atoms and makes them free. Likewise, a single free electron generates multiple free electrons. Therefore, adding a small number of impurity phosphorus atoms generates millions of free electrons. In extrinsic photoresistors, we already have large number of charge carriers.

Hence, providing a small amount of light energy generates even more number of charge carriers. Thus, the electric current increases rapidly. Therefore, the resistance of the extrinsic photoresistor decreases rapidly with the small increase in applied light energy.

Extrinsic photoresistors are reliable for the practical applications. The American standard symbol and the international standard symbol of the photoresistor is shown in the below figure. This area has to be made relatively large because the resistance of the contact to the active area needs to be minimised.

This type of structure is widely used for many small photoresistors or light dependent resistors that are seen. The interdigital pattern is quite recognisable. Each material gives different properties in terms of the wavelength of sensitivity, etc. In view of the environmental concerns of using Cadmium, this material is not used for product in Europe.

Regardless of the type of light dependent resistor or photoresistor, both types exhibit an increase in conductivity or fall in resistance with increasing levels of incident light. The sensitivity of photoresistors is shown to vary with the wavelength of the light that is impacting the sensitive area of the device. The effect is very marked and it is found that if the wavelength is outside a given range then there is no noticeable effect.

Devices made from different materials respond differently to light of different wavelengths, and this means that the different electronics components can be used for different applications. It is also found that extrinsic photoresists tend to be more sensitive to longer wavelength light and can be used for infrared.

However when working with infrared, care must be taken to avoid heat build-up caused but he elating effect of the radiation. One important aspect associated with photoresistors or light dependent resistors is that of the latency, or the time taken for the electronic component to respond to any changes. This aspect can be particularly important for a circuit design. However when the light changes take place over a period of time they are more than adequate.

The rate at which the resistance changes is called the resistance recovery rate. It is for this reason that one of the specifications normally quoted in the electronic component datasheets for photo-resistors is the dark resistance after a given time, typically in seconds. Often two values are quoted, one for one second and another for five seconds. These given an indication of the latency of the resistor. Photoresistors are found in many different applications and can be seen in many different electronic circuit designs.

They have a very simple structure and they are low cost and rugged devices. They are widely used in many different items of electronic equipment and circuit designs including photographic light meters, fire or smoke alarms as well as burglar alarms, and they also find uses as lighting controls for street lamps. Extrinsic photoresistors are provide sensitivity for longer wavelengths and as a result they are popular in various electronic circuit designs as info-red photodetectors.

Photoresistors can also be used to detect nuclear radiation. LDRs are very useful electronic components that can be used for a variety of light sensing applications and their associated electronic circuit designs. As the LDR resistance varies over such a wide range, they are particularly useful, and there are many LDR circuit designs available beyond any shown here. In order to utilise these electronic components, it is necessary to know something of how an LDR works, which has been explained above.

Typical leaded light dependent resistor What is light dependent resistor, LDR or photoresistor A photoresistor or light dependent resistor is an electronic component that is sensitive to light. The basic format for a photoresistor is that shown below: Photoresistor structure The active semiconductor region is normally deposited onto a semi-insulating substrate and the active region is normally lightly doped. Photoresistor structure showing interdigital pattern to maximise exposed area.

Types of photoresistor Light dependent resistors, LDRs or photoresistors fall into one of two types or categories: Intrinsic photoresistors: Intrinsic photoresistors use un-doped semiconductor materials including silicon or germanium.

Photons fall on the LDR excite electrons moving them from the valence band to the conduction band.