What is a Light Dependent Resistor?

A Light Dependent Resistor (LDR) or a photograph electrical device could be a device whose impedance could be a perform of the incident nonparticulate radiation. Hence, they're lightweight sensitive devices. they're conjointly known as as ikon conductors, ikon conductive  cells or just photocells. they're created of semiconductor materials having high resistance. There ar many various symbols wont to indicate a LDR, one among the foremost unremarkably used image. The arrow indicates lightweight falling thereon. lightweight Dependent electrical device or Photoresistor, that could be a passive electronic part, essentially a electrical device that encompasses a resistance that varies relying of the sunshine intensity. A photoresistor is created of a high resistance semiconductor that absorbs photons and supported the number and frequency of the absorbed photons the semiconductor material offer certain electrons enough energy to leap into the physical phenomenon band. The ensuing free electrons conduct electricity leading to lowering resistance of the photoresistor. the quantity of electrons relies of the photons frequency.

Working Principle of LDR:

A light dependent electrical device works on the principle of ikon physical phenomenon. ikon physical phenomenon is AN physical phenomenon during which the fabrics physical phenomenon is increased  once lightweight is absorbed by the material. once lightweight falls i.e. once the photons fall on the device, the electrons within the valence band of the semiconductor material ar excited to the physical phenomenon band. These photons within the incident lightweight ought to have energy bigger than the band gap of the semiconductor material to form the electrons jump from the valence band to the physical phenomenon band. thus once lightweight having enough energy strikes on the device, a lot of and a lot of electrons ar excited to the physical phenomenon band which ends in sizable amount of charge carriers. The results of this method is a lot of and a lot of current starts flowing throgh the device once the circuit is closed and thus it's aforesaid that the resistance of the device has been weakened. this can be the foremost common working rule of LDR.


Characteristics of LDR:

LDR’s ar lightweight dependent devices whose resistance is weakened once lightweight falls on them which is increased  within the dark. once a light-weight dependent electrical device is unbroken in dark, its resistance is extremely high. This resistance is termed as dark resistance. It is as high as 1012 Ω and if the device is allowed to soak up lightweight its resistance are going to be weakened drastically. If a continuing voltage is applied thereto and intensity of sunshine is increased  this starts increasing. Figure below shows resistance vs. illumination curve for a selected LDR.

Photocells or LDR’s ar non linear devices. There sensitivity varies with the wavelength of sunshine incident on them. Some photocells won't the least bit response to an explicit vary of wavelengths. supported the fabric used completely different|completely different} cells have different spectral response curves.

When lightweight is incident on a photoconductive cell it always takes regarding eight to twelve ms for the modification in resistance to require place, whereas it takes one or a lot of seconds for the resistance to rise back once more to its initial price once removal of sunshine. This development is termed as resistance recovery rate. This property is employed in audio compressors. Also, LDR’s ar less sensitive than ikon diodes and ikon electronic transistor. (A ikon diode and a photoconductive cell (LDR) aren't constant, a photo-diode could be a tangency semiconductor unit that converts lightweight to electricity, whereas a photoconductive cell could be a passive device, there's no tangency during this nor it “converts” lightweight to electricity). varieties of lightweight Dependent Resistors: supported the materials used they're classified as:

1. Intrinsic ikon resistors (Un doped semiconductor): These ar product of pure semiconductor materials like semiconductor or semiconducting material. Electrons get excited from framework band to physical phenomenon band once photons of enough energy fall thereon and variety charge carriers is increased .

2. Accidental ikon resistors: These ar semiconductor materials doped with impurities that ar known as as dopants. Theses dopants produce new energy bands higher than the valence band that ar full of electrons. thus this reduces the band gap and fewer energy is needed in exciting them. accidental ikon resistors ar usually used for long wavelengths.


Construction of a Photocell:

The structure of a light-weight dependent electrical device consists of a light-weight sensitive material that is deposited on AN insulating substrate like ceramic. the fabric is deposited in zigzag pattern so as to get the required resistance & power rating. This zigzag space separates the metal deposited areas into 2 regions. Then the resistance unit contacts ar created on the either sides of the realm. The resistances of those contacts ought to be as less as doable to form certain that the resistance in the main changes as a result of the result of sunshine solely. Materials commonly used ar mineral, metallic element selenide, In antimonide and metallic element sulphonide. the employment of lead and metallic element is avoided as they're harmful to the surroundings.

How will the LDR (photoresistor) works:

The resistance is extremely high darkly, nearly high as 1MΩ however once there's lightweight that falls on the LDR, the resistance is falling all the way down to a number of KΩ (10-20kΩ @ ten illumination unit, 2-4kOmega; @ a hundred lux) looking on the model.
Light dependent resistors are available in totally different shapes and colours.LDRs ar terribly helpful in several electronic circuits, particularly in alarms, change devices, clocks, street lights and a lot of. There ar some audio application uses like audio limiters or compressors. it's wont to activate or OFF a tool consistent with the close lightweight.

On electroschematics.com we've got some circuits that uses the photoresistor.
They are all labeled  as LDR. Here ar a number of them.

Conclusion of LDR:

LDR’s have low price and straightforward structure. they're usually used as lightweight sensors. they're used once there's a requirement to find absences or presences of sunshine like during a camera photographic equipment. utilized in street lamps, watch, thief alarm circuits, intensity meters, for numeration the packages moving on a conveyer, etc...