Light Dependent Resistor, Photoresistor, or Photocell

- notes on the basics of a photoresistor, light dependent resistor or photocell including its construction, operation, circuit symbol, and circuit applications

The light dependent resistor, LDR, is known by many names including the photoresistor, photo resistor, photoconductor, photoconductive cell, or simply the photocell. It is probably the term photocell that is most widely used in data and instruction sheets for domestic equipment.

The photoresistor, or light dependent resistor, LDR, finds many uses as a low cost photo sensitive element and was used for many years in photographic light meters as well as in other applications such as flame, smoke and burglar detectors, card readers and lighting controls for street lamps. Often within the literature the photoresistor is called the photocell as a more generic term.

Photoresistor discovery

Photoresistors, or light dependent resistors have been in use for very many years. Photoresistors have been seen in early forms since the nineteenth century when photoconductivity in selenium was discovered by Smith in 1873. Since then many variants of photoconductive devices have been made.

Much useful work was conducted by T. W. Case in 1920 when he published a paper entitled "Thalofide Cell - a new photo-electric cell".

Other substances including PbS, PbSe and PbTe were studied in the 1930s and 1940s, and then in 1952, Rollin and Simmons developed their photoconductors using silicon and germanium.

Light dependent resistor symbol

The circuit symbol used for the light dependent resistor or photoresistor combines its resistor action while indicating that it is sensitive to light. The basic light dependent resistor symbol has the rectangle used to indicate its resistor action, and then has two incoming arrows - the same as those used for photodiodes and phototransistors to indicate its light sensitivity.

Light dependent resistor symbol used in circuit diagrams

For most applications, the light dependent resistor symbol used will be that with the resistor with arrows, but in some instances those drawing circuit diagrams prefer to encase the resistor in a circle. The more commonly used photoresistor symbol is the resistor without the circle around it.

Photoresistor mechanism

A photoresistor or photocell is a component that uses a photconductor between two contacts. When this is exposed to light a change in resistance is noted.

Photoconductivity - the mechanism behind the photoresistor - results from the generation of mobile carriers when photons are absorbed by the semiconductor material used for the photoconductor. While the different types of material used for light dependent resistors are semiconductors, when used as a photo-resistor, they are used only as a resistive element and there are no PN junctions. Accordingly the device is purely passive.

There are two types of photoconductor and hence photoresistor:

• Intrinsic photoresistor:   This type of photoresistor uses a photoconductive material that involves excitation of charge carriers from the valence bands to the conduction band.
• Extrinsic photoresistor:   This type of photoresistor uses a photoconductive material that involves excitation of charge carriers between an impurity and the valence band or conduction band. It requires shallow impurity dopants that are not ionised in the presence of light.
  
  Extrinisc photoresistors or photocells are generally designed for long wavelength radiation - often infra-red, but to avoid thermal generation they need to be operated at low temperatures.

Basic photoresistor structure

Although there are many ways in which light dependent resistors, or photo resistors can be manufactured, there are naturally a few more common methods that are seen. Essentially the photoresisitor or photocell consists of a resistive material sensitive to light that is exposed to light. The photo resistive element comprises section of the material with contacts at either end.

A typical structure for a light dependent or photo resistor uses an active semiconductor layer that is deposited on an insulating substrate. The semiconductor is normally lightly doped to enable it to have the required level of conductivity. Contacts are then placed either side of the exposed area.

One form of photoresistor structure

Within the basic photoresistor or photocell structure, the resistance of the material itself is a key issue. To ensure the resistance changes resulting from the light dominate, contact resistance is minimised. To achieve this, the area around the contacts is normally heavily doped to reduce the resistance in this region.

In many instances the area between the contacts is in the form of a zig zag, or interdigital pattern. This maximises the exposed area and by keeping the distance between the contacts small it reduces the spurious resistance levels and enhances the gain.

Photoresistor or photocell with interdigital contact pattern

It is also possible to use a polycrystalline semiconductor that is deposited onto a substrate such as ceramic. This makes for a very low cost light dependent resistor

Photoresistor applications

The photoresistor or light dependent resistor is attractive in many electronic circuit designs because of its low cost, simple structure and rugged features. While it may not have some of the features of the photo-diode and photo-transistor, it is ideal for many applications. As a result the photo-resistor is widely used in circuits such as photographic meters, flame or smoke detectors, burglar alarms, card readers, controls for street lighting and many others.

The properties of photoresistors can vary quite widely dependent upon the type of material used. Some have very long time constants, for example. It is therefore necessary to carefully choose the type of photoresistor for any given circuit or application.

By Ian Poole

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