A photoresistor or Light Dependent Resistor (LDR) is a resistor whose resistance decreases with increasing incident light intensity. Light detection operation depends on the fact that the resistance of certain materials like Cadmium Sulphide (CdS) varies with the intensity of light falling on the surface of the film.

A light-emitting diode (LED) is a semiconductor light source. When a light-emitting diode is forward biased, electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the color of the light is determined by the energy gap of the semiconductor. Here we are finding the relationship between the input voltage V and forward current I. The biasing voltage is increased from zero to V in suitable steps by varying the biasing supply. The corresponding reading in the ammeter connected to the LED is noted at each step. Agraph plotted between V and I, which represents the V-I characteristics.

In a phototransistor, the base region is enlarged and generally does not have a lead attached to it. The collector-base junction is sensitive to light falling on it. When light falls on the base junction, a base current proportional to light intensity is produced. This initiates a collector current proportional to the light intensity.

A photodiode is a type of photo detector capable of converting light into either current or voltage, depending upon the mode of operation. When a photon of sufficient energy strikes the diode, it excites an electron, thereby creating a free electron and a positively charged electron hole. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. This photocurrent is the sum of both the dark current (without light) and the light current, so the dark current must be minimized to enhance the sensitivity of the device.

A solar cell is a semiconductor device, which generates an emf when illuminated by light. When light hits the surface of a solar cell near the p-n junction, the crystal is ionized and new electron hole pairs are generated. These electrons and holes created in the n and p-region diffuse towards the junction and if they have no time to recombine in the transit, they fall under the influence of the internal electric field existing in the junction. The electric field forces the electrons to move to n-region and holes to the p-region. This produces an excess of hole in the p-region and electrons in the n-region. These electrons and holes buildup charges in their respective regions and a potential difference called photo emf appear across the cell.

The opto-coupler or opt isolator is essentially a device that uses a short optical path to couple an electrical signal from one area to another. Typically the opto coupler is housed within a single small package, often around the size of a small integrated circuit, although sizes vary according to the application and the specification.

Graphs:

Fig. V - I Characteristics

Photoelectric or photovoltaic effect, used in :

    Photo diodes

    Solar cells

    Photo transistors

Photo conductivity, used in :

    Light Dependent Resistor (LDR)

Lossev effect or radiative recombination, used in :

    Light - Emitting Diodes or LED

Optical isolation, used in :

    Opto - Coupler