The electronics industry produces many different products, among which light-sensitive elements occupy a special place. The simplest of them are represented by a radio component called "photoresistor", which belongs to the category of semiconductors.
When radiant energy from any light source hits its surface, the resistance of the material decreases, which causes a change in the current flowing through the element. The figure below shows the appearance of the photoresistor and its generally accepted designation on electrical circuits.
Operating principle
To understand the principle of operation of this radioelement, you first need to familiarize yourself with its device. Inside the product, between two conductors (electrodes), there is a semiconductor layer that is sensitive to the intensity of light exposure.
For its formation, such common combinations of elements of the periodic table are used as:
- Cadmium sulfide.
- Lead sulphide.
- Cadmium selenite and the like.
Additional Information: The spectral characteristic of the device depends on the selected material.
In other words, the range of wavelengths depends on the type of semiconductor, under the influence of which the resistance of the material changes.
When the light does not irradiate it, it is large enough and reaches tens of megohms. When light energy hits the semiconductor, a sharp increase in the concentration of charge carriers (electrons and holes) occurs. The resistance of the material decreases sharply, which leads to an increase in the current flowing along the chain. The latter is possible in a situation where the photoresistor is connected to an external power source, as shown in the photo below.
Due to the absence of an electron-hole junction as such, the polarity of the photoresistor switching on does not matter.
Application area
Let's make a reservation right away that the resistance of photocells of this class cannot change instantly, that is, they have a certain inertness. This drawback limits the scope of application of photoresistors to circuits with low speed.
Taking into account the noted limitations, the considered elements are widely used in the following electronic devices and systems:
- In standard photo relays designed for automatic control of existing lighting networks.
- In light sensors, where photoresistors are used as sensitive recorders of the luminous flux level.
- In modern systems, alarms in which photocells are installed that are sensitive to waves from the UV range.
In the latter case, the principle of operation of the system with a photoresistor is based on interrupting the radiation flux from an ultraviolet source directed towards the sensitive element. Examples of its use in radiation receivers are shown in the photo below.