What are photoresistors, how do they work and where are they used
Basic concepts and device
A photoresistor is a semiconductor device whose resistance (if convenient - conductivity) varies depending on how much its sensitive surface is illuminated. Structurally found in various designs. The most common elements of this design, as shown in the figure below. In this case, to work in specific conditions, you can find photoresistors enclosed in a metal case with a window through which light enters the sensitive surface. Below you see its graphic symbol in the diagram.
Interesting: a change in resistance under the influence of light flux is called the photoresistive effect.
The principle of operation is as follows: between the two conductive electrodes there is a semiconductor (shown in red in the figure), when the semiconductor is not lit - its resistance is high, up to several megohms. When this area is illuminated, its conductivity increases sharply, and the resistance decreases accordingly.
Such materials as cadmium sulfide, lead sulfide, cadmium selenite and others can be used as a semiconductor. The spectral characteristic depends on the choice of material in the manufacture of the photoresistor. In simple words - a range of colors (wavelengths) when illuminated by which the resistance of an element will correctly change. Therefore, choosing a photoresistor, you need to consider in which spectrum it works. For example, for UV-sensitive elements, you need to select those types of emitters whose spectral characteristics are suitable for photoresistors. A figure that describes the spectral characteristics of each of the materials is shown below.
One frequently asked question is “Is there a polarity in the photoresistor?” The answer is no. Photoresistors do not have a pn junction, so it does not matter in which direction the current flows. You can check the photoresistor with a multimeter in the resistance measurement mode by measuring the resistance of the lighted and darkened element.
You can see an approximate dependence of resistance on illumination in the graph below:
Here, it is shown how the current changes at a certain voltage depending on the amount of light, where Ф = 0 is darkness, and Ф3 is bright light.The following graph shows the change in current at constant voltage, but changing illumination:
In the third graph, you see the dependence of resistance on illumination:
In the figure below, you can see how popular photoresistors made in the USSR look like:
Modern photoresistors, which are widely used in the practice of do-it-yourselfers, look a little different:
An element is usually marked with lettering.
Photoresistor Characteristics
So, photoresistors have the main characteristics that are paid attention to when choosing:
- Dark resistance. As the name implies, this is the resistance of the photoresistor in the dark, that is, in the absence of light flux.
- Integral photosensitivity - describes the response of an element, the change in current through it to a change in light flux. Measured at a constant voltage in A / lm (or mA, µA / lm). It is designated as S. S = Iph / F, where Iph is the photocurrent, and F is the light flux.
In this case, the photocurrent is indicated. This is the difference between the dark current and the current of the illuminated element, that is, the part that arose due to the photoconductivity effect (the same as the photoresistive effect).
Note: dark resistance is, of course, characteristic of each specific model, for example, for FSK-G7 - it is 5 MΩ, and the integral sensitivity is 0.7 A / lm.
Remember that photoresistors have a certain inertia, that is, its resistance does not change immediately after exposure to light flux, but with a slight delay. This parameter is called the cutoff frequency. This is the frequency of the sinusoidal signal modulating the light flux through the element at which the sensitivity of the element decreases by a factor of 2 (1.41). The speed of components usually lies within tens of microseconds (10 ^ (- 5) s). Thus, the use of a photoresistor in circuits where a fast response is needed is limited, and often unjustified.
Where is used
When we learned about the device and the parameters of photoresistors, let's talk about why it is needed with specific examples. Although the use of photo resistances is limited by their speed, the scope has not become less.
- Twilight relays. They are also called photorelay - these are devices for automatically turning on the light in the dark. The diagram below shows the simplest version of such a circuit, on analog components and an electromechanical relay. Its disadvantage is the absence of hysteresis and the possible occurrence of rattling at cross-border illumination values, as a result of which the relay will rattle or turn on or off with slight fluctuations in illumination.
- Light sensors. Using photoresistors, a weak luminous flux can be detected. Below is an implementation of such a device based on ARDUINO UNO.
- Alarms. Such circuits primarily use elements that are sensitive to ultraviolet radiation. The sensitive element is illuminated by the emitter, in the event of an obstacle between them, an alarm or actuator is triggered. For example, a turnstile in the subway.
- Sensors of the presence of something. For example, in the printing industry using photoresistors, you can control the breakage of the paper tape or the number of sheets fed to the printing machine. The principle of operation is similar to that discussed above. In the same way, the quantity of products that have passed along the conveyor belt, or its size (at a known speed) can be considered.
We briefly talked about what a photoresistor is, where it is used and how it works. The practical use of the element is very wide, therefore it is rather difficult to describe all the features within one article. If you have any questions - write them in the comments.
Finally, we recommend watching a useful video on the topic:
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