Photoresistor: device, principle of operation, characteristics

In industry and consumer electronics, photoresistors are used to measure light, count something, identify obstacles, and more. Its main purpose is to convert the amount of light falling on the sensitive area into a useful electrical signal. The signal can then be processed by an analog, digital logic circuit or a microcontroller-based circuit. In this article, we will tell you how a photoresistor works and how its properties change under the influence of light.

Basic concepts and device

A photoresistor is a semiconductor device whose resistance (conductivity, if convenient) changes depending on how much its sensitive surface is illuminated. Structurally, they are found in various designs. The most common elements of this design are as shown in the figure below. At the same time, for 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 can see its conventional graphic designation on the diagram.

Interesting: the change in resistance under the influence of a light flux is called the photoresistive effect.

The principle of operation is as follows: between two conducting electrodes there is a semiconductor (on the figure is shown in red), when the semiconductor is not illuminated - its resistance is high, up to units MOhm. When this area is illuminated, its conductivity rises sharply, and the resistance falls accordingly.

Materials such as Cadmium sulfide, Lead sulfide, Cadmium Selenite and others can be used as a semiconductor. The choice of material in the manufacture of a photoresistor depends on its spectral characteristic. In simple words - the range of colors (wavelengths) when illuminated by which the resistance of the element will change correctly. Therefore, when choosing a photoresistor, you need to take into account in what spectrum it works. For example, for UV-sensitive elements it is necessary to select those types of emitters whose spectral characteristics are suitable for photoresistors. The figure that describes the spectral characteristics of each of the materials is shown below.

One of the frequently asked questions is "Does the photoresistor have polarity?" 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 using a multimeter in resistance measurement mode by measuring the resistance of the illuminated and darkened element.

You can see the approximate dependence of resistance on illumination in the graph below:

It shows how the current changes at a certain voltage depending on the amount of light, where Ф = 0 - darkness, and Ф3 - bright light. The following graph shows the change in current at constant voltage, but varying illumination:

In the third graph, you can see the dependence of resistance on illumination:

In the picture below you can see how popular photoresistors made in the USSR look like:

Modern photoresistors, which are widespread in the practice of home-made, look a little different:

Letter marking is usually used to designate an element.

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 a luminous flux.
• Integral photosensitivity - describes the response of an element, a change in current through it to a change in luminous flux. Measured at constant voltage in A / lm (or mA, μA / lm). Designated as S. S = Iph / F, where Iph is the photocurrent, and F is the luminous flux.

In this case, it is the photocurrent that 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 photoconduction effect (the same as the photoresistive effect).

Note: dark resistance, of course, is typical for 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 irradiation with a light flux, but with a slight delay. This parameter is called the cutoff frequency. This is the frequency of the sinusoidal signal modulating the luminous flux through the element, at which the sensitivity of the element decreases by the root of 2 times (1.41). The speed of the components is usually within tens of microseconds (10 ^ (- 5) s). Thus, the use of a photoresistor in circuits where fast response is required is limited and often unnecessary.

Where is used

When we learned about the device and parameters of photoresistors, let's talk about what it is for using specific examples. Although the use of photoresistors is limited by their speed, this has not made the field of application smaller.

1. Twilight relays. They are also called photo relays - these are devices for automatically turning on the light at night. The diagram below shows the simplest version of such a circuit, using analog components and an electromechanical relay. Its disadvantage is the absence of hysteresis and the possible occurrence of rattling at near-border values. illumination, as a result of which the relay will rattle or turn on / off with slight fluctuations illumination.
2. Light sensors. With the help of photoresistors, weak luminous flux can be detected. Below is an implementation of such a device based on ARDUINO UNO.
3. Alarms. Such circuits predominantly 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 an actuator is triggered. For example, a turnstile in the subway.
4. Sensors for the presence of something. For example, in the printing industry, photoresistors can be used to control ribbon breaks or the number of sheets fed into the press. The principle of operation is similar to that discussed above. In the same way, you can count the amount of products passed along the conveyor belt, or its size (at a known speed).

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, so 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:

You probably don't know:

• How to make a photo relay with your own hands
• How to connect a motion sensor for lighting
• What is a resistor and what is it for