Do-it-yourself photo relay assembly method. Do-it-yourself relay repair

The operation of various semiconductor radiation receivers (photoresistors, photodiodes, phototransistors, photothyristors) is based on the use of the internal photoelectric effect, which consists in the fact that under the influence of radiation, pairs of charge carriers - electrons and holes - are generated in semiconductors. These additional carriers increase electrical conductivity. This additional conductivity due to the action of photons is called photoconductivity. In metals, the phenomenon of photoconductivity is practically absent, since their concentration of conduction electrons is huge (approximately 1022 cm -3) and cannot noticeably increase under the influence of radiation. In some devices, due to the photogeneration of electrons and holes, an emf arises, which is commonly called photo-emf, and then these devices work as current sources. And as a result of the recombination of electrons and holes in semiconductors, photons are formed, and under certain conditions, semiconductor devices can act as radiation sources.

A phototransistor is a photosensitive semiconductor radiation receiver, similar in structure to a transistor and providing internal signal amplification. It can be thought of as consisting of a photodiode and a transistor. The photodiode is the illuminated part of the base-collector junction, the transistor is the part of the structure located directly under the emitter. Since the photodiode and the collector junction of the transistor are structurally combined, the photocurrent is summed with the collector current. The supply voltage is supplied so that the collector junction is closed and the emitter junction is open. The base may be disabled.

Unlike a bipolar transistor, a phototransistor does not have an electrical contact to the base, and the base current is controlled by changing its illumination. For this reason, the phototransistor has only two terminals - an emitter and a collector.

Figure 2.1 - a) Schematic of a phototransistor with a p-n-p structure;

b) band diagram of a phototransistor in active mode

In Fig. Figure 2.1 shows the phototransistor switching circuit and the band diagram in the active operating mode.

When a light flux hits the n-region of the base, nonequilibrium electrons and holes are generated in it. The holes will be minority carriers; an increase in their concentration will lead to an increase in the drift component of the current from the base to the collector. The magnitude of the primary “seed” photocurrent will be expressed in the same ratios as the photocurrent of a diode based on a p-n junction. The only difference is that nonequilibrium carriers participating in the photocurrent in the phototransistor are collected from the base region, the width of which W is less than the diffusion length L p. Therefore, the density of the primary “seed” photocurrent will be:

Due to the fact that nonequilibrium holes move from the base to the collector, the base is negatively charged relative to the emitter, which is equivalent to forward bias of the emitter junction of the phototransistor. When the emitter pn junction is forward biased, an injection current component appears from the emitter to the base. At the emitter current transfer coefficient b, (1-b) injected carriers are recombined in the base or a factor of one less than the number of injected carriers. Under stationary current conditions, the number of recombined carriers in the base must be equal to the number that left with the initial photocurrent. Therefore, the injection current must be several times greater than the primary photocurrent. The collector current I K will consist of three components: the primary photocurrent I f, the injection current I K0 and the thermal current I K0.

I K = I f+v I f =(v+1) I f + I K0 (2.2)

Using the expression for the gain in the base current through the design and technological parameters of the bipolar transistor, we obtain:

The magnitude of the primary photocurrent I Ф is expressed through the parameters of the luminous flux and the characteristics of the semiconductor material in the standard way:

When the base is illuminated, electron-hole pairs appear in it. Just as in a photodiode, pairs that reach the collector junction as a result of diffusion are separated by the junction field, minority carriers from the base move to the collector, and its current increases. The majority carriers remain in the base, lowering its potential relative to the emitter. In this case, an additional forward voltage is created at the emitter junction, causing additional injection from the emitter to the base and a corresponding increase in the collector current.

Figure 2.2 - Energy diagram of the phototransistor (a) and current-voltage characteristics of the phototransistor at different lighting levels (b).

Operation of a phototransistor with a common emitter

Consider, for example, the operation of a phototransistor in a circuit with a common emitter with the base turned off. The photocurrent of the collector junction is summed with the reverse collector current, therefore in the formula for the transistor current, instead of J K0, one should put

J K0 + J Ф /J = (J K0 + J Ф)/(1-b).

When J K 0>>J Ф J =J Ф /(1-b) ? inJ Ф, i.e. The photocurrent of the phototransistor is amplified several times compared to the current of the photodiode. Accordingly, the sensitivity increases several times. The current can be amplified 1000 times, so the sensitivity of a phototransistor is many times greater than that of a photodiode. However, since the product of the gain and the frequency band is constant, the limiting frequency decreases by a factor of several times.

Figure 2.3 - Equivalent circuit of a phototransistor.

The presence of carrier diffusion causes significant inertia of the device f = 10-5 -10-6 s. As the base narrows, the diffusion time decreases, but the sensitivity also decreases. For germanium phototransistors SI = 0.2-0.5 A/lm, V slave = 3 V, I dark = 300 μA, f = 0.2 ms. A transparent window is provided in the device body, through which the light flux usually falls on the base area of ​​the phototransistor. The area of ​​the photosensitive area is 1-3 mm 2 .

Photoresistors are semiconductor resistors whose resistance changes under the influence of electromagnetic radiation in the optical range.

The photosensitive element in such devices is a rectangular or round tablet pressed from a semiconductor material, or a thin layer of a semiconductor deposited on a glass plate - a substrate. The semiconductor layer on both sides has leads for connecting a photoresistor to the circuit. On circuit diagrams, a photoresistor is indicated by a resistor sign in a circle with side arrows.
The electrical conductivity of a photoresistor depends on the illumination. The brighter the lighting of the device, the lower the resistance of the photoresistor and the greater the circuit current.
These devices are used in automatic control circuits.

Photodiodes are a type of semiconductor diode. Until the photocell is refreshed, the blocking layer prevents the mutual exchange of electrons and holes between the semiconductor layers. When irradiated, light penetrates the “p” layer and knocks electrons out of it. The released electrons pass into the “n” layer and neutralize holes there. A potential difference arises between the photodiode terminals, which can be amplified by an electronic circuit to turn on automation and telemechanics devices.
Photodiodes are used to assemble power batteries in everyday life and on spacecraft.

Phototransistors are photocells based on transistors. This photo lighting relay uses a direct conduction phototransistor. To ensure that the light flux reaches the semiconductor crystal, the transistor cover is removed by simply removing it with pliers.

The photo relay in the figure above is used to automatically turn off or turn on actuators when the lighting changes.

Resistor R1, R2 and phototransistor VT1 represent a voltage divider based on transistor VT2. When phototransistor VT1 is illuminated, the voltage at the base of transistor VT2 decreases, transistor VT2 closes, and VT3 opens.

Relay K1 is triggered by the passage of current and opens contacts K 1-2, power supply to the load stops. Diode VD2 protects transistor VT3 from pulse noise that occurs when switching current in the winding of relay K1.

Relay contacts can be used to switch automation and telemechanics actuators.
Resistor R1 sets the sensitivity threshold, and R4 the illumination threshold.

LED HL1 indicates power on and operation mode of relay K1. Capacitor C1 prevents the relay from operating in the presence of interference. The power supply of the relay circuit is stabilized by the DA1 analog microcircuit. Capacitors C2, C3 are included in the anti-aliasing filter. Diode bridge VD1 is selected for a current of up to 1 ampere and a voltage of 50-100 Volts.
The device is equipped with a power switch S1 and a fuse F1.
The design of the VT1 phototransistor is simple: the “cap” of the transistor is removed with pliers, the transistor is glued to the M.8 nut, and the nut with the transistor is to a piece of glass and attached to the device.

A correctly assembled device should work immediately. When the slider of resistor R1 is in the upper position and resistor R4 is in the middle position, when lighting is applied to phototransistor VT1, relay K1 should operate. First check the relay by directly turning on the 12 volt power supply. Use resistor R1 to “adjust” the sensitivity of the photo relay at a given lighting R4.

List of radioelements

Photovoltaic devices are electronic devices that can change certain of their characteristics under the influence of light. The importance of these devices in almost all areas of radio engineering and electronics is difficult to overestimate, so we will devote today’s conversation to them.

Photoresistors. In principle, the name of the device speaks for itself - they change their resistance under the influence of light. Typically, a darkened resistor has a resistance of about 1 - 200 MOhm; when illuminated, this figure decreases by 2-3 orders of magnitude. The main advantage of a photoresistor is the almost linear dependence of resistance on illumination, so they are convenient to use in analog devices - sensors and illumination meters.

The disadvantages of photoresistors are the following: fairly high resistances (both dark and light), which are not always convenient to work with. For example, TTL microcircuits of digital technology cannot be directly controlled by such a resistor - their too “coarse” inputs will not be able to work with dividers assembled on high-value resistances:

Only CMOS microcircuits assembled on field-effect transistors are capable of this. The next drawback is the rather low (compared, of course, with other types of photocells) sensitivity. And the main drawback that makes the use of photoresistors in digital technology impractical is the low speed of reaction to light. At a frequency of light pulses above a kilohertz, the shape of the electrical signal on the photoresistor is unsatisfactory, and if you increase the frequency further, the resistor will no longer see the light “blinking” at all.

If you remember at what frequencies today’s digital technology operates, it will be obvious that a photoresistor as an “eye” for a digital device is a bad option. A photoresistor is a non-polar device, and therefore there is no need to monitor which output is connected where.

Photodiodes. This semiconductor device, in its characteristics, is very similar to a regular diode, so you will have to monitor the polarity of its connection.

When turned on in reverse (the “plus” of the power supply is applied to the cathode), the photodiode behaves in the same way as a photoresistor, but unlike the latter, it has a much lower light resistance and is able to withstand a decent current. This allows you to control powerful transistors and TTL microcircuits directly, without additional amplifiers:

Another advantage of the photodiode is its fairly high reaction speed, due to which these devices are widely used for transmitting digital information. Computer IR communications, remote controls for radio and television equipment are all photodiodes. Based on their sensitivity range, photodiodes are divided into infrared and visible radiation devices. The former “see” mainly IR radiation and are little sensitive to the visible part of the radiation, the latter, on the contrary, see well the light that our eyes see, but are “blind” in the IR range.

And one more interesting property of the photodiode is that when connected directly, it can work as a generator. If you illuminate a photodiode, voltage will appear at its terminals. It can be amplified if the device works as a light sensor, or it can also be used to power equipment by connecting many LEDs into a solar battery.

Phototransistor. In essence, this is an ordinary transistor, but without a lid in the literal sense. Of course, there is a cover covering the crystal of the device, but it is made of transparent material and visible light can fall on the crystal. For what? First of all, let's remember.

By applying some voltage to the base, you can control the resistance of the emitter-collector junction. But it turns out that the junction resistance can also be controlled by ordinary light. So, a phototransistor is an ordinary transistor that has one more, additional “base” - a light one. We illuminate - we open the transistor. In such a connection, the base output of the phototransistor can not be used at all - its role is played by light.

But by applying one or another bias voltage to the base, you can change the sensitivity of the phototransistor (experts usually say “shift, shift its operating point”), opening it slightly to one degree or another, and therefore adjusting the parameters of the entire circuit:

Phototransistors are solid-state semiconductors with internal amplification used to transmit digital and analog signals. This device is made on the basis of a conventional transistor. Analogues of phototransistors are photodiodes, which are inferior to them in many properties and are not compatible with the operation of modern electronic devices and radio devices. Their operating principle is similar to that of a photoresistor.

The sensitivity of a phototransistor is much higher than that of a photodiode. They have found application in various devices that rely on luminous flux dependence. Such devices are laser radars, remote controls, smoke detectors and others. Phototransistors can respond to both ordinary lighting and ultraviolet and infrared radiation.

Phototransistors. Device

The most popular are bipolar phototransistors of the n-p-n structure.

F transistors are more sensitive to light than simple bipolar transistors because they are optimized to interact better with light rays. In their design, the collector and base area has a large area. The body is made of dark opaque material, with a window for light transmission.

Most of these semiconductors are made from single crystals of germanium and silicon. There are also phototransistors based on complex materials.

Operating principle

A transistor includes a base, collector and emitter. When a phototransistor operates, the base is not turned on because the light creates an electrical signal that allows current to flow through the semiconductor junction.

When the base is not working, the collector junction of the transistor is biased in the opposite direction, and the emitter junction is biased in the forward direction. The device remains inactive until a beam of light illuminates its base. Illumination activates the semiconductor, creating pairs of holes and conduction electrons, that is, charge carriers. As a result, current passes through the collector and emitter.

Gain property

Phototransistors have an operating range, the size of which depends on the intensity of the incident light, as this is related to the positive potential of its base.

The base current from the incident light is amplified hundreds and thousands of times. Additional current amplification is provided by a special Darlington transistor, which is a semiconductor whose emitter is connected to the base of another bipolar transistor. The diagram shows this type of phototransistor.

This makes it possible to create increased sensitivity in low light, since double amplification occurs by two semiconductors. With two transistors, amplification of hundreds of thousands of times can be achieved. It must be taken into account that the Darlington transistor responds more slowly to light, unlike a conventional phototransistor.

Connection diagrams

Common emitter circuit

This circuit creates an output signal that goes from a high state to a low state when light rays fall.

This circuit is made by connecting a resistance between the collector of the transistor and the power supply. The output voltage is removed from the collector.

Common collector circuit

An amplifier connected to a common collector produces an output signal that goes from low to high when light hits the semiconductor.

This circuit is formed by connecting a resistance between the negative supply terminal and the emitter. The output signal is removed from the emitter.

In both options, the transistor can operate in 2 modes:

1. Active mode.
2. Switching mode.

Active mode

In this mode, the phototransistor produces an output signal that depends on the intensity of the incident light. When the light level exceeds a certain limit, the transistor becomes saturated, and the output signal will no longer increase, even if the intensity of the light rays increases. This mode of operation is recommended for devices with a function for comparing two light flux thresholds.

Switching mode

Operating a semiconductor in this mode means that the transistor will respond to light by turning off or on. This mode is necessary for devices that require the output signal to be received in digital form. By changing the resistor value in the amplifier circuit, you can select one of the operating modes.

To operate a phototransistor as a switch, a resistance of more than 5 kOhm is most often used. The high-level output voltage in switching mode will be equal to the supply voltage. The low level output voltage should be less than 0.8 V.

Checking the phototransistor

Such a transistor can be easily checked, even without the presence of a transistor base. If you connect a multitester to the emitter-collector section, then its resistance at any polarity will be high, since the transistor is closed. If a beam of light hits the sensitive element, the measuring device will show a low resistance value, since the transistor in this case opened, thanks to the light, with the correct polarity of the power supply.

This is how a normal transistor behaves, but it is opened by an electric current signal, not by a beam of light. In addition to luminous intensity, the spectral composition of light plays an important role.

Application

• Security systems (infrared f-transistors are often used).
• Photo relay.
• Data calculation systems and level sensors.
• Automatic switching systems for lighting devices (infrared f-transistors are also used).
• Computer control logic systems.
• Coders.

Advantages

• They produce more current than photodiodes.
• Capable of creating an instantaneous high output current.
• The main advantage is the ability to create increased voltage, unlike photoresistors.
• Low cost.

Flaws

F-transistors are an analogue of photodiodes, but they have serious disadvantages that create conditions for the narrow specialization of this semiconductor.

• Many types of phototransistors are made of silicone, so they cannot operate at voltages higher than 1 kV.
• Such photosensitive semiconductors are highly dependent on supply voltage fluctuations in the electrical circuit. In such modes, the photodiode behaves much more reliably.
• F-transistors are not compatible with operation in lamps due to the low speed of charge carriers.

Symbols on diagrams

Transistors controlled by the light flux are designated in the diagrams as ordinary transistors.

VT1 and VT2 are f-transistors with a base, VT3 are transistors without a base. The pinout is shown as for simple transistors.

Just like other devices based on semiconductors with an n-p-n junction, used to convert light flux, phototransistors can be called optocouplers. They are depicted in the diagrams as an LED in a housing, or as optocouplers with arrows. The amplifier in many circuits is designated as a base and a collector.

One of the main elements of automation in street lighting, along with timers and motion sensors, is a photo relay or twilight relay. The purpose of this device is to automatically connect the payload when it gets dark, without human intervention. This device has also gained immense popularity due to its low cost, availability and ease of connection. In this article we will analyze in detail the principle of operation of the twilight switch and the nuances of its connection, and also tell you how to make a photo relay with your own hands. This will not take much time and effort, but you will be pleased to use the self-assembled device.

Relay design

The main element of the relay is a photosensor; diodes, transistors, and photoelectric elements can be used in circuits. When the illumination on a photocell changes, its properties, such as resistance, states of the P-N junction in diodes and transistors, as well as the voltage at the contacts of the photosensitive element, change accordingly. Next, the signal is amplified and the power element switching the load occurs. Relays or triacs are used as output control elements.

Almost all purchased elements are assembled according to a similar principle and have two inputs and two outputs. A mains voltage of 220 Volts is supplied to the input, which, depending on the set parameters, also appears at the output. Sometimes a photo relay has only 3 wires. Then zero is common, a phase is supplied to one wire, and at the required illumination it is connected to the remaining wire.

If necessary, read the instructions, pay special attention to the maximum power of the connected load, the type of lighting lamps (incandescent, gas-discharge, LED lamps). It is important to know that lighting relays with a thyristor output will not be able to work with energy-saving lamps, as well as with some types due to design features. This nuance must be taken into account in order not to damage the equipment.

Let's look at several schemes for self-assembling a twilight switch at home. For example, let's look at how to make a triac night light with a photocell.

Assembly instructions

This is the most elementary photo relay circuit consisting of several parts: a Quadrac Q60 triac, reference resistor R1, and a photo of the FSK element:

In the absence of light, the triac key opens completely and the lamp in the night light shines at full intensity. As the illumination in the room increases, the voltage shifts at the control contact and the brightness of the lamp changes, until the light bulb goes out completely.

Please note that the circuit contains dangerous voltage. It must be connected and tested with extreme care. And the finished device must be in a dielectric housing.

The following circuit with relay output:

Transistor VT1 amplifies the signal from the voltage divider, which consists of photoresistor PR1 and resistor R1. VT2 controls the electromagnetic relay K1, which can have both normally open and normally closed contacts, depending on its purpose. Diode VD1 shunts voltage pulses when the coil is turned off, protecting transistors from failure due to reverse voltage surges. Having examined this circuit, you can find that part of it (highlighted in red) is close in functionality to ready-made relay module assemblies for Arduino.

Having slightly altered the circuit and supplemented it with one transistor and a solar photocell from an old calculator, a prototype of a twilight switch was assembled - a homemade photo relay on a transistor. When solar cell PR1 is illuminated, transistor VT1 opens and sends a signal to the output relay module, which switches its contacts to control the payload.