How to make an inductive sensor for an oscilloscope. Inductive sensors of the electromagnetic field in microcircuits. Color coding of sensor outputs

What are capacitive sensors? This is the most common electronic relay that is triggered by a change in capacitance. The sensing element of many of the circuits discussed here are high frequency oscillators from hundreds of kilohertz or more. If an additional capacitance is connected in parallel with the circuit of this generator, then either the frequency of the generator will change, or its oscillations will stop altogether. In any case, a threshold device will work, which includes a sound or light signaling device. These schemes can be used in various models that, when they encounter various obstacles, will change their movement, in everyday life - they sat down in a computer chair, a laptop turned on or a music center played, devices can also be used to turn on lights in rooms to build alarm systems, etc. .

The circuit operates at audio frequencies. To increase the sensitivity, a field-effect transistor is added to the low-frequency oscillator circuit.

Rectangular pulse generator with a repetition rate of the latter 1 kHz made on the elements DD1.1 and DD1.2. As an output stage DD1.3, whose load is the telephone speaker.

In order to increase the sensitivity of the circuit, you can add the number of radio components introduced into RC-chain.

The circuit should start working immediately after switching on. Sometimes you need to adjust the resistance R1 for threshold sensitivity.

When adjusting the relay, two options for its functioning are possible: a breakdown or the occurrence of generation when a capacitance appears. The installation of the circuitry option we need is selected by selecting the nominal value of the variable resistance R1. When the hand approaches E1 by adjusting the resistance R1, they make it so that the distance from which the circuit was started is 10 - 20 centimeters.

To turn on various actuators in a capacitive relay, we use the signal from the output of the element DD1.3.

To turn on the light, they pass next to the second capacitive transducer, and to turn off the lighting in the room with the first one.

The operation of the converter leads to the switching of the RS flip-flop built on logical elements. Capacitive sensors are made from pieces of coaxial cable, from the end of which a screen is removed for a length of about 50 centimeters. The edge of the screen needs to be isolated. Sensors are installed on the door frame. The length of the unshielded part of the sensors and the resistance values ​​R5 and R6 are selected when debugging the circuit so that the trigger is reliably triggered when a biological object passes at a distance of 10 centimeters from the sensor.

While the capacitance between the sensor and the housing is small, at the resistance R2, and at the input of the element DD1.3 short pulses of positive polarity are formed, and at the output of the element the same pulses but already inverted. The capacitance C5 is slowly charged through the resistance R3 when the output of the element has a logical one level, and is quickly discharged through the diode VD1 at a logical zero. Since the discharge current is higher than the charging current, the voltage across the capacitance C5 has a logic zero level, and the DD1.4 element is locked for an audio frequency signal.

When approaching an element of any biological object, its capacitance relative to the common wire increases, the amplitude of the pulses at the resistance R2 falls below the switching threshold DD1.3. At its output there will be a constant logical unit, up to this level the capacitor C5 will be filled with capacity. Element DD1.4 will begin to pass the audio frequency signal, and a beep will sound in the speaker. The sensitivity of the capacitive relay can be adjusted by the trimmer C3.

The sensor is made by hand using a metal mesh with dimensions of 20 x 20 centimeters, for a good level of relay sensitivity.

In this capacitive relay circuit, a transistor VT1 is connected to the logic element DD1.4, in the collector circuit of which the thyristor VS1 is connected to control a powerful load.

The device, assembled according to the diagram below, reacts to the presence of any conductive object, including a person. The sensitivity of the sensor can be adjusted with a potentiometer. The circuit does not allow detecting the movement of objects, but it is good precisely as a presence sensor. One obvious solution to using a capacitive presence sensor in everyday life is a homemade automatic door opening circuit. For these purposes, the device diagram must be placed from the front of the door.

The basis of this capacitive device is an oscillator with T1 and a single vibrator. The oscillator is a typical Clapp oscillator of a stable frequency. The surface of the capacitive pickup acts as a capacitor for the oscillating circuit, and in this configuration the frequency will be around 1 MHz.

The switching time of the circuit can be changed over a wide range using a variable resistor P2. It is not necessary to bring metal objects close to the sensor, because the capacitive relay will remain in the closed state. This circuit can also be used as a detector of aggressive liquids. The main advantage here is that the surface of the capacitive sensor does not come into direct contact with the liquid.

A low-power generator with a pulse repetition rate of 465 kHz is made on a field-effect transistor, and an electronic key is used on a bipolar transistor to operate relay K1, the contacts of which turn on the actuator. The diode is used in the circuit when the polarity of the connected power source is accidentally reversed.

The range of the capacitive relay and sensitivity depends on the adjustment of C1 and the design of the sensor, if you are interested in this development, then you can download the modeler designer magazine from the link just above.

The basis of the circuit is a low-power RF generator. To the oscillatory circuit L1C4 metal plate attached. The palm of the hand raised to it or another part of the human body is the second plate of the capacitor C d. the higher, the larger the area of ​​its plates and the smaller the distance between them. L1 wind on the frame ∅ 8-9 mm glued from paper. The coil CONSISTS OF 22-25 turns of wire PEV-1 0.3-0.4, wound round to round. The withdrawal must be made from the 5-7th turn, counting from the beginning.

Connect to the collector circuit of the bipolar transistor V1 milliammeter at 10 mA and between the connection point of the milliammeter with the coil L1 and the emitter of the second transistor to connect a capacitor 0.01-0.5 uF. Temporarily disconnect the metal plate from the generator. Following the readings of the milliammeter, briefly close L1C4. Collector current V1 drops sharply: from 2.5-3 to 0.5-0.8 mA. The maximum indications correspond to generation, the smallest - to its absence. If the generator is excited, attach a plate to it and slowly bring your palm up. The collector current should drop to 0.5-0.8 mA.

Weak current changes are amplified using a two-stage ULF on V2, V3. And in order to be able to control the load by a non-contact method, the final stage of the circuit is built on a trinistor V5.

Variable resistance engine R4 set to the lowest position. And then it is slowly moved up until the indicator turns on. H1. Now we bring our palm to the plate and check the operation of the device.

Diode V4 in the trinistor circuit V5 eliminates the appearance of a reverse voltage pulse. BUT V6 and resistance R7 protect the trinistor from breakdown. For trinistor with U o6p. = 400 V cells V6 and R7 can be removed from the diagram.

Greetings dear radio amateurs. The inductive sensor proposed for your consideration can be used in many devices - opening doors or removing goods from shelves, in tachometers, in intrinsically safe liquid level indicators, instead of breakers in gasoline engines, in automation elements, for example, in turning off the water intake valve in tanks ... The scheme is taken from its classic prototypes, but simplified and balanced. It is quite simple, but at the same time reliable, and distinguished by the clarity of its work, it is easy to manufacture, adjust and integrate into various devices.

Schematic diagram of the sensor

For a clearer view of the picture - save it to your PC and enlarge it.

The circuit is built as an oscillator with inductive feedback. The oscillatory circuit on the elements: L2, C2 sets the frequency, the feedback coil L1 and the feedback capacitance C1 provide generation, the resistors: R2, R4 set the transistor's DC mode and stabilize it. High frequency decoupling is provided by the chain: R1, C3.

Important! Capacitance C3 must be pulsed, of good quality and denominated as indicated in the diagram.

The output signal conditioner is made according to the voltage doubling scheme on the elements: C4, C5, VD1, VD2, R3 any high-frequency diodes, the resistor R3 is selected depending on the required rate of decrease in the output voltage during generation failure. In the presence of a metal petal between the coils, the generation breaks down.

The printed circuit board is made of foil fiberglass, 2 mm is used for its fastening. a hole into which a bolt is inserted with a limiting boss put on it (or just a piece of PVC pipe from a dropper) and clamped with a nut, or the bolt is screwed into a thread cut on some basis ...

The project file and drawing can be downloaded from the link. Coils L1 and L2 without cores. L2 contains 30 turns of PEV-1 wire (0.1-0.12 mm). L1 20-30 turns of PEV-1 wire (0.1-0.12 mm.) depending on the gap-distance in the sensor (selected empirically, but with a gap of about 2 mm. 23-26 turns). Coils are wound on a mandrel (a small 1-1.5 mm drill, or a needle, a piece of wire) between two cardboard cheeks, then they are fixed with glue and removed from the mandrel, the cheeks are also discarded. The thickness of the coils is two to three wire diameters, they are wound in bulk. Both finished coils are put on a plastic rod, which can then be removed, a polyethylene or fluoroplastic gasket of suitable thickness is placed between the coils (polyethylene and fluoroplastic lag behind the cured epoxy resin).

A cruciform reamer of the box is cut out of the pressboard, four holes are pierced in its bottom, into which flexible stranded wires for coil leads are threaded, the ends of the coils are soldered to them, the reamer is bent to obtain a box, wrapped with tape or electrical tape, another plastic pin is threaded through (plastic after is removed and a hole for fastening is obtained), the pin with coils is also centered and fastened and, finally, poured with epoxy. Each coil is soldered into its place with flexible leads, phased to obtain generation, the sensor is attached to its place, next to it is the generator board.

Nowadays, such coils or similar ones can be found in many no longer needed, broken or obsolete devices, for example, in floppy drives. There are also ready-made coils and sensors, but they are not always available, and it is not always cheap. Well, doing it yourself with your own hands is also a pleasure for someone, especially if it works no worse, but somewhere better than finished products.

There are no photos of the finished device, since the moped was sold, and the device was in it. As well as the board of the ignition itself, to which this sensor is connected. Now only the most detailed description and answers to questions of those interested in the forum are possible. But the ignition, together with this sensor, was really an order of magnitude better than the industrial one. Sparks in a laboratory test even ignited bale paper. The guys joked - why do you need gasoline now? You will ride on waste paper ... In general, the scheme is excellent, I recommend it! Article author - PNP.

Discuss the article UNIVERSAL INDUCTION SENSOR

Work in manufacturing plants requires partial or complete automation of the system. To do this, various devices are used to ensure uninterrupted operation. Metal devices quite often control inductive proximity sensors, which have their own advantages and disadvantages. They are small in size and perform their function well, provided they are connected correctly.

General information

The inductive sensor is a special non-contact device. This means that in order to determine the location of an object in space, it does not require direct contact with it. Thanks to this technology, it is possible to automate the production process.

As a rule, the fixture is used in various lines and systems in large plants and factories. It can also be used as a limit switch. The device is of high quality and reliability., works even in difficult conditions. It affects only metal objects, since other materials are insensitive to it.

The device is quite resistant to aggressive chemicals and is widely used in the machine-building, food and textile industries. The aerospace, military and railway industries also cannot do without these sensors.

The importance of the device makes it in demand, so many companies around the world produce various models with standard and advanced features, in different price categories.

Device device

The inductive sensor consists of several interconnected nodes, which ensure its uninterrupted operation. . The main parts of the device are as follows:

All elements are located in a case made of brass or polyamide. These materials are considered very strong in order to protect the core from the adverse effects of manufacturing conditions. Due to the robust design, the sensor is able to withstand a significant load and still function correctly.

Principle of operation

Thanks to a special generator that produces special vibrations, the operation of the device is carried out. When an object made of metal enters its field of action, a signal is sent to the control unit.

The work of the device begins after switching on, which gives impetus to the formation of a magnetic field. This field, in turn, affects the eddy currents, which change the amplitude of the oscillations of the generator, which is the first to respond to any changes.

As soon as a signal arrives, it starts processing in other nodes of the device. The strength of this signal largely depends on the size of the object that has fallen into the field of action of the device, as well as the distance at which it is located. The next step is to convert the analog signal to logic. Only in this way is it possible to accurately determine its value.

Such sensors play a special role in production., where the metal parts must follow a line in a certain position. The device can fix it and if it detects any, even a slight deviation, it signals to the main control panel.

As a rule, reading the results of the operation of the device is carried out by a specialist who also performs the role of a controller who monitors the smooth operation of the entire system.

Basic definitions

There are several definitions for controlling the operation of a device and reading its signals. The most important are the following:

Thanks to these definitions, it is possible to adjust the fixture to obtain the most accurate data that plays an important role in the production process.

Advantages and disadvantages

Inductive sensors have their advantages and disadvantages, like any other device. The main advantage is the simplicity of the design, which does not require complex settings and does not require special conditions for mounting. The device has no sliding contacts, is made of durable material and can work for a long time without interruption.

It is also worth noting that the device very rarely fails, and its repair is not difficult. That is why it is often installed in enterprises where almost round-the-clock control over the production process is necessary. The non-contact connection allows you to easily connect to an industrial voltage system.

An important advantage is considered to be high sensitivity, which allows installing sensors in production, where they work with metal objects from different alloys.

Despite all the advantages of the device, there are some disadvantages. The most important are the errors that the device produces in operation. The non-linear type of error is manifested due to the fact that the device has its own inductive value, which may differ from the value of those objects to which it reacts. That is why the sensor may react incorrectly to the metal and give incorrect signals.

Often there is a temperature error associated with a significant decrease or increase in temperature in the production room. The instruction for the device assumes its correct functioning at +25 degrees. If the value deviates in one direction or another, the operation of the device is disrupted.

One of the random errors is considered to be a change in the readings of the sensor due to the influence of the electromagnetic field of other devices on it. In order to avoid such situations, the frequency standard for electrical installations, which is 50 Hz, is set in all industries. In this way, the risk of error due to extraneous electromagnetic radiation is reduced to a minimum. It is possible to exclude any violations in the operation of the device by preliminary study of the details.

Connection methods

Depending on the type of device, the methods of connecting it also differ, since certain varieties have a different number of wires. Two-wire are considered the simplest, but also the most problematic option. They are connected directly to the current load circuit. For proper manipulation, a nominal load resistance is required. In the event of a decrease or increase, the device begins to function incorrectly. An important point will be the connection to the network, in which it is necessary to observe the polarity.

Three-wire are considered the most popular and easiest to connect. One wire is connected to the load, and the other two to the voltage source. This eliminates the possibility of the device reacting to the nominal resistance in the form of incorrect operation.

There are also sensors with four and five wires. When they are installed, two wires are connected to a voltage source, two - to the load. If there is a fifth cord, then it is possible to select the appropriate operating mode.

Typically, the wires are marked with different colors in order to facilitate installation and subsequent maintenance of the sensor. Minus and plus are marked in blue and red, respectively. The output is always marked black. There are devices that have two outputs. The second is usually white and can also serve as an entrance. These nuances are indicated in the instruction manual for the inductive sensor.

The body of the device can be made of different materials, have a cylindrical, square or rectangular shape. The first option is considered the most common.

Selection rules

The inductive sensor is considered an important element in many enterprises, so its choice should be approached very responsibly. It is recommended to observe the following rules:

An important parameter is the cost of the device. It depends most often on the manufacturer and some additional functions that are built into the sensor. However, there is no significant difference in the operation of devices from different price categories.

Popular Models

There are many models of inductive sensors on the market today. The most popular are various devices from the Russian company TECO. They are distinguished by good quality, excellent technical characteristics, ease of installation and operation. The main advantage of the company's devices is the democratic price.

The cost of simple models starts from 850 rubles, and for this money the device works flawlessly. More expensive sensors are also produced with a price of 2 to 5 thousand rubles. They are usually installed in large-scale industries where high accuracy and trouble-free operation are required.

The inductive sensor is considered one of the best non-contact devices used in various plants, factories and other enterprises. The high quality and accuracy of the device makes it in demand and necessary.

- these are sensors that work without physical and mechanical contact. They work through an electric and magnetic field, and optical sensors are also widely used. In this article, we will analyze all three types of sensors: optical, capacitive and inductive, and at the end we will do an experiment with an inductive sensor. By the way, the people also call contactless sensors proximity switches, so don't be afraid if you see such a name ;-).

optical sensor

So, a few words about optical sensors ... The principle of operation of optical sensors is shown in the figure below

barrier

Do you remember any shots from films where the main characters had to go through optical beams and not hit any of them? If the beam was touched by any part of the body, an alarm was triggered.

The beam is emitted by some source. And there is also a “beam receiver”, that is, the thing that receives the beam. As soon as there is no beam on the beam receiver, the contact will immediately turn on or off in it, which will directly control the alarm or something else at your discretion. Basically, a beam source and a receiver, properly called a "photodetector", come in pairs.

SKB IS optical motion sensors are very popular in Russia.

These types of sensors have both a light source and a photodetector. They are located right in the body of these sensors. Each type of sensor is a complete design and is used in a number of machines where increased processing accuracy is needed, up to 1 micrometer. Basically, these are machines with a system H logical P software At board ( CNC) that work according to the program and require minimal human intervention. These non-contact sensors are built on this principle

These types of sensors are denoted by the letter “T” and are called barrier. As soon as the optical beam was interrupted, the sensor worked.

Pros:

• range can reach up to 150 meters
• high reliability and noise immunity

Minuses:

• at large sensing distances, fine adjustment of the photodetector to the optical beam is required.

Reflex

The reflective type of sensors is indicated by the letter R. In these types of sensors, the emitter and receiver are located in the same housing.

The principle of operation can be seen in the figure below.

Light from the emitter is reflected from some reflector (reflector) and enters the receiver. As soon as the beam is interrupted by any object, the sensor is triggered. This sensor is very convenient on conveyor lines when counting products.

diffusion

And the last type of optical sensors - diffusion - denoted by the letter D. They may look different:

The principle of operation is the same as that of the reflex, but here the light is already reflected from objects. Such sensors are designed for a small sensing distance and are unpretentious in their work.

Capacitive and inductive sensors

Optics are optics, but inductive and capacitive sensors are considered the most unpretentious in their work and very reliable. This is how they look like

They are very similar to each other. The principle of their operation is associated with a change in the magnetic and electric fields. Inductive sensors are triggered when any metal is brought to them. They do not “peck” on other materials. Capacitive ones work on almost any substance.

How an inductive sensor works

As they say, it's better to see once than hear a hundred times, so let's do a little experiment with inductive sensor.

So, our guest is a Russian-made inductive sensor

We read what is written on it

WBI sensor brand blah blah blah blah, S - sensing distance, here it is 2 mm, U1 - version for a temperate climate, IP - 67 - protection level(in short, the level of protection here is very steep), U b - voltage at which the sensor operates, here the voltage can be in the range from 10 to 30 volts, I load - load current, this sensor can deliver up to 200 milliamps of current to the load, I think this is decent.

On the reverse of the tag is a wiring diagram for this sensor.

Well, let's evaluate the work of the sensor? To do this, we cling to the load. The load we will have is an LED connected in series with a resistor with a nominal value of 1 kOhm. Why do we need a resistor? The LED at the moment of inclusion begins to frantically eat current and burns out. To prevent this, a resistor is placed in series with the LED.

On the brown wire of the sensor we supply a plus from the Power supply, and on the blue wire - a minus. The voltage I took was 15 volts.

The moment of truth is coming ... We bring a metal object to the working area of ​​​​the sensor, and the sensor immediately works, as the LED built into the sensor tells us, as well as our experimental LED.

The sensor does not respond to materials other than metals. A jar of rosin means nothing to him :-).

Instead of an LED, a logic circuit input can be used, that is, the sensor, when triggered, outputs a logic one signal that can be used in digital devices.

Conclusion

In the world of electronics, these three types of sensors are in increasing use. Every year the production of these sensors is growing and growing. They are used in absolutely different areas of industry. Automation and robotics would not be possible without these sensors. In this article, I have analyzed only the simplest sensors that give us only an “on-off” signal or, to put it in a professional language, one bit of information. More sophisticated types of sensors can provide different parameters and can even connect directly to computers and other devices.

Buy inductive sensor

In our radio store, inductive sensors cost 5 times more than if they were ordered from China from Aliexpress.

Here You can look at a variety of inductive sensors.

Sensors for automatic devices of electronic equipment are the most important elements in electrical circuits. Sensors widely known to radio amateurs are used everywhere in professional industrial and non-professional equipment made by radio amateurs themselves. For example, a shock sensor in car alarms, or a noise sensor in acoustic devices, or an IR radiation sensor in devices for remote control of parameters. The options for sensor designs are endless. For example, a roll (or shock) sensor in a car alarm can be made with the same efficiency in several different ways. The sensors themselves are only part of the electronic circuit, so let's not forget that the reliable operation of the entire device also depends on the control circuit. What is a sensor?

There are many definitions, but the simplest is a device that changes its state depending on external influences. Below we will consider several options for sensors that can be made at home with your own hands without large time and financial costs. These sensors send single pulses or bursts of pulses (contact bounce) to the electronic control device, depending on the impact on them.

At present, the radio-electronic industry offers radio amateurs industrial versions of sensors, it would seem, for all occasions - even Geiger counters that register radiation have become available. The following material is relevant due to the high retail cost of most industrial sensors and is aimed at encouraging the creativity of a radio amateur in domestic and "field" conditions, when it is possible to manufacture sensors on their own, without losing their quality.

Mechanical sensors

On fig. 3.28 shows a variant of the manufacture of a flat mechanical sensor. By mounting one or more of these sensors under the carpet, linoleum, or under the wallpaper, it is possible to control the lighting, for example, in a hallway, without being noticed.

When you press your foot on the plane of the sensor (or press your hand on a certain place on the wall of an apartment, office), the foil contacts close, and the impulse goes through the connecting wires to the control circuit. The sensitivity of the sensor is high - it reacts even to a small load.

Rice. 3.28. Mechanical sensor design

As can be seen from the figure, a cardboard with a hole cut inside is superimposed on a dense foil, and another layer of foil is superimposed on it. Thin flexible conductors are carefully soldered to the conductive foil (dense paper-based foil is quite suitable). The entire resulting "sandwich" is then laminated. As a material for lamination, a plastic folder-pocket for papers or school supplies is used - it must be cut to the size of the sensor, put foil and cardboard inside and iron it through a rag. You can just glue the sensor with tape. If the control circuit uses noise-immune elements (MOSFETs or microcircuits), the length of the connecting wires from the sensors to the circuit can be several meters. If more distance is required, then bus amplifiers and level converters on microcircuits are used (for example, on elements K561PUZ, K561PU4, K561LP1, K561LN2 and others). If the sensor is made in several layers, alternating the conductor and the dielectric, then the resulting “thick sandwich” can be used as a force sensor (pressure) - that is, as a sensor for weighing. There are many options for using such a sensor; its feature is the possibility of easy masking. The flat sensor is reliable, durable and described in detail by the author in several developments of automatic household devices.

Acoustic sensors

On fig. 3.29 and 3.30 there are two sensitive circuits that perform the functions of an acoustic sensor that generates series (bursts) of pulses under sound exposure that is different from a quiet acoustic background. The operational amplifier circuit (Fig. 3.29) uses a piezoelectric element as an impact sensor.

Rice. 3.29. Acoustic sensor built on the op-amp

This option has been repeatedly published in combination with other types of OS and therefore it does not claim to be original. The primer ZP-22 was used as a piezoelectric element, and due to relative sensitivity - it reacts only to shocks - it is not very effective, but it can be successfully used in security devices, for example, for protecting windows. To do this, the capsule must be securely glued to the glass and the sensor will produce single pulses when it hits the glass. The larger the glass area (protected area) - the more sensitive the sensor. It can be used to protect from the side of external glasses and showcases in stores. The greater the resistance of resistors R4 and R2 at the input of the comparator, the more sensitive the circuit. From the output of the comparator (pin 6), the pulse goes to the key or forming circuit. Capacitor C1 (K50-24) filters power supply noise.

On fig. 3.30 shows a more sensitive, though more old-fashioned, version. As BM1, any carbon microphone from old telephones (MK-10 and similar) is used. The transistor amplifier is assembled according to the series amplification scheme in such a way that the gain of the second stage is twice that of the first, and so on. In the figure, we see a three-stage amplifier, which allows using this circuit as an ultra-sensitive one, however, if such sensitivity is not so necessary, you can get by with only one stage on a composite transistor. The amplifier is operable within a wide range of supply voltage of the circuit. From the collector of the last transistor, bursts of pulses are fed to a key or pulse sequence-forming circuit (for example, a single vibrator). The gain is effectively regulated by the resistor R1 (the greater its resistance, the more sensitive the circuit) and, to a small extent, by the resistor R6. As you know, such microphones contain carbon powder, which is very sensitive to concussions and sound waves, it changes the resistance of the microphone to direct current. These pulses are captured by the amplifier on transistors VT1-VT4. A negative feature of the circuit is its inertia, due to the properties of carbon microphones. But for many amateur radio developments, such a sensitive circuit is almost indispensable in its simplicity and efficiency. Positive qualities - ease of manufacture, non-criticality to reverse switching on and power supply voltage drops, reliability. Conductors from the microphone to the electrical circuit must be kept to a minimum length. Transistors can be used any of the KT3107, KT361 series. In the author's practice, the device shown in Fig. 3.30 has been successfully and consistently used as a sound sensor for supplying air to fish in an aquarium. The microphone, together with the sensor circuit, is installed in a compact plastic case, which is securely attached to the aquarium wall in such a way as to ensure a snug fit of the working surface of the carbon microphone to the glass. Practice has shown that any movement behind the wall of the aquarium, even a small fish near the microphone-sensor, and even more so when the fish floats to the upper edge of the water for air, is captured by the sensor and it gives out a pack of pulses. The carbon microphone changes its impedance depending on external acoustic influences. This change is then sensed by the transistor amplifying circuit. The number of pulses in a pack is proportional to the strength of the impact of the sound wave on the microphone. The impulses are converted by the control circuit, and the compressor is automatically turned on for 1 ... 2 hours (time determined by the additional timer).

This sensor can also find other applications, such as an acoustic sensor that reacts to conversations in the room and turns on the backlight. If the body of the device together with the microphone is mounted on the floor, the circuit will notify of the approach of a person long before he approaches the sensor. Since the steps of a person on the floor, as practice shows in city apartments, affect its surface and are transmitted over a long distance. Thus, there are a lot of options for using such a sensor.

Inductive sensor

On fig. 3.31 shows a simple sensor that responds to magnetic induction. When a small current appears near the winding of the AND coil (for example, in the wires of a communication line), it is induced in the coil and transmitted to the amplifying stage on a composite transistor. The amplifier for this circuit can be of any configuration, with a large gain. An alternating voltage induced in coil I is removed from the positive lining of capacitor C2. If a magnetic antenna is used as a coil, a device can be obtained that responds to

Rice. 3.31. Amplifier for induction sensor

radio waves of a certain length, that is, to control the radio. The sensitivity of the circuit is regulated by the resistor R1, which sets the bias to the composite transistor.

The greater the resistance of the variable resistor, the more sensitive the circuit. For the optimal amplification mode (since the supply voltage of the circuit can vary significantly), the value of the resistor R2 is selected so that the current consumed by this node from the power source is within 2 mA. In practice, the sensor picks up an alternating current of 50 mA in wires up to 5 cm away. The length of the wires from the L1 coil to the input stage of the circuit should be kept to a minimum to eliminate interference.

The coil is wound with PEV or PEL wire with a diameter of 0.1 ... 0.15 mm in bulk and contains 2500 turns on any suitable cardboard, wooden or plastic frame with a diameter of 8 mm. A core made of ferrite grade 600 - 2000NN is inserted inside the frame. The length of the frame corresponds to the length of the core and is in the range of 25…40 mm.

current sensor

The design of the device is shown in Figure 3.32.

The sensor is a reed switch with a wire with a diameter of 0.08 ... 0.1 mm wound along the length of its glass body. Bulk winding (300-400 turns) - depending on the purpose of the sensor. When an electric current flows through the winding of such a sensor, the reed switch closes (opens) its contacts under the influence of magnetic induction, switching the peripheral circuit. Based on this sensor, a radio amateur can independently make a "current relay" by connecting one of the contacts of the reed switch to the end of the winding, as shown in fig. 3.33.

Immediately after switching on, the current flowing through the load creates a voltage drop across the winding L1. The voltage drop across the winding is directly proportional to the current in the circuit. The induced voltage will create a small electromagnetic field, which will be sufficient to affect the contacts of the reed switch, which will block the electrical circuit. When the load is de-energized (or the current in its circuit decreases, which can happen for very different reasons), the voltage drop across L1 will decrease, the magnetic field will decrease, and the reed switch contacts will open. The sensitivity of such a sensor depends on the number of turns L1 and the current strength in the circuit. The current relay, like the electromagnetic sensor, has many applications in radio engineering structures.

Low current sensor

Rice. 3.36. optical sensor

parallel to the transmitter and at an angle to it are photodetectors (block 2), also facing into space. In the absence of a reflective object, the energy emitted by the LED is dissipated without reaching the sensitive surface of the photodetectors. When an object appears within the active radiation range, the reflected light beam is captured by one or more sensors-receivers, as a result of which a pulse is sent from the photodetector to the control circuit. The distance from the signal emitter to the receiver (sensor) in the radiation plane should not exceed 4 ... 5 centimeters. However, if a mirror surface (even without a focusing lens) with a radius of curvature of 50–80 mm is used as a reflecting object, then the device can effectively operate at a distance of up to 25 cm from the reflecting object.

Based on this principle, a special sensor was created and tested in the life support system of aquariums and as a rain sensor for cars. Consider the operation of the node (the schematic diagram is shown in Fig. 3.36, b) using the example of an aquarium. The sensor (optocoupler AORS113A - an optocoupler with an open optical channel, in this scheme, its emitting LEDs and receiving photoresistors are connected in parallel) is mounted on the outside on one of the walls of the aquarium and the working surface is facing inside the aquarium. The anodes of the emitting diodes inside the optocoupler housing are combined and have a common pin 8. The AOP113A and AORS113A case is metal, with sixteen pins, based on a planar-type ceramic substrate, with a glass window. This makes it easier to mount to a flat controlled surface.

The difference between AOP113A and AOP113A is that AOP113A contains two identical transceivers (similar to one in AOP113A). The AORS113A optocoupler allows you to control two coordinates, respectively, and turn on differential photodetectors in series or in parallel.

In large aquariums (more than 60 liters), there are some difficulties with changing the water. Therefore, compressors are installed there to filter, purify water and constantly supply air to the water area. Water in large aquariums is replaced partially, and completely very rarely, in case of emergency. As a result, various organic deposits accumulate on the bottom and on the walls of the aquarium, polluting the water. In some cases, grass begins to bloom inside the water area and the water completely loses its transparency. For responsible owners, this is unacceptable. The original sensor considered here practically does not reflect radiation in the case of clean aquarium walls and clear water, and begins to reflect the beam if there is contamination. The impulse from the sensors is fed to the parameter control circuit (implemented on a composite transistor), then when power is applied to the load (alarm device), the latter signals the contamination of the aquarium with a sound. The control scheme should provide for a delay in the alarm signaling (a timer for several minutes) in order to exclude false alarms in the event of fish appearing within the active zone of the sensors or, for example, snails crawling. Practice has proven that living organisms in an aquarium are not harmed by a small radiation from the sensor. Rather, we can state the opposite fact - fish often appear in the working area of ​​the sensors and are keenly interested in what is happening.

The principle of operation of the rain sensor for a car is similar to the one above. The sensors themselves (emitters and receivers) are connected to the control circuit by shielded conductors of the shortest length. The rain sensor actuator is designed to close the circuit of automotive electronics - the contacts of the wiper switch. The car does not need a delay to turn on the load devices. At night, in the dark, the device behaves stably. The sensitivity of the device is adjusted only once when installed on a car window to eliminate false positives from the solar radiation spectrum in clear weather. The power supply of the circuit is stabilized, it can be in the range of 10 ... 18 V. If the accuracy of the operation of the circuit is not fundamental, then any car relay for a voltage of 12 V can be used as a load.

The difference with the previous version consists in attaching the case with the device to the windshield of the car (from the inside). In clear weather, constant radiation freely passes through clean glass and is scattered in space. During rain, the glass from the outside is polluted with raindrops, which slightly reflect the rays. The reflected signal, accordingly, changes the resistance of the photoresistors in the case of the optocoupler with an open optical channel. This leads to a change in the mode of the composite transistor and the appearance of a current pulse at the output. As in the first case, photodetectors (photoresistors) are connected in parallel (their total resistance decreases faster when exposed to light - the node sensitivity increases). When there is no reflective signal, the total resistance of the photoresistors of the optocoupler is high, about a hundred kOhm. At the output of the circuit, the voltage tends to zero relative to the negative pole of the power source. The reflected light radiation will reduce the total resistance of the photoresistors and open VT1, VT2. At the output of the circuit, a high level voltage will appear, almost equal to the supply voltage. The sensitivity of the circuit is adjusted by a variable resistor R1, which should be selected with a linear characteristic. From the circuit output, a control signal can be applied to a comparator that compares the base voltage with the input voltage (assembled according to any standard circuit, for example, on K521SAZ). The comparator at its output will give a constant positive signal when the voltage at its input changes. The signal from the output of the comparator through any transistor switch will turn on the executive relay, which will close the alarm (load) circuit with its contacts.

A few words about mounting to the aquarium wall. The transparent window of the optocoupler housing is mounted to the glass with instant glue, while making sure that the glue does not get on the working surface of the optocoupler. Instead of AORS113A, two AOP113A devices can be used (in Fig. 3.36, c, the pinout and the difference between these optocouplers are shown). They have similar electrical parameters. The use of only one element from a pair will not be slow to affect the operation of the entire circuit in the direction of reducing sensitivity.

When using the circuit as a rain sensor for a car, the following fact must be taken into account. The device works well in the temperature range of 0 ... 50 ° C, so in winter, if the car is not put in a warm garage at a negative air temperature, at the first moments of the car’s movement, until the temperature in the cabin rises to zero degrees, the rain sensor may not respond correctly to external factors.

Fire sensor

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Photo sensors and electronic devices implemented on their basis that control various household appliances have long been popular among radio amateurs. It would seem that it is already impossible to find anything new in the circuit design for such devices. That…….

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