Below are simple light and sound circuits, mainly assembled on the basis of multivibrators, for beginner radio amateurs. All schemes use the simplest element base, no complicated setup is required and it is possible to replace elements with similar ones within a wide range.

Electronic duck

A toy duck can be equipped with a simple “quack” simulator circuit using two transistors. The circuit is a classic multivibrator with two transistors, one arm of which includes an acoustic capsule, and the load of the other is two LEDs that can be inserted into the eyes of the toy. Both of these loads work alternately - either a sound is heard, or the LEDs flash - the eyes of a duck. A reed sensor can be used as a power switch SA1 (can be taken from sensors SMK-1, SMK-3, etc., used in systems burglar alarm like door sensors). When a magnet is brought to the reed switch, its contacts close and the circuit begins to work. This can happen when the toy is tilted towards a hidden magnet or a kind of “ magic wand"with a magnet.

Transistors in the circuit can be any p-n-p type, low or medium power, for example MP39 - MP42 (old type), KT 209, KT502, KT814, with a gain of more than 50. Transistors can also be used n-p-n structures, for example KT315, KT 342, KT503, but then you need to change the polarity of the power supply, turn on the LEDs and the polar capacitor C1. As an acoustic emitter BF1, you can use a TM-2 type capsule or a small-sized speaker. Setting up the circuit comes down to selecting resistor R1 to obtain the characteristic quack sound.

The sound of a metal ball bouncing

The circuit quite accurately imitates such a sound; as capacitor C1 discharges, the volume of the “beats” decreases, and the pauses between them decrease. At the end, a characteristic metallic rattle will be heard, after which the sound will stop.

Transistors can be replaced with similar ones as in the previous circuit.
The total duration of the sound depends on capacity C1, and C2 determines the duration of pauses between “beats”. Sometimes, for a more believable sound, it is useful to select transistor VT1, since the operation of the simulator depends on its initial collector current and gain (h21e).

Engine sound simulator

They can, for example, voice a radio-controlled or other model of a mobile device.

Options for replacing transistors and speakers - as in previous schemes. Transformer T1 is the output from any small-sized radio receiver (a speaker is also connected through it in the receivers).

There are many schemes for simulating the sounds of birdsong, animal voices, steam locomotive whistles, etc. The diagram proposed below is assembled using just one digital chip K176LA7 (K561 LA7, 564LA7) and allows you to simulate many different sounds depending on the value of the resistance connected to the X1 input contacts.

It should be noted that the microcircuit here operates “without power,” that is, no voltage is supplied to its positive terminal (pin 14). Although in fact the microcircuit is still powered, this happens only when a resistance sensor is connected to the X1 contacts. Each of the eight inputs of the chip is connected to the internal power bus through diodes that protect against static electricity or incorrect connections. The microcircuit is powered through these internal diodes due to the presence of positive power feedback through the input resistor-sensor.

The circuit consists of two multivibrators. The first one (on elements DD1.1, DD1.2) immediately begins to produce square pulses with a frequency of 1 ... 3 Hz, and the second (DD1.3, DD1.4) is switched on when the logical level “1” arrives at pin 8 from the first multivibrator. It produces tone pulses with a frequency of 200 ... 2000 Hz. From the output of the second multivibrator, pulses are supplied to the power amplifier (transistor VT1) and a modulated sound is heard from the dynamic head.

If you now connect to the input jacks X1 variable resistor resistance up to 100 kOhm, then power feedback occurs and this transforms the monotonous intermittent sound. By moving the slider of this resistor and changing the resistance, you can achieve a sound reminiscent of the trill of a nightingale, the chirping of a sparrow, the quack of a duck, the croaking of a frog, etc.

Details
The transistor can be replaced with KT3107L, KT361G, but in this case you need to install R4 with a resistance of 3.3 kOhm, otherwise the sound volume will decrease. Capacitors and resistors - any type with ratings close to those indicated in the diagram. It must be borne in mind that the K176 series microcircuits of early releases do not have the above protective diodes and such copies will not work in this circuit! It’s easy to check the presence of internal diodes - just measure the resistance with a tester between pin 14 of the microcircuit (“+” power supply) and its input pins (or at least one of the inputs). As with diode testing, the resistance should be low in one direction and high in the other.

There is no need to use a power switch in this circuit, since in idle mode the device consumes a current of less than 1 µA, which is significantly less than even the self-discharge current of any battery!

Setup
A correctly assembled simulator does not require any adjustment. To change the tone of the sound, you can select capacitor C2 from 300 to 3000 pF and resistors R2, R3 from 50 to 470 kOhm.

Flashing light

The flashing frequency of the lamp can be adjusted by selecting elements R1, R2, C1. The lamp can be from a flashlight or a car 12 V. Depending on this, you need to select the supply voltage of the circuit (from 6 to 12 V) and the power of the switching transistor VT3.

Transistors VT1, VT2 - any low-power corresponding structures (KT312, KT315, KT342, KT 503 (n-p-n) and KT361, KT645, KT502 (p-n-p), and VT3 - medium or high power (KT814, KT816, KT818).

A simple device for listening to the sound of TV broadcasts on headphones. Does not require any power and allows you to move freely within the room.

Coil L1 is a “loop” of 5...6 turns of PEV (PEL)-0.3...0.5 mm wire, laid around the perimeter of the room. It is connected in parallel to the TV speaker via switch SA1 as shown in the figure. For normal operation of the device, the output power of the TV audio channel must be within 2...4 W, and the loop resistance must be 4...8 Ohms. The wire can be laid under the baseboard or in the cable channel, and it should be located, if possible, no closer than 50 cm from the wires of the 220 V network to reduce alternating voltage interference.

The L2 coil is wound onto a frame made of thick cardboard or plastic in the form of a ring with a diameter of 15...18 cm, which serves as a headband. It contains 500...800 turns of PEV (PEL) wire 0.1...0.15 mm secured with glue or electrical tape. A miniature volume control R and an earphone (high-impedance, for example TON-2) are connected in series to the coil terminals.

Automatic light switch

This one differs from many circuits of similar machines in its extreme simplicity and reliability, and in detailed description does not need. It allows you to turn on the lighting or some electrical appliance for a specified short time, and then automatically turns it off.

To turn on the load, just briefly press switch SA1 without latching. In this case, the capacitor manages to charge and opens the transistor, which controls the relay switching on. The turn-on time is determined by the capacitance of capacitor C and with the nominal value indicated in the diagram (4700 mF) it is about 4 minutes. An increase in the on-state time is achieved by connecting additional capacitors in parallel with C.

The transistor can be any n-p-n type of medium power or even low-power, such as KT315. This depends on the operating current of the relay used, which can also be any other with an operating voltage of 6-12 V and capable of switching the load of the power you need. Can also be used pnp transistors type, but you will need to change the polarity of the supply voltage and turn on capacitor C. Resistor R also affects the response time to a small extent and can be rated 15 ... 47 kOhm depending on the type of transistor.

List of radioelements

Designation	Type	Denomination	Quantity	Note	Shop	My notepad
	Electronic duck
VT1, VT2	Bipolar transistor	KT361B	2	MP39-MP42, KT209, KT502, KT814		To notepad
HL1, HL2	Light-emitting diode	AL307B	2			To notepad
C1		100uF 10V	1			To notepad
C2	Capacitor	0.1 µF	1			To notepad
R1, R2	Resistor	100 kOhm	2			To notepad
R3	Resistor	620 Ohm	1			To notepad
BF1	Acoustic emitter	TM2	1			To notepad
SA1	Reed switch		1			To notepad
GB1	Battery	4.5-9V	1			To notepad
	Simulator of the sound of a bouncing metal ball
	Bipolar transistor	KT361B	1			To notepad
	Bipolar transistor	KT315B	1			To notepad
C1	Electrolytic capacitor	100uF 12V	1			To notepad
C2	Capacitor	0.22 µF	1			To notepad
	Dynamic head	GD 0.5...1W 8 Ohm	1			To notepad
GB1	Battery	9 Volt	1			To notepad
	Engine sound simulator
	Bipolar transistor	KT315B	1			To notepad
	Bipolar transistor	KT361B	1			To notepad
C1	Electrolytic capacitor	15uF 6V	1			To notepad
R1	Variable resistor	470 kOhm	1			To notepad
R2	Resistor	24 kOhm	1			To notepad
T1	Transformer		1	From any small radio receiver		To notepad
	Universal sound simulator
DD1	Chip	K176LA7	1	K561LA7, 564LA7		To notepad
	Bipolar transistor	KT3107K	1	KT3107L, KT361G		To notepad
C1	Capacitor	1 µF	1			To notepad
C2	Capacitor	1000 pF	1			To notepad
R1-R3	Resistor	330 kOhm	1			To notepad
R4	Resistor	10 kOhm	1			To notepad
	Dynamic head	GD 0.1...0.5Watt 8 Ohm	1			To notepad
GB1	Battery	4.5-9V	1			To notepad
	Flashing light
VT1, VT2	Bipolar transistor

You can make simple electronic circuits for home use with your own hands, even without deep knowledge of electronics. Actually on household level radio is very simple. Knowledge of the elementary laws of electrical engineering (Ohm, Kirchhoff), general principles operation of semiconductor devices, schematic reading skills, ability to work with electric soldering iron quite enough to assemble a simple circuit.

Radio amateur workshop

No matter how complex the scheme may have to be completed, you must have a minimum set of materials and tools in your home workshop:

• Side cutters;
• Tweezers;
• Solder;
• Flux;
• Circuit boards;
• Tester or multimeter;
• Materials and tools for making the device body.

You shouldn't buy expensive ones to begin with. professional tools and devices. Expensive Soldering Station or a digital oscilloscope will be of little help to a novice radio amateur. At first creative path The simplest instruments are quite enough, on which you need to hone your experience and skills.

Where to start

Do-it-yourself radio circuits for the home should not exceed the level of complexity that you have, otherwise it will only mean wasted time and materials. If you lack experience, it is better to limit yourself to the simplest schemes, and as you gain skills, improve them, replacing them with more complex ones.

Typically, most literature in the field of electronics for beginner radio amateurs leads classic example manufacturing of simple receivers. This especially applies to classical old literature, which does not contain so many fundamental errors compared to modern literature.

Note! These schemes were designed for the enormous power of transmitting radio stations in the past. Today, transmitting centers use less power to transmit and try to move to shorter wavelengths. Don't waste time trying to make a working radio using a simple circuit.

Radio circuits for beginners should contain a maximum of two or three active elements - transistors. This will make it easier to understand the operation of the circuit and increase the level of knowledge.

What can be done

What can be done so that it is not difficult and can be used in practice at home? There can be many options:

• Apartment call;
• Christmas tree garland switch;
• Backlight for modding the computer system unit.

Important! Devices that operate from a household network should not be designed alternating current, there is not enough experience yet. This is dangerous both for life and for others.

Quite simple circuits have amplifiers for computer speakers, made on specialized integrated circuits. Devices assembled on their basis contain a minimum number of elements and require virtually no adjustment.

You can often find circuits that need basic modifications and improvements that simplify manufacturing and configuration. But this should be done by an experienced master so that the final version is more accessible to a beginner.

What to use for the design

Most literature recommends designing simple circuits on circuit boards. Nowadays this is quite simple. There is a wide variety of circuit boards with various configurations mounting holes and printed tracks.

The installation principle is that the parts are installed on the board in free spaces, and then the necessary pins are connected to each other by jumpers, as indicated on the circuit diagram.

With due care, such a board can serve as the basis for many circuits. The power of the soldering iron for soldering should not exceed 25 W, then the risk of overheating radio elements and printed conductors will be minimized.

The solder should be low-melting, like POS-60, and as a flux it is best to use pure pine rosin or its solution in ethyl alcohol.

Highly qualified radio amateurs can develop a drawing themselves printed circuit board and perform it on foil material, on which radioelements are then soldered. The design developed in this way will have optimal dimensions.

Design of the finished structure

Looking at the creations of beginners and experienced craftsmen, we can come to the conclusion that assembling and adjusting the device is not always the most difficult part of the design process. Sometimes a properly functioning device remains a set of parts with soldered wires, not covered by any housing. Nowadays, you no longer have to worry about making a case, because on sale you can find all kinds of sets of cases of any configuration and size.

Before you start manufacturing the design you like, you should fully think through all the stages of the work: from the availability of tools and all radio elements to the design of the housing. It will be completely uninteresting if during the work it turns out that one of the resistors is missing, and there are no replacement options. It is better to carry out the work under the guidance of an experienced radio amateur, and, as a last resort, periodically monitor the manufacturing process at each stage.

Video

Schemes of homemade measuring instruments

A device circuit developed on the basis of a classic multivibrator, but instead of load resistors, transistors with opposite main conductivity are included in the collector circuits of the multivibrator.

It's good if you have an oscilloscope in your laboratory. Well, if it is not there and it is not possible to buy it for one reason or another, do not be upset. In most cases, it can be successfully replaced by a logic probe, which allows you to monitor the logical levels of signals at the inputs and outputs of digital integrated circuits, determine the presence of pulses in the controlled circuit and reflect the received information visually (light-color or digital) or audio (tone signals of various frequencies ) forms. When setting up and repairing structures based on digital integrated circuits, it is not always so necessary to know the characteristics of pulses or the exact values ​​of voltage levels. Therefore, logic probes make the setup process easier, even if you have an oscilloscope.

A huge selection of different pulse generator circuits is presented. Some of them generate a single pulse at the output, the duration of which does not depend on the duration of the triggering (input) pulse. Such generators are used for a wide variety of purposes: simulating input signals of digital devices, when testing the functionality of digital integrated circuits, the need to supply a certain number of pulses to some device with visual inspection processes, etc. Others generate sawtooth and square pulses of various frequencies, duty cycles and amplitudes

Repair of various components and devices of low-frequency electronic equipment and technology can be significantly simplified if you use a function generator as an assistant, which makes it possible to study the amplitude-frequency characteristics of any low-frequency device, transient processes and nonlinear characteristics of any analog devices, and also has the ability to generate pulses rectangular shape and simplifying the process of setting up digital circuits.

When setting up digital devices, you definitely need one more device - a pulse generator. An industrial generator is a rather expensive device and is rarely on sale, but its analogue, although not as accurate and stable, can be assembled from available radio elements at home

However, creating a sound generator that produces a sinusoidal signal is not easy and quite painstaking, especially in terms of setup. The fact is that any generator contains at least two elements: an amplifier and a frequency-dependent circuit that determines the oscillation frequency. It is usually connected between the output and input of the amplifier, creating a positive feedback(POS). In the case of an RF generator, everything is simple - just an amplifier with one transistor and an oscillating circuit that determines the frequency. For the audio frequency range, winding a coil is difficult, and its quality factor is low. Therefore, in the audio frequency range, RC elements are used - resistors and capacitors. They filter the fundamental harmonics quite poorly, and therefore the sine wave signal turns out to be distorted, for example, limited by peaks. To eliminate distortion, amplitude stabilization circuits are used that support low level generated signal when distortion is not yet noticeable. It is the creation of a good stabilizing circuit that does not distort the sinusoidal signal that causes the main difficulties.

Often, after assembling the structure, the radio amateur sees that the device does not work. Humans do not have sense organs that allow them to see. electricity, electromagnetic field or processes occurring in electronic circuits. Radio measuring instruments - the eyes and ears of a radio amateur - help to do this.

Therefore, we need some means of testing and checking telephones and loudspeakers, audio amplifiers, and various sound recording and sound reproducing devices. Such a tool is amateur radio circuits of audio frequency signal generators, or, more simply, a sound generator. Traditionally, it produces a continuous sine wave whose frequency and amplitude can be varied. This allows you to check all ULF stages, find faults, determine the gain, take amplitude-frequency characteristics (AFC) and much more.

We consider a simple homemade amateur radio attachment that turns your multimeter into a universal device for testing zener diodes and dinistors. PCB drawings available

So. Life has turned out in such a way that I have a house in the village with gas heating. It is not possible to live there permanently. The house is used as a summer house. For a couple of winters I stupidly left the boiler on with the minimum coolant temperature.
But there are two disadvantages.
1. Gas bills are astronomical.
2. If there is a need to come to the house in the middle of winter, the temperature in the house is around 12 degrees.
Therefore, it was necessary to invent something.
I'll clarify right away. The presence of a WI-FI access point in the relay coverage area is mandatory. But, I think, if you get confused, you can put a connected mobile phone next to the sensor and give out a signal from the phone.

Connecting a 4-pin motion sensor with your own hands (diagram)

DIY motion sensor connection diagram

It happens that you need to install lighting in your dacha or in your home. will be triggered by movement or a person or someone else.

A motion sensor, which I ordered from Aliexpress, works well with this function. The link to which will be below. By connecting light through a motion sensor, when a person passes through his field of vision, the light turns on and stays on for 1 minute. and turns off again.

In this article I’ll tell you how to connect such a sensor if it doesn’t have 3 contacts, but 4 like this one.

DIY power supply from an energy-saving light bulb

When to get 12 Volt for LED strip , or for some other purpose, there is an option to make such a power supply with your own hands.

DIY fan speed controller

This regulator allows for smooth adjustment variable resistor fan speed.

The circuit of the floor fan speed controller turned out to be the simplest. To fit into the case from an old Nokia phone charger. The terminals from a regular electrical outlet also fit in there.

The installation is quite tight, but this was due to the size of the case..

DIY plant lighting

DIY plant lighting

There may be a problem with lack of lighting plants, flowers or seedlings, and there is a need for artificial light for them, and this is the kind of light we can provide on LEDs with your own hands.

DIY brightness control

DIY brightness control

It all started with the fact that after I installed at home halogen lamps for lighting. When turned on, they often burned out. Sometimes even 1 light bulb a day. Therefore, I decided to make a smooth switching on of the lighting based on a brightness control with my own hands, and I am attaching a diagram of the brightness control.

DIY refrigerator thermostat

DIY refrigerator thermostat

It all started when I returned from work and opened the refrigerator to find it warm. Turning the thermostat control did not help - the cold did not appear. So I decided not to buy new block, which is also rare, and make an electronic thermostat yourself on the ATtiny85. The difference with the original thermostat is that the temperature sensor is on the shelf and not hidden in the wall. In addition, 2 LEDs appeared - they signal that the unit is turned on or the temperature is above the upper threshold.

DIY soil moisture sensor

DIY soil moisture sensor

This device can be used for automatic watering in greenhouses, flower conservatories, flower beds and indoor plants. Below is a diagram on which you can make a simple sensor (detector) of soil moisture (or dryness) with your own hands. When the soil dries out, voltage is applied with a current of up to 90 mA, which is quite enough, turn on the relay.

Also suitable for automatic switching on drip irrigation to avoid excess moisture.

Fluorescent lamp power supply circuit

Power supply circuit for a fluorescent lamp.

Often when it fails energy saving lamps, in it burns power supply circuit, and not the lamp itself. As is known, LDS with burnt filaments, it is necessary to supply the network with rectified current using a starterless starting device. In this case, the filaments of the lamp are shunted with a jumper and to which the high voltage to turn on the lamp. There is an instantaneous cold ignition of the lamp, with a sharp increase in voltage across it, upon start-up without preheating the electrodes. In this article we will look at starting an LDS lamp with your own hands.

USB keyboard for tablet

USB keyboard for tablet

Somehow, suddenly, I took something and decided to buy a new keyboard for my PC. The desire for novelty is irresistible. Changed the background color from white to black, and the letter color from red-black to white. A week later, the desire for novelty naturally disappeared like water into sand (an old friend is better than two new ones) and the new thing was sent to the closet for storage - until better times. And now they came for her, she didn’t even imagine that it would happen so quickly. And therefore the name would be even better suited not which is, but how to connect a usb keyboard to a tablet.

Those who do radio electronics at home are usually very inquisitive. Amateur radio circuits and homemade products will help you find a new direction in your creativity. Perhaps someone will find it for themselves original solution one problem or another. Some homemade products use ready-made devices, connecting them in different ways. For others, you need to completely create the circuit yourself and make the necessary adjustments.

One of the most simple homemade products. More suitable for those who are just starting to craft. If you have an old but working cell phone with a button to turn on the player, you can use it, for example, to make a doorbell for your room. The advantages of such a call:

First you need to make sure that the selected phone is capable of producing a sufficiently loud melody, after which it must be completely disassembled. Basically, the parts are secured with screws or staples, which are carefully folded back. When disassembling, you will need to remember what goes with what, so that later you can put everything back together.

The player's power button is unsoldered on the board, and two short wires are soldered in its place. These wires are then glued to the board so the solder doesn't come off. The phone is going. All that remains is to connect the phone to the call button via a two-wire wire.

Homemade products for cars

Modern cars are equipped with everything you need. However, there are times when it is simply necessary homemade devices. For example, something broke, they gave it to a friend, and the like. That’s when the ability to create electronics with your own hands at home will be very useful.

The first thing you can tamper with without fear of damaging your car is the battery. If you don’t have a battery charger at hand at the right time, you can quickly assemble it yourself. To do this you will need:

A transformer from a tube TV is ideal. Therefore, those who are interested in homemade electronics never throw away electrical appliances in the hope that they will be needed someday. Unfortunately, two types of transformers were used: with one and with two coils. To charge a battery at 6 volts, any will do, but for 12 volts only two.

The wrapping paper of such a transformer shows the winding terminals, the voltage for each winding and the operating current. To power filaments vacuum tubes a voltage of 6.3 V with high current is used. The transformer can be remade by removing the extra secondary windings, or you can leave everything as is. In this case, the primary and secondary windings are connected in series. Each primary is rated at 127 V, so combining them produces 220 V. The secondary are connected in series to produce an output of 12.6 V.

Diodes must withstand a current of at least 10 A. Each diode requires a radiator with an area of ​​at least 25 square centimeters. They connect in diode bridge. Any electrical insulating plate is suitable for fastening. A 0.5 A fuse is included in the primary circuit, and a 10 A fuse in the secondary circuit. The device does not tolerate short circuit, so when connecting the battery, do not confuse the polarity.

Simple heaters

During the cold season, it may be necessary to warm up the engine. If the car is parked where there is electrical current, this problem can be solved using a heat gun. To make it you will need:

• asbestos pipe;
• nichrome wire;
• fan;
• switch.

The diameter of the asbestos pipe is selected according to the size of the fan that will be used. The performance of the heater will depend on its power. The length of the pipe is everyone's preference. You can collect it a heating element and a fan, only a heater is possible. If you choose the latter option, you will have to think about how to let air flow to the heating element. This can be done, for example, by placing all components in a sealed housing.

Nichrome wire is also selected according to the fan. The more powerful the latter, the larger diameter nichrome can be used. The wire is twisted into a spiral and placed inside the pipe. For fastening, bolts are used that are inserted into the drilled holes in the pipe. The length of the spiral and their number are selected experimentally. It is advisable that the coil does not become red hot when the fan is running.

The choice of fan will determine what voltage needs to be supplied to the heater. When using a 220 V electric fan, you will not need to use additional source nutrition.

The entire heater is connected to the network via a cord with a plug, but it itself must have its own switch. It can be either just a toggle switch or an automatic machine. The second option is more preferable; it allows you to protect the general network. To do this, the operation current of the machine must be less than the operation current of the room machine. A switch is also needed to quickly turn off the heater in case of problems, for example, if the fan does not work. This heater has its disadvantages:

• harmful to the body from asbestos pipes;
• noise from a running fan;
• smell from dust falling on the heated coil;
• fire hazard.

Some problems can be solved by using another homemade product. Instead of an asbestos pipe, you can use a coffee can. To prevent the spiral from closing on the jar, it is attached to a textolite frame, which is fixed with glue. A cooler is used as a fan. To power it, you will need to collect another electronic device- a small rectifier.

Homemade products bring those who do them not only satisfaction, but also benefits. With their help, you can save energy, for example, by turning off electrical appliances that you forgot to turn off. A time relay can be used for this purpose.

The simplest way to create a time-setting element is to use the charging or discharging time of a capacitor through a resistor. Such a chain is included in the base of the transistor. The circuit will require the following parts:

• high-capacity electrolytic capacitor;
• transistor p-n-p type;
• electromagnetic relay;
• diode;
• variable resistor;
• fixed resistors;
• DC source.

First you need to determine what current will be switched through the relay. If the load is very powerful, you will need a magnetic starter to connect it. The starter coil can be connected via a relay. It is important that the relay contacts can operate freely without sticking. Based on the selected relay, a transistor is selected and it is determined what current and voltage it can operate with. You can focus on KT973A.

The base of the transistor is connected through a limiting resistor to a capacitor, which, in turn, is connected through a bipolar switch. The free contact of the switch is connected through a resistor to the power supply negative. This is necessary to discharge the capacitor. The resistor acts as a current limiter.

The capacitor itself is connected to the positive bus of the power source through a variable resistor with high resistance. By selecting the capacitance of the capacitor and the resistance of the resistor, you can change the delay time interval. The relay coil is shunted by a diode, which turns on in the opposite direction. This circuit uses KD 105 B. It closes the circuit when the relay is de-energized, protecting the transistor from breakdown.

The scheme works as follows. In the initial state, the base of the transistor is disconnected from the capacitor, and the transistor is closed. When the switch is turned on, the base is connected to the discharged capacitor, the transistor opens and supplies voltage to the relay. The relay operates, closes its contacts and supplies voltage to the load.

The capacitor begins to charge through a resistor connected to the positive terminal of the power source. As the capacitor charges, the base voltage begins to rise. At a certain voltage value, the transistor closes, de-energizing the relay. The relay switches off the load. In order for the circuit to work again, you need to discharge the capacitor; to do this, switch the switch.