home Usage Asymmetric multivibrator on transistors with adjustable frequency. Transistor multivibrator. Work description. The principle of operation of the multivibrator

Asymmetric multivibrator on transistors with adjustable frequency. Transistor multivibrator. Work description. The principle of operation of the multivibrator

Multivibrator.

The first circuit is the simplest multivibrator. Despite its simplicity, its scope is very wide. none electronic device can't do without it.

The first figure shows its schematic diagram.

LEDs are used as load. When the multivibrator is running, the LEDs are switched.

Assembly requires a minimum of parts:

1. Resistors 500 Ohm - 2 pieces

2. Resistors 10 kOhm - 2 pieces

3. Electrolytic capacitor 47 uF at 16 volts - 2 pieces

4. KT972A transistor - 2 pieces

5. LED - 2 pieces

KT972A transistors are composite transistors, that is, there are two transistors in their case, and it has high sensitivity and can withstand significant current without heat sink.

When you get all the parts, arm yourself with a soldering iron and start assembling. To conduct experiments, you should not make a printed circuit board, you can assemble everything by surface mounting. Solder as shown in the pictures.

And how to use the assembled device, let your imagination tell you! For example, instead of LEDs, you can put a relay, and this relay can switch a more powerful load. If you change the values of resistors or capacitors, the switching frequency will change. By changing the frequency, you can achieve very interesting effects, from a squeak in the dynamics, to a pause for many seconds ..

Photorelay.

And this is a diagram of a simple photorelay. This device can be successfully used anywhere, for automatic illumination of the DVD tray, for turning on the light or for signaling against intrusion into a dark cabinet. Two variants of the scheme are provided. In one embodiment, the circuit is activated by light, and the other by its absence.

It works like this: when the light from the LED hits the photodiode, the transistor will open and LED-2 will start to glow. The tuning resistor adjusts the sensitivity of the device. As a photodiode, you can use a photodiode from an old ball mouse. LED - any infrared LED. The use of an infrared photodiode and an LED will avoid interference from visible light. As LED-2, any LED or a chain of several LEDs is suitable. You can also use an incandescent lamp. And if instead of an LED we put an electromagnetic relay, then it will be possible to control powerful incandescent lamps, or some mechanisms.

The figures show both circuits, the pinout (location of the legs) of the transistor and the LED, as well as the wiring diagram.

In the absence of a photodiode, you can take an old MP39 or MP42 transistor and cut off its case opposite the collector, like this:

Instead of a photodiode, it will be necessary to include in the circuit p-n junction transistor. Which one will work better - you have to determine experimentally.

Power amplifier on a TDA1558Q chip.

This amplifier has an output power of 2 x 22 watts and is simple enough for beginners to repeat. This scheme will help you homemade speakers, or for homemade music center, which can be made from an old MP3 player.

To assemble it, you need only five parts:

1. Chip - TDA1558Q

2. Capacitor 0.22uF

3. Capacitor 0.33 uF - 2 pieces

4. Electrolytic capacitor 6800 uF at 16 volts

The microcircuit has a fairly high output power and a radiator is needed to cool it. You can use a heatsink from the processor.

The entire assembly can be done by surface mounting without the use of printed circuit board. First, pins 4, 9 and 15 must be removed from the microcircuit. They are not used. The pin count goes from left to right, if you hold it with the pins facing you and the markings up. Then carefully straighten the leads. Next, bend pins 5, 13 and 14 up, all these pins are connected to the power plus. The next step is to bend pins 3, 7 and 11 down - this is the power minus, or "ground". After these manipulations, screw the chip to the heat sink using thermally conductive paste. The pictures show the installation from different angles, but I'll explain anyway. Pins 1 and 2 are soldered together - this is the input of the right channel, a 0.33 uF capacitor must be soldered to them. The same must be done with pins 16 and 17. The common wire for the input is the power minus or ground.

A transistorized multivibrator is a square wave generator. Below in the photo is one of the oscillograms symmetrical multivibrator.

The symmetrical multivibrator generates rectangular pulses with a duty cycle of two. You can read more about duty cycle in the article frequency generator. We will use the principle of operation of a symmetrical multivibrator to turn on the LEDs in turn.

The scheme consists of:

- two KT315B (possible with any other letter)

- two capacitors with a capacity of 10 microfarads

- four, two of 300 ohms and two of 27 kilo ohms

- two Chinese LEDs for 3 Volts

This is what the device looks like on the breadboard:

And this is how it works:

To change the blinking duration of the LEDs, you can change the values of capacitors C1 and C2, or resistors R2 and R3.

There are also other types of multivibrators. You can read more about them. It also describes the principle of operation of a symmetrical multivibrator.

If you are too lazy to assemble such a device, you can buy a ready-made one ;-) I even found a ready-made device on Alik. You can look at it this link.

Here is a video detailing how the multivibrator works:

Hello dear friends and all readers of my blog site. Today's post will be about a simple but interesting device. Today we will consider, study and assemble an LED flasher, which is based on a simple rectangular pulse generator - a multivibrator.

When I go to my blog, I always want to do something like that, something that will make the site memorable. So I present to your attention a new "secret page" on the blog.

This page is now called - "It's interesting."

You might be asking, “How do you find it?” And very simple!

You may have noticed that a peeling corner appeared on the blog with the inscription "Hurry here."

Moreover, one has only to move the mouse cursor to this inscription, as the corner begins to flake off even more, exposing the inscription - the link "This is interesting."

Leads to a secret page where a small but a pleasant surprise- a gift prepared by me. Moreover, in the future on this page will be placed useful materials, amateur radio software and something else - has not yet been invented. So, periodically look around the corner - suddenly I hid something there.

Okay, a little distracted, now let's continue ...

In general, there are many multivibrator circuits, but the most popular and discussed is the unstable symmetrical multivibrator circuit. She is usually portrayed in this way.

For example, I soldered this multivibrator flasher somewhere a year ago from improvised parts and, as you can see, it flashes. Blinking despite clumsy wiring done on a prototyping board.

This scheme is working and unpretentious. You just need to figure out how it works?

The principle of operation of the multivibrator

If we assemble this circuit on a breadboard and measure the voltage between the emitter and collector with a multimeter, what will we see? We will see that the voltage across the transistor rises to almost the voltage of the power supply, then drops to zero. This suggests that the transistors in this circuit operate in a key mode. I note that when one transistor is open, the second is necessarily closed.

Switching transistors is as follows.

When one transistor is open, let's say VT1, the capacitor C1 is discharged. Capacitor C2 - on the contrary, it is quietly charged by the base current through R4.

Capacitor C1 in the process of discharging keeps the base of the transistor VT2 under negative voltage - locks it. Further discharge brings the capacitor C1 to zero and then charges it in the other direction.

Now the voltage at the base of VT2 increases by opening it. Now the capacitor C2, once charged, is being discharged. Transistor VT1 is locked negative voltage on the base.

And all this pandemonium continues non-stop until the power is turned off.

Multivibrator in its performance

Having once made a multivibrator flasher on a breadboard, I wanted to ennoble it a little - make a normal printed circuit board for the multivibrator and at the same time make a scarf for LED indication. I developed them in the Eagle CAD program, which is not much more complicated than Sprintlayout, but has a rigid binding to the scheme.

The printed circuit board of the multivibrator is on the left. Electrical diagram on the right.

Printed circuit board. Electric scheme.

PCB drawings using laser printer I printed on photo paper. Then, in full accordance with folk etched scarves. As a result, after soldering the parts, we got such scarves.

To be honest, after the complete installation and power connection, there was a small bug. The plus sign typed from the LEDs did not wink. It simply and evenly burned as if there was no multivibrator at all.

I had to be pretty nervous. Replacing the four-point indicator with two LEDs corrected the situation, but as soon as everything was put back in place, the flasher did not blink.

It turned out that the two LED arms were closed with a jumper, apparently when I was tinning the scarf, I overdid it with solder. As a result, the LED "shoulders" did not burn alternately, but synchronously. Well, nothing, a few movements with a soldering iron corrected the situation.

The result of what happened, I captured on video:

It didn't turn out bad in my opinion. 🙂 By the way, I leave links to circuits and boards - use it to your health.

Board and circuit of the multivibrator.

Board and diagram of the Plus indicator.

In general, the use of multivibrators is diverse. They are suitable not only for simple LED flashers. By playing with the values of resistors and capacitors, you can output signals to the speaker audio frequency. Wherever you may need a simple pulse generator, a multivibrator will definitely fit.

Like everything I planned to say. If I missed something, then write in the comments - I will add what is needed, and what is not needed - I will correct it. Comments are always welcome!

I write new articles spontaneously and not according to a schedule, and therefore I suggest subscribing to updates by email or by e-mail. Then new articles will come directly to your mailbox or straight to the RSS reader.

That's all for me. I wish you all success and good spring mood!

Sincerely, Vladimir Vasiliev.

Also, dear friends, you can subscribe to site updates and receive new materials and gifts directly to your mailbox. To do this, just fill out the form below.

In this article we will talk about the multivibrator, how it works, how to connect the load to the multivibrator and the calculation of a transistor symmetrical multivibrator.

multivibrator- This is a simple square-wave generator that operates in oscillator mode. It only needs battery power or other power source to operate. Consider the simplest symmetrical transistor multivibrator. Its scheme is shown in the figure. The multivibrator can be complicated depending on the required functions to be performed, but all the elements shown in the figure are mandatory, without them the multivibrator will not work.

The operation of a symmetrical multivibrator is based on the charge-discharge processes of capacitors, which together with resistors form RC chains.

I wrote about how RC chains work earlier in my article Capacitor, which you can read on my website. On the Internet, if you find material about a symmetrical multivibrator, then it is presented briefly and not intelligibly. This circumstance does not allow novice radio amateurs to understand anything, but only helps experienced electronic engineers to remember something. At the request of one of the visitors to my site, I decided to eliminate this gap.

How does a multivibrator work?

At the initial moment of power supply, the capacitors C1 and C2 are discharged, so their current resistance is small. The low resistance of the capacitors leads to the fact that there is a "fast" opening of the transistors, caused by the flow of current:

- VT2 along the way (shown in red): "+ power supply> resistor R1> low resistance of discharged C1> base-emitter junction VT2> - power supply";

- VT1 along the way (shown in blue): "+ power supply> resistor R4> low resistance of discharged C2> base-emitter junction VT1> - power supply".

This is the "unsteady" mode of operation of the multivibrator. It lasts for a very short time, determined only by the speed of the transistors. And two absolutely identical transistors do not exist. Which transistor opens faster, that one will remain open - the "winner". Suppose that in our diagram it turned out to be VT2. Then, through the low resistance of the discharged capacitor C2 and the low resistance of the collector-emitter junction VT2, the base of the transistor VT1 will be closed to the emitter VT1. As a result, the transistor VT1 will be forced to close - "become defeated."

Since the transistor VT1 is closed, there is a “fast” charge of the capacitor C1 along the path: “+ power source> resistor R1> low resistance of the discharged C1> base-emitter junction VT2> - power source”. This charge occurs almost up to the voltage of the power supply.

At the same time, the capacitor C2 is charged with a current of reverse polarity along the path: “+ power source> resistor R3> low resistance of the discharged C2> collector-emitter junction VT2> - power source”. The duration of the charge is determined by the values of R3 and C2. They determine the time at which VT1 is in the closed state.

When the capacitor C2 is charged to a voltage approximately equal to a voltage of 0.7-1.0 volts, its resistance will increase and the transistor VT1 will open with the voltage applied along the path: “+ power supply> resistor R3> base-emitter junction VT1> - power source". In this case, the voltage of the charged capacitor C1, through the open collector-emitter junction VT1, will be applied to the emitter-base junction of the transistor VT2 with reverse polarity. As a result, VT2 will close, and the current that previously passed through the open collector-emitter junction VT2 will run through the circuit: “+ power supply> resistor R4> low resistance C2> base-emitter junction VT1> - power source”. This circuit will quickly recharge the capacitor C2. From this moment, the "steady" mode of autogeneration begins.

The operation of a symmetrical multivibrator in the "steady" generation mode

The first half-cycle of operation (oscillation) of the multivibrator begins.

With the transistor VT1 open and VT2 closed, as I just wrote, capacitor C2 is quickly recharged (from a voltage of 0.7 ... 1.0 volts of one polarity to the power supply voltage of the opposite polarity) along the circuit: “+ power supply> resistor R4 > low resistance C2 > base-emitter junction VT1 > - power supply. In addition, the capacitor C1 is slowly recharged (from the voltage of the power supply of one polarity to a voltage of 0.7 ... 1.0 volts of the opposite polarity) along the circuit: “+ power supply> resistor R2> right plate C1> left plate C1> collector- emitter junction of the transistor VT1> - power supply".

When, as a result of overcharging C1, the voltage at the base of VT2 reaches a value of +0.6 volts relative to the emitter of VT2, the transistor will open. Therefore, the voltage of the charged capacitor C2, through the open collector-emitter junction VT2, will be applied to the emitter-base junction of the transistor VT1 with reverse polarity. VT1 will close.

The second half-cycle of operation (oscillation) of the multivibrator begins.

When the transistor VT2 is open and VT1 is closed, the capacitor C1 is quickly recharged (from a voltage of 0.7 ... 1.0 volts of one polarity to the power supply voltage of the opposite polarity) along the circuit: “+ power supply> resistor R1> low resistance C1> base- emitter junction VT2 > - power supply". In addition, there is a slow recharge of the capacitor C2 (from the voltage of the power supply of one polarity, to a voltage of 0.7 ... 1.0 volts of the opposite polarity) along the circuit: “right plate C2> collector-emitter junction of the transistor VT2> - power supply> + source power > resistor R3 > left plate C2. When the voltage at the base of VT1 reaches +0.6 volts relative to the emitter of VT1, the transistor will open. Therefore, the voltage of the charged capacitor C1, through the open collector-emitter junction VT1, will be applied to the emitter-base junction of the transistor VT2 with reverse polarity. VT2 will close. On this, the second half-cycle of the multivibrator oscillation ends, and the first half-cycle begins again.

The process is repeated until the multivibrator is disconnected from the power source.

Ways to connect the load to a symmetrical multivibrator

Rectangular pulses are taken from two points of a symmetrical multivibrator- collectors of transistors. When there is a “high” potential on one collector, then there is a “low” potential on the other collector (it is absent), and vice versa - when there is a “low” potential on one output, then “high” on the other. This is clearly shown in the timeline below.

The multivibrator load must be connected in parallel with one of the collector resistors, but in no case in parallel with the collector-emitter transistor junction. You can not shunt the transistor with a load. If this condition is not met, then at least the duration of the pulses will change, and as a maximum, the multivibrator will not work. The figure below shows how to connect the load correctly, and how not to do it.

In order for the load not to affect the multivibrator itself, it must have sufficient input impedance. For this, buffer transistor stages are usually used.

The example shows connecting a low-resistance dynamic head to a multivibrator. An additional resistor increases the input resistance of the buffer stage, and thereby eliminates the influence of the buffer stage on the multivibrator transistor. Its value must be at least 10 times the value of the collector resistor. Connecting two transistors in a "composite transistor" scheme greatly increases the output current. In this case, it is correct to connect the base-emitter circuit of the buffer stage in parallel with the collector resistor of the multivibrator, and not in parallel with the collector-emitter junction of the multivibrator transistor.

For connecting a high-impedance dynamic head to a multivibrator buffer stage is not needed. The head is connected instead of one of the collector resistors. The only condition that must be met is that the current flowing through the dynamic head must not exceed the maximum collector current of the transistor.

If you want to connect ordinary LEDs to the multivibrator- to make a flasher, then buffer cascades are not required for this. They can be connected in series with collector resistors. This is due to the fact that the current of the LED is small, and the voltage drop across it during operation is not more than one volt. Therefore, they do not have any effect on the operation of the multivibrator. True, this does not apply to super-bright LEDs, in which the operating current is higher and the voltage drop can be from 3.5 to 10 volts. But in this case, there is a way out - to increase the supply voltage and use transistors with high power, providing sufficient collector current.

Please note that oxide (electrolytic) capacitors are connected with pluses to the collectors of transistors. This is due to the fact that on the bases of bipolar transistors, the voltage does not rise above 0.7 volts relative to the emitter, and in our case, emitters are a minus of power. But on the collectors of transistors, the voltage changes almost from zero to the voltage of the power source. Oxide capacitors are not able to perform their function when they are connected with reverse polarity. Naturally, if you use transistors of a different structure (not N-P-N, a P-N-P structures), then in addition to changing the polarity of the power source, it is necessary to turn the LEDs with cathodes "up the circuit", and the capacitors - pluses to the bases of the transistors.

Let's figure it out now what parameters of the multivibrator elements set the output currents and generation frequency of the multivibrator?

What are the collector resistor values? I have seen in some incompetent Internet articles that the values of the collector resistors are insignificant, but they affect the frequency of the multivibrator. All this is complete nonsense! With the correct calculation of the multivibrator, the deviation of the values \u200b\u200bof these resistors by more than five times from the calculated one will not change the frequency of the multivibrator. The main thing is that their resistance should be less than the base resistors, because the collector resistors provide a fast charge of the capacitors. But on the other hand, the values of the collector resistors are the main ones for calculating the power consumption from the power source, the value of which should not exceed the power of the transistors. If you figure it out, then correct connection they do not even directly affect the output power of the multivibrator. But the duration between switching (multivibrator frequency) is determined by the "slow" recharge of the capacitors. The recharge time is determined by the values of RC chains - basic resistors and capacitors (R2C1 and R3C2).

The multivibrator, although it is called symmetrical, refers only to the circuitry of its construction, and it can produce both symmetrical and non-symmetrical output pulses. The duration of the pulse (high level) on the VT1 collector is determined by the values of R3 and C2, and the duration of the pulse (high level) on the VT2 collector is determined by the values of R2 and C1.

The duration of the recharge of capacitors is determined by a simple formula, where Tau is the pulse duration in seconds, R is the resistance of the resistor in ohms, FROM is the capacitance of the capacitor in Farads:

Thus, if you have not already forgotten what was written in this article a couple of paragraphs earlier:

If equal R2=R3 and C1=C2, at the outputs of the multivibrator there will be a “meander” - rectangular pulses with a duration equal to the pauses between the pulses, which you see in the figure.

The total period of oscillation of the multivibrator is T is equal to the sum of the pulse and pause durations:

Oscillation frequency F(Hz) related to period T(sec) through the ratio:

As a rule, if there are any calculations of radio circuits on the Internet, they are scarce. That's why we will calculate the elements of a symmetrical multivibrator using an example .

Like any transistor cascades, the calculation must be carried out from the end - the output. And at the output we have a buffer stage, then there are collector resistors. Collector resistors R1 and R4 perform the function of loading transistors. Collector resistors have no effect on the generation frequency. They are calculated based on the parameters of the selected transistors. Thus, we first calculate the collector resistors, then the base resistors, then the capacitors, and then the buffer stage.

The order and example of calculating a transistor symmetrical multivibrator

Initial data:

Supply voltage Ui.p. = 12 V.

Required multivibrator frequency F = 0.2 Hz (T = 5 seconds), and the pulse duration is equal to 1 (one) second.

An incandescent car light bulb is used as a load. 12 volts, 15 watts.

As you guessed, we will calculate the flasher, which will flash once every five seconds, and the duration of the glow will be 1 second.

Choosing transistors for the multivibrator. For example, we have the most common transistors in Soviet times KT315G.

For them: Pmax=150 mW; Imax=150 mA; h21>50.

Transistors for the buffer stage are selected based on the load current.

In order not to depict the circuit twice, I have already signed the values of the elements on the diagram. Their calculation is given later in the Decision.

Solution:

1. First of all, it is necessary to understand that the operation of a transistor at high currents in the key mode is the safest for the transistor itself than operation in the amplifying mode. Therefore, there is no need to calculate the power for the transition state at the moments of the passage of an alternating signal, through the operating point "B" of the static mode of the transistor - the transition from the open state to the closed state and vice versa. For pulse circuits, built on bipolar transistors, usually calculate the power for transistors that are in the open state.

First, we determine the maximum power dissipation of the transistors, which should be a value that is 20 percent less (a factor of 0.8) than the maximum power of the transistor indicated in the reference book. But why should we drive the multivibrator into a rigid frame of high currents? Yes, and from increased power, energy consumption from the power source will be large, but there will be little benefit. Therefore, having determined the maximum power dissipation of transistors, we will reduce it by 3 times. A further reduction in dissipated power is undesirable because the operation of a multivibrator on bipolar transistors in the low current mode is an “unstable” phenomenon. If the power supply is used not only for the multivibrator, or it is not quite stable, the frequency of the multivibrator will also “float”.

Determine the maximum power dissipation: Pras.max = 0.8 * Pmax = 0.8 * 150mW = 120mW

We determine the rated power dissipation: Pras.nom. = 120 / 3 = 40mW

2. Determine the collector current in the open state: Ik0 = Pras.nom. / Ui.p. = 40mW / 12V = 3.3mA

Let's take it as the maximum collector current.

3. Find the value of the resistance and power of the collector load: Rk.total = Ui.p. / Ik0 = 12V / 3.3mA = 3.6 kOhm

We select resistors as close as possible to 3.6 kOhm in the existing nominal range. In the nominal series of resistors there is a nominal value of 3.6 kOhm, therefore, we first consider the value of the collector resistors R1 and R4 of the multivibrator: Rk \u003d R1 \u003d R4 \u003d 3.6 kOhm.

The power of the collector resistors R1 and R4 is equal to the rated power dissipation of the transistors Pras.nom. = 40 mW. We use resistors with a power exceeding the specified Pras.nom. - MLT-0.125 type.

4. Let's proceed to the calculation of the basic resistors R2 and R3. Their value is found based on the gain of the transistors h21. At the same time, for reliable operation of the multivibrator, the resistance value must be within: 5 times the resistance of the collector resistors, and less than the product Rk * h21. In our case Rmin \u003d 3.6 * 5 \u003d 18 kOhm, and Rmax \u003d 3.6 * 50 \u003d 180 kOhm

Thus, the resistance values Rb (R2 and R3) can be in the range of 18...180 kOhm. We pre-select the average value = 100 kOhm. But it is not final, since we need to provide the required frequency of the multivibrator, and as I wrote earlier, the frequency of the multivibrator directly depends on the base resistors R2 and R3, as well as on the capacitance of the capacitors.

5. Calculate the capacitances of capacitors C1 and C2 and, if necessary, recalculate the values of R2 and R3.

The values of the capacitance of the capacitor C1 and the resistance of the resistor R2 determine the duration of the output pulse on the collector VT2. It is during the action of this pulse that our light bulb should light up. And in the condition, the pulse duration was set to 1 second.

determine the capacitance of the capacitor: C1 \u003d 1 sec / 100kOhm \u003d 10 uF

A capacitor with a capacity of 10 microfarads is available in the nominal range, so it suits us.

The values of the capacitance of the capacitor C2 and the resistance of the resistor R3 determine the duration of the output pulse on the collector VT1. It is during the action of this pulse that a "pause" operates on the VT2 collector and our light should not light up. And in the condition, a full period of 5 seconds was set with a pulse duration of 1 second. Therefore, the duration of the pause is 5 seconds - 1 second = 4 seconds.

By transforming the recharge duration formula, we determine the capacitance of the capacitor: C2 \u003d 4sec / 100kOhm \u003d 40 uF

A 40 uF capacitor is not in the nominal series, so it does not suit us, and we will take a 47 uF capacitor as close as possible to it. But as you understand, the “pause” time will also change. To prevent this from happening, we recalculate the resistance of the resistor R3 based on the duration of the pause and the capacitance of the capacitor C2: R3 = 4sec / 47uF = 85kΩ

According to the nominal series, the nearest value of the resistance of the resistor is 82 kOhm.

So, we got the values of the elements of the multivibrator:

R1 = 3.6 kΩ, R2 = 100 kΩ, R3 = 82 kΩ, R4 = 3.6 kΩ, C1 = 10 uF, C2 = 47 uF.

6. Calculate the value of the resistor R5 of the buffer stage.

The resistance of the additional limiting resistor R5 to eliminate the influence on the multivibrator is selected at least 2 times the resistance of the collector resistor R4 (and in some cases more). Its resistance, together with the resistance of the emitter-base junctions VT3 and VT4, in this case will not affect the parameters of the multivibrator.

R5 = R4 * 2 = 3.6 * 2 = 7.2 kΩ

According to the nominal series, the nearest resistor is 7.5 kOhm.

With the value of the resistor R5 = 7.5 kOhm, the buffer stage control current will be equal to:

I ex. \u003d (Ui.p. - Ube) / R5 \u003d (12v - 1.2v) / 7.5 kOhm \u003d 1.44 mA

In addition, as I wrote earlier, the value of the collector load of the multivibrator transistors does not affect its frequency, so if you do not have such a resistor, then you can replace it with another "close" value (5 ... 9 kOhm). It is better if this is in the direction of decreasing, so that there is no drop in the control current at the buffer stage. But keep in mind that the additional resistor is an additional load on the VT2 transistor of the multivibrator, so the current flowing through this resistor adds up to the current of the collector resistor R4 and is a load for the VT2 transistor: Itotal \u003d Ik + Iupr. = 3.3mA + 1.44mA = 4.74mA

The total load on the collector of the transistor VT2 is within normal limits. If it exceeds the maximum collector current specified in the reference book and multiplied by a factor of 0.8, increase the resistance R4 until the load current is sufficiently reduced, or use a more powerful transistor.

7. We need to provide current to the light bulb In \u003d Rn / Ui.p. = 15W / 12V = 1.25 A

But the buffer stage control current is 1.44mA. The multivibrator current must be increased by a value equal to the ratio:

In / I ex. = 1.25A / 0.00144A = 870 times.

How to do it? For a significant increase in output current use transistor cascades built according to the "composite transistor" scheme. The first transistor is usually low-power (we will use KT361G), it has the highest gain, and the second must provide sufficient load current (let's take the no less common KT814B). Then their gains h21 are multiplied. So, for the transistor KT361G h21> 50, and for the transistor KT814B h21=40. And the overall transfer coefficient of these transistors, connected according to the "composite transistor" scheme: h21 = 50 * 40 = 2000. This figure is more than 870, so these transistors are enough to drive a light bulb.

Well, that's all!

This article describes a device designed simply so that a novice radio amateur (electrician, electronics engineer, etc.) can better understand circuit diagrams and gain experience during assembly this device. Although it is possible for this simplest multivibrator, which is described below, you can also find practical use. Consider the schema:

Figure 1 - The simplest multivibrator on a relay

When power is applied to the circuit, the capacitor begins to charge through the resistor R1, while the contacts K1.1 are open, when the capacitor is charged to a certain voltage, the relay will operate and the contacts will close, with the contacts closed, the capacitor will start to discharge through these contacts and resistor R2, when the capacitor is discharged to a certain voltage, the contacts will open and the process will continue to repeat cyclically. This multivibrator works because the relay operation current is greater than the holding current. The resistance of the resistors CANNOT be changed over a wide range and this is a disadvantage of this circuit. The resistance of the power supply affects the frequency and because of this, this multivibrator will not work from all power sources. The capacitance of the capacitor can be increased, while the frequency of contact closure will decrease. If the relay has a second group of contacts and use huge values of the capacitance of the capacitor, then you can use this circuit to periodically automatically turn on / off the devices. The assembly process is shown in the photos below:

Connecting resistor R2

Capacitor connection

Connecting resistor R1

Connection of relay contacts with its winding

Connecting wires for power supply

Relays can be bought at a radio parts store or obtained from old broken equipment, for example, you can solder relays from refrigerator circuit boards:

If the relay has bad contacts, then they can be cleaned a little.