Symmetrical multivibrator, calculation and scheme of the multivibrator. Symmetrical multivibrator for LEDs Where is the multivibrator used?
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."
It leads to a secret page where a small but pleasant surprise awaits you - a gift prepared by me. Moreover, in the future, useful materials, amateur radio software and something else will be posted on this page - I have not yet come up with it. 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.
Using a laser printer, I printed the circuit board drawings 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 returned to its 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. Playing with the values of resistors and capacitors, you can output audio frequency signals to the speaker. 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!
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Sincerely, Vladimir Vasiliev.
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If you look, all electronics consists of a large number of individual bricks. These are transistors, diodes, resistors, capacitors, inductive elements. And from these bricks you can add anything you want.
From a harmless children's toy that emits, for example, the sound "meow", to a ballistic missile guidance system with an eight-megaton multiple warhead.
One of the very well-known and often used circuits in electronics is a symmetrical multivibrator, which is an electronic device that generates (generates) oscillations in shape approaching a rectangular one.
The multivibrator is assembled on two transistors or logic circuits with additional elements. In fact, this is a two-stage amplifier with a positive feedback circuit (POS). This means that the output of the second stage is connected through a capacitor to the input of the first stage. As a result, the amplifier, due to positive feedback, turns into a generator.
In order for the multivibrator to start generating pulses, it is enough to connect the supply voltage. Multivibrators can be symmetrical And asymmetrical.
The figure shows a diagram of a symmetrical multivibrator.
In a symmetrical multivibrator, the values of the elements of each of the two arms are exactly the same: R1=R4, R2=R3, C1=C2. If you look at the waveform of the output signal of a symmetrical multivibrator, it is easy to see that the rectangular pulses and the pauses between them are the same in time. t pulse ( t and) = t pauses ( t p). Resistors in the collector circuits of transistors do not affect the pulse parameters, and their value is selected depending on the type of transistor used.
The pulse repetition rate of such a multivibrator is easily calculated using a simple formula:
Where f is the frequency in hertz (Hz), C is the capacitance in microfarads (uF) and R is the resistance in kiloohms (kΩ). For example: C \u003d 0.02 uF, R \u003d 39 kOhm. We substitute into the formula, perform actions and get a frequency in the audio range approximately equal to 1000 Hz, or rather 897.4 Hz.
By itself, such a multivibrator is uninteresting, since it produces one unmodulated “peep”, but if we select the frequency of 440 Hz with the elements, and this is the note A of the first octave, then we will get a miniature tuning fork, with which you can, for example, tune a guitar on a hike. The only thing to do is to add a single transistor amplifier stage and a miniature speaker.
The following parameters are considered to be the main characteristics of the pulse signal:
Frequency. Unit of measurement (Hz) Hertz. 1 Hz is one oscillation per second. The frequencies perceived by the human ear are in the range of 20 Hz - 20 kHz.
Pulse duration. Measured in fractions of a second: miles, micro, nano, pico, and so on.
Amplitude. In the multivibrator under consideration, amplitude adjustment is not provided. In professional devices, both step and smooth amplitude adjustment are used.
duty cycle. The ratio of the period (T) to the pulse duration ( t). If the pulse length is 0.5 period, then the duty cycle is two.
Based on the above formula, it is easy to calculate a multivibrator for almost any frequency, with the exception of high and ultra-high frequencies. There are several other physical principles at work.
In order for the multivibrator to give out several discrete frequencies, it is enough to put a two-section switch and five to six capacitors of different capacities, naturally the same in each arm, and select the required frequency using the switch. Resistors R2, R3 also affect the frequency and duty cycle and can be made variable. Here is another multivibrator circuit with adjustable switching frequency.
Reducing the resistance of resistors R2 and R4 less than a certain value depending on the type of transistors used can cause generation failure and the multivibrator will not work, therefore, in series with resistors R2 and R4, you can connect a variable resistor R3, which can select the switching frequency of the multivibrator.
The practical application of the symmetrical multivibrator is very extensive. Pulse computer technology, radio measuring equipment in the production of household appliances. A lot of unique medical equipment is built on circuits based on the same multivibrator.
Due to its exceptional simplicity and low cost, the multivibrator has found wide application in children's toys. Here is an example of a conventional LED flasher.
With the values of electrolytic capacitors C1, C2 and resistors R2, R3 indicated on the diagram, the pulse frequency will be 2.5 Hz, which means that the LEDs will flash approximately twice per second. You can use the circuit proposed above and include a variable resistor together with resistors R2, R3. Thanks to this, it will be possible to see how the flash frequency of the LEDs will change when the resistance of the variable resistor changes. You can put capacitors of different ratings and observe the result.
While still a schoolboy, I assembled a Christmas tree garland switch on a multivibrator. Everything worked out, but when I connected the garlands, my device began to switch them at a very high frequency. Because of this, in the next room, the TV began to show with wild noise, and the electromagnetic relay in the circuit crackled like a machine gun. It was both joyful (it works!) and a little scary. The parents were outraged.
Such an annoying blunder with too frequent switching did not give me peace of mind. And I checked the circuit, and the capacitors at face value were the ones that were needed. I did not take into account only one.
The electrolytic capacitors were very old and dried up. Their capacity was small and did not at all correspond to the one indicated on their case. Due to the low capacitance, the multivibrator operated at a higher frequency and switched garlands too often.
At that time I did not have any instruments that could measure the capacitance of capacitors. Yes, and I used a tester with a pointer, and not a modern digital multimeter.
Therefore, if your multivibrator produces an overestimated frequency, then first check the electrolytic capacitors. Fortunately, now you can buy a universal radio component tester for little money, with which you can measure the capacitance of a capacitor.
A multivibrator (from Latin I hesitate a lot) is a non-linear device that converts a constant supply voltage into almost rectangular pulse energy. The multivibrator is based on a positive feedback amplifier.
There are self-oscillating and waiting multivibrators. Let's consider the first type.
On fig. 1 shows a generalized circuit of a feedback amplifier.
The circuit contains an amplifier with a complex gain k=Ke-ik, an OOS circuit with a gain m, and a PIC circuit with a complex gain B=e-i. It is known from the theory of generators that for the occurrence of oscillations at any frequency, it is necessary that the condition Bk>1 be satisfied at it. A pulsed periodic signal contains a set of frequencies that form a line spectrum (see the 1st lecture). That. to generate pulses, it is necessary to fulfill the condition Bk>1 not at one frequency, but in a wide frequency band. Moreover, the shorter the pulse and with shorter fronts, the signal is required to be received, for a wider frequency band, the condition Vk>1 is required. The given condition is divided into two:
amplitude balance condition - the module of the total generator transfer coefficient must exceed 1 in a wide frequency range - K>1;
phase balance condition - the total phase shift of the oscillations in the closed circuit of the generator in the same frequency range must be a multiple of 2 - to + = 2n.
Qualitatively, the process of abrupt voltage growth occurs as follows. Let at some point in time, as a result of fluctuations, the voltage at the generator input increased by a small amount u. As a result of the fulfillment of both generation conditions, a voltage increment will appear at the output of the device: uout = Inkin > uin, which is transmitted to the input in phase with the initial uin. Accordingly, this increase will lead to a further increase in the output voltage. There is an avalanche-like process of voltage growth in a wide frequency range.
The task of constructing a practical circuit of the pulse generator is reduced to applying to the input of a broadband amplifier part of the output signal with a phase difference =2. Since one resistive amplifier shifts the phase of the input voltage by 1800, using two amplifiers connected in series, the phase balance condition can be satisfied. The amplitude balance condition will look like this in this case:
One of the possible schemes that implements this method is shown in Fig.2. This is a circuit of a self-oscillating multivibrator with collector-base connections. The circuit uses two amplifying stages. The output of one amplifier is connected to the input of the second by capacitor C1, and the output of the latter is connected to the input of the first by capacitor C2.
Qualitatively, we consider the operation of the multivibrator using the time voltage diagrams (diagrams) shown in Fig. 3.
Let the multivibrator switch at the time t=t1. Transistor VT1 enters saturation mode, and VT2 - in cutoff mode. From this moment, the processes of recharging capacitors C1 and C2 begin. Until the moment t1, the capacitor C2 was completely discharged, and C1 was charged to the supply voltage Ep (the polarity of the charged capacitors is shown in Fig. 2). After unlocking VT1, it starts charging from the source En through the resistor Rk2 and the base of the unlocked transistor VT1. The capacitor is charged almost to the supply voltage En with a constant charge
zar2 = С2Rк2
Since C2 is connected in parallel to VT2 through open VT1, the rate of its charging determines the rate of change of the output voltage Uout2 .. Assuming the charging process is completed when Uout2 = 0.9Up, it is easy to get the duration
t2-t1= С2Rк2ln102,3С2Rк2
Simultaneously with the charging of C2 (starting from the moment t1), the capacitor C1 is recharged. Its negative voltage applied to the base of VT2 maintains the locked state of this transistor. Capacitor C1 is recharged along the circuit: En, resistor Rb2, C1, E-K of the open transistor VT1. case with time constant
razr1 \u003d C1Rb2
Since Rb >> Rk, then the charge<<разр. Следовательно, С2 успевает зарядиться до Еп пока VT2 еще закрыт. Процесс перезарядки С1 заканчивается в момент времени t5, когда UC1=0 и начинает открываться VT2 (для простоты считаем, что VT2 открывается при Uбє=0). Можно показать, что длительность перезаряда С1 равна:
t3-t1 = 0.7C1Rb2
At the time t3, the collector current VT2 appears, the voltage Uke2 drops, which leads to the closing of VT1 and, accordingly, to an increase in Uke1. This voltage increment is transmitted through C1 to the VT2 base, which entails an additional opening of VT2. The transistors go into active mode, an avalanche-like process occurs, as a result of which the multivibrator goes into another quasi-stationary state: VT1 is closed, VT2 is open. The duration of the multivibrator rollover is much less than all other transients and can be considered equal to zero.
From the moment t3, the processes in the multivibrator will proceed similarly to the one described, it is only necessary to swap the indices of the circuit elements.
Thus, the duration of the pulse front is determined by the processes of charging the coupling capacitor and is numerically equal to:
The duration of the multivibrator in a quasi-stable state (duration of the pulse and pause) is determined by the process of discharging the coupling capacitor through the base resistor and is numerically equal to:
With a symmetrical multivibrator circuit (Rk1 = Rk2 = Rk, Rb1 = Rb2 = Rb, C1 = C2 = C), the pulse duration is equal to the pause duration, and the pulse repetition period is equal to:
T \u003d u + n \u003d 1.4CRb
Comparing the duration of the pulse and the front, it must be taken into account that Rb / Rk \u003d h21e / s (h21e for modern transistors is 100, and s2). Therefore, the rise time is always less than the pulse duration.
The output voltage frequency of a symmetrical multivibrator does not depend on the supply voltage and is determined only by the circuit parameters:
To change the duration of the pulses and their repetition period, it is necessary to vary the values of Rb and C. But the possibilities here are small: the limits of change in Rb are limited on the larger side by the need to maintain an open transistor, on the smaller side - shallow saturation. It is difficult to smoothly change the value of C even within small limits.
To find a way out of the difficulty, let's turn to the time period t3-t1 in Fig. 2. It can be seen from the figure that the specified time interval, and, consequently, the pulse duration can be adjusted by changing the slope of the direct discharge of the capacitor. This can be achieved by connecting the base resistors not to the power supply, but to an additional voltage source Ecm (see Fig. 4). Then the capacitor tends to recharge not to En, but to Esm, and the steepness of the exponent will change with a change in Esm.
The pulses generated by the considered circuits have a long rise time. In some cases, this value becomes unacceptable. To shorten f, cut-off capacitors are introduced into the circuit, as shown in Fig. 5. Capacitor C2 is charged in this circuit not through Rz, but through Rd. Diode VD2, remaining closed, "cuts off" the voltage on C2 from the output and the voltage on the collector increases almost simultaneously with the closing of the transistor.
In multivibrators, an operational amplifier can be used as an active element. The self-oscillating multivibrator on the op-amp is shown in fig. 6.
The OU is covered by two OS circuits: positive
and negative
Xc/(Xc+R) = 1/(1+wRC).
Let the generator be turned on at time t0. At the inverting input, the voltage is zero, at the non-inverting input, it is equally likely positive or negative. For definiteness, we take the positive. Due to the POS, the maximum possible voltage will be set at the output - Uout m. The settling time of this output voltage is determined by the frequency properties of the op amp and can be set to zero. Starting from the moment t0, the capacitor C will be charged with the time constant =RC. Up to the time t1, Ud = U+ - U- >0, and positive Uoutm is kept at the output of the op-amp. At t=t1, when Ud = U+ - U- = 0, the output voltage of the amplifier will change its polarity to - Uout m. After the moment t1, the capacitance C is recharged, tending to the level - Uout m. Up to the moment t2 Ud = U+ - U-< 0, что обеспечивает квазиравновесное состояние системы, но уже с отрицательным выходным напряжением. Т.о. изменение знака Uвых происходит в моменты уравнивания входных напряжений на двух входах ОУ. Длительность квазиравновесного состояния системы определяется постоянной времени =RC, и период следования импульсов будет равен:
T=2RCln(1+2R2/R1).
The multivibrator shown in Fig. 6 is called symmetrical, because. the times of positive and negative output voltages are equal.
To obtain an asymmetric multivibrator, the resistor in the OOS should be replaced with a circuit, as shown in Fig. 7. Different duration of positive and negative pulses is provided by different time constants for recharging capacities:
R "C, - \u003d R" C.
An op amp multivibrator can easily be turned into a single vibrator or a standby multivibrator. First, in the OOS circuit, in parallel with C, we connect the diode VD1, as shown in Fig. 8. Thanks to the diode, the circuit has one stable state when the output voltage is negative. Indeed, since Uout = - Uout m, then the diode is open and the voltage at the inverting input is approximately equal to zero. While the voltage at the non-inverting input is
U+ =- Uout m R2/(R1+R2)
and the stable state of the circuit is maintained. To generate one pulse, a trigger circuit should be added to the circuit, consisting of a diode VD2, C1 and R3. Diode VD2 is maintained in the closed state and can only be opened by a positive input pulse that arrives at the input at time t0. With the opening of the diode, the sign changes and the circuit goes into a state with a positive voltage at the output. Uout = Uout m. After that, the capacitor C1 begins to charge with a time constant =RC. At time t1, the input voltages are compared. U- \u003d U + \u003d Uout m R2 / (R1 + R2) and \u003d 0. At the next moment, the differential signal becomes negative and the circuit returns to a steady state. Diagrams are shown in fig. 9.
Schemes of waiting multivibrators on discrete and logical elements are used.
The scheme of the considered multivibrator is similar to that considered earlier.
This lesson will be devoted to a rather important and popular topic, about multivibrators and their application. If I tried to just list where and how self-oscillating symmetrical and asymmetrical multivibrators are used, this would require a decent number of book pages. There is, perhaps, no such branch of radio engineering, electronics, automation, impulse or computer technology, where such generators would not be used. This tutorial will give you some theoretical information about these devices, and at the end, I will give some examples of their practical use in relation to your creativity.
Self-oscillating multivibrator
Multivibrators are electronic devices that generate electrical vibrations that are close in shape to a rectangular one. The spectrum of oscillations generated by the multivibrator contains many harmonics - also electrical oscillations, but multiples of the fundamental frequency oscillations, which is reflected in its name: "multi - many", "vibration - oscillate".
Consider the circuit shown in (Fig. 1a). Do you recognize? Yes, this is a 3H two-stage transistor amplifier circuit with headphone output. What happens if the output of such an amplifier is connected to its input, as shown by the dashed line in the diagram? A positive feedback arises between them and the amplifier will self-excite and become an audio frequency oscillation generator, and in telephones we will hear a low-pitched sound. A decisive struggle is waged against such a phenomenon in receivers and amplifiers, but for automatically operating devices it turns out to be useful.
Now look at (Fig. 1b). On it you see a circuit of the same amplifier, covered positive feedback , as in (Fig. 1, a), only its outline is somewhat changed. This is how the circuits of self-oscillatory, i.e., self-excited multivibrators, are usually drawn. Experience is perhaps the best method of understanding the essence of the action of an electronic device. You have proven this many times. And now, in order to better understand the operation of this universal device - an automatic machine, I propose to conduct an experiment with it. You can see a schematic diagram of a self-oscillating multivibrator with all the data of its resistors and capacitors in (Fig. 2, a). Mount it on a breadboard. Transistors must be low-frequency (MP39 - MP42), since high-frequency transistors have a very small breakdown voltage of the emitter junction. Electrolytic capacitors C1 and C2 - type K50 - 6, K50 - 3 or their imported counterparts for a nominal voltage of 10 - 12 V. The resistance of the resistors may differ from those indicated in the diagram up to 50%. It is only important that the ratings of the load resistors Rl, R4 and the base resistors R2, R3 are possibly the same. For power, use the Krona battery or PSU. In the collector circuit of any of the transistors, turn on a milliammeter (PA) for a current of 10 - 15 mA, and connect a high-resistance DC voltmeter (PU) to a voltage of up to 10 V to the emitter-collector section of the same transistor. After checking the installation and especially carefully the polarity of switching on electrolytic capacitors, connect a power source to the multivibrator. What do the meters show? Milliammeter - sharply increasing to 8 - 10 mA, and then also sharply decreasing almost to zero, the current of the collector circuit of the transistor. The voltmeter, on the contrary, either decreases almost to zero, or increases to the voltage of the power source, the collector voltage. What do these measurements say? The fact that the transistor of this arm of the multivibrator operates in the switching mode. The largest collector current and at the same time the smallest voltage on the collector correspond to the open state, and the smallest current and the largest collector voltage correspond to the closed state of the transistor. The transistor of the second arm of the multivibrator works exactly the same way, but, as they say, with 180° phase shift : when one of the transistors is open, the other is closed. It is easy to verify this by including the same milliammeter in the collector circuit of the transistor of the second arm of the multivibrator; the arrows of the measuring instruments will alternately deviate from the zero marks of the scales. Now, using a clock with a second hand, count how many times per minute the transistors go from open to closed. Approximately 15 - 20 times. This is the number of electrical oscillations generated by the multivibrator per minute. Therefore, the period of one oscillation is 3 - 4 s. Continuing to follow the arrow of the milliammeter, try to depict these fluctuations graphically. On the horizontal axis of ordinates, plot on a certain scale the time intervals for the transistor to be in the open and closed states, and along the vertical axis, the collector current corresponding to these states. You will get approximately the same graph as the one shown in Fig. 2b.
So, it can be considered that the multivibrator generates electrical oscillations of a rectangular shape. In a multivibrator signal, regardless of which output it is taken from, current pulses and pauses between them can be distinguished. The time interval from the moment a single current (or voltage) pulse appears until the next pulse of the same polarity appears is usually called the pulse repetition period T, and the time between pulses with a pause duration Tn - Multivibrators generating pulses whose duration Tn is equal to the pauses between them are called symmetrical . Therefore, the experienced multivibrator you have assembled - symmetric. Replace capacitors C1 and C2 with other 10 to 15 uF capacitors. The multivibrator remained symmetrical, but the frequency of the oscillations generated by it increased by 3-4 times - up to 60-80 per 1 min, or, what is the same, up to about a frequency of 1 Hz. The arrows of the measuring instruments barely have time to follow the changes in currents and voltages in the transistor circuits. And if capacitors C1 and C2 are replaced with paper capacitances of 0.01 - 0.05 microfarads? How will the arrows of measuring instruments now behave? Having deviated from the zero marks of the scales, they stand still. Maybe the generation is broken? No! It's just that the oscillation frequency of the multivibrator has increased to several hundred hertz. These are fluctuations in the audio frequency range, which DC devices can no longer fix. You can detect them using a frequency meter or headphones connected through a capacitor with a capacity of 0.01 - 0.05 microfarads to any of the outputs of the multivibrator or by connecting them directly to the collector circuit of any of the transistors instead of a load resistor. On the phones, you will hear a low tone sound. What is the working principle of a multivibrator? Let's return to the diagram in Fig. 2, a. At the moment the power is turned on, the transistors of both arms of the multivibrator open, since negative bias voltages are applied to their bases through the corresponding resistors R2 and R3. At the same time, the coupling capacitors begin to charge: C1 - through the emitter junction of the transistor V2 and the resistor R1; C2 - through the emitter junction of transistor V1 and resistor R4. These capacitor charging circuits, being voltage dividers of the power supply, create on the bases of the transistors (relative to the emitters) negative voltages that increase in value, tending to open the transistors more and more. Opening a transistor causes the negative voltage at its collector to drop, which causes the negative voltage at the base of the other transistor to drop, turning it off. Such a process occurs immediately in both transistors, however, only one of them closes, on the basis of which a higher positive voltage, for example, due to the difference in the current transfer coefficients h21e of the resistor and capacitor ratings. The second transistor remains open. But these states of transistors are unstable, because the electrical processes in their circuits continue. Let's assume that after some time after turning on the power, the transistor V2 turned out to be closed, and the transistor V1 turned out to be open. From this moment, the capacitor C1 begins to discharge through the open transistor V1, the resistance of the emitter-collector section of which is low at this time, and the resistor R2. As the capacitor C1 discharges, the positive voltage at the base of the closed transistor V2 decreases. As soon as the capacitor is completely discharged and the voltage at the base of the transistor V2 becomes close to zero, a current appears in the collector circuit of this now opening transistor, which acts through the capacitor C2 on the base of the transistor V1 and lowers the negative voltage on it. As a result, the current flowing through the transistor V1 begins to decrease, and through the transistor V2, on the contrary, increases. This causes transistor V1 to turn off and transistor V2 to turn on. Now the capacitor C2 will start to discharge, but through the open transistor V2 and the resistor R3, which ultimately leads to the opening of the first and closing of the second transistor, etc. Transistors interact all the time, as a result of which the multivibrator generates electrical oscillations. The oscillation frequency of the multivibrator depends both on the capacitance of the coupling capacitors, which you have already checked, and on the resistance of the base resistors, as you can see right now. Try, for example, to replace the basic resistors R2 and R3 with high-resistance resistors. The oscillation frequency of the multivibrator will decrease. Conversely, if their resistances are less, the oscillation frequency will increase. Another experience: disconnect the upper (according to the diagram) terminals of the resistors R2 and R3 from the negative conductor of the power source, connect them together, and between them and the negative conductor, turn on a variable resistor with a resistance of 30 - 50 kOhm with a rheostat. By turning the axis of the variable resistor, you can change the oscillation frequency of the multivibrators within a fairly wide range. The approximate oscillation frequency of a symmetrical multivibrator can be calculated using the following simplified formula: F = 700 / (RC), where f is the frequency in hertz, R is the resistance of the base resistors in kiloohms, C is the capacitance of the coupling capacitors in microfarads. Using this simplified formula, calculate what frequencies your multivibrator generated. Let's return to the initial data of the resistors and capacitors of the experimental multivibrator (according to the diagram in Fig. 2, a). Replace capacitor C2 with a capacitor with a capacity of 2 - 3 μF, turn on a milliammeter in the collector circuit of transistor V2, following its arrow, graphically depict the current fluctuations generated by the multivibrator. Now the current in the collector circuit of transistor V2 will appear in shorter pulses than before (Fig. 2, c). The duration of the pulses Th will be approximately as many times less than the pauses between the pulses Th, by how much the capacitance of the capacitor C2 has decreased compared to its previous capacitance. And now turn the same (or such) milliammeter into the collector circuit of transistor V1. What does the meter show? Also current pulses, but their duration is much longer than the pauses between them (Fig. 2, d). What happened? By reducing the capacitance of capacitor C2, you violated the symmetry of the arms of the multivibrator - it became asymmetrical . Therefore, the vibrations generated by it became asymmetrical : in the collector circuit of the transistor V1, the current appears in relatively long pulses, in the collector circuit of the transistor V2, in short pulses. From Output 1 of such a multivibrator, you can take short, and from Output 2 - long voltage pulses. Temporarily swap capacitors C1 and C2. Now short voltage pulses will be at Output 1, and long voltage pulses at Output 2. Count (by the clock with a second hand) how many electrical impulses per minute this version of the multivibrator generates. About 80. Increase the capacitance of capacitor C1 by connecting a second electrolytic capacitor with a capacity of 20 - 30 microfarads in parallel with it. The pulse repetition rate will decrease. And if, on the contrary, the capacitance of this capacitor is reduced? The pulse repetition rate should increase. There is, however, another way to regulate the pulse repetition rate - by changing the resistance of the resistor R2: with a decrease in the resistance of this resistor (but not less than 3 - 5 kOhm, otherwise the transistor V2 will be open all the time and the self-oscillating process will be disrupted), the pulse repetition frequency should increase, and with an increase in its resistance, on the contrary, decrease. Check it out empirically - is it so? Choose a resistor of such a value that the number of pulses in 1 minute is exactly 60. The milliammeter needle will oscillate at a frequency of 1 Hz. The multivibrator in this case will become, as it were, an electronic clock mechanism that counts seconds.
Waiting multivibrator
Such a multivibrator generates current (or voltage) pulses when triggering signals are applied to its input from another source, for example, from a self-oscillating multivibrator. In order to turn the self-oscillating multivibrator, with which you have already conducted experiments in this lesson (according to the diagram in Fig. 2, a), into a waiting multivibrator, you must do the following: remove the capacitor C2, and instead of it, connect a resistor between the collector of the transistor V2 and the base of the transistor V1 (in Fig. 3 - R3) with a resistance of 10 - 15 kOhm; between the base of the transistor V1 and the grounded conductor, connect a series-connected element 332 (G1 or another constant voltage source) and a resistor with a resistance of 4.7 - 5.1 kOhm (R5), but so that the positive pole of the element is connected to the base (through R5); connect a capacitor (in Fig. 3 - C2) with a capacity of 1 - 5 thousand pF to the base circuit of transistor V1, the second output of which will act as a contact for the input control signal. The initial state of the transistor V1 of such a multivibrator is closed, the transistor V2 is open. Check - is it true? The voltage on the collector of a closed transistor should be close to the voltage of the power source, and on the collector of an open transistor it should not exceed 0.2 - 0.3 V. , turn on between the Uin contact and the grounded conductor, literally for a moment, one or two 332 elements connected in series (in the GB1 diagram) or a 3336L battery. Just do not confuse: the negative pole of this external electrical signal must be connected to the contact Uin. In this case, the arrow of the milliammeter should immediately deviate to the value of the highest current of the collector circuit of the transistor, freeze for a while, and then return to its original position in order to wait for the next signal. Repeat this experience several times. The milliammeter with each signal will show an instantaneous increase to 8 - 10 mA and after a while, the collector current of the transistor V1 will also instantly decrease almost to zero. These are single current pulses generated by a multivibrator. And if the battery GB1 is longer to keep connected to the clamp Uin. The same thing will happen as in the previous experiments - only one impulse will appear at the output of the multivibrator. Try it!
And one more experiment: touch the output of the base of the transistor V1 with some metal object taken in your hand. Perhaps, in this case, the waiting multivibrator will work - from the electrostatic charge of your body. Repeat the same experiments, but by including a milliammeter in the collector circuit of the transistor V2. When a control signal is applied, the collector current of this transistor should sharply decrease to almost zero, and then just as sharply increase to the value of the current of the open transistor. This is also a current pulse, but of negative polarity. What is the principle of operation of a waiting multivibrator? In such a multivibrator, the connection between the collector of transistor V2 and the base of transistor V1 is not capacitive, as in a self-oscillating one, but resistive - through resistor R3. A negative bias voltage is applied to the base of the transistor V2 through the resistor R2. The transistor V1 is securely closed by the positive voltage of the G1 element at its base. This state of the transistors is very stable. They can stay in this state for as long as they like. But on the basis of transistor V1, a voltage pulse of negative polarity appeared. From this point on, the transistors go into an unstable state. Under the influence of the input signal, the transistor V1 opens, and the changing voltage on its collector through the capacitor C1 closes the transistor V2. The transistors are in this state until the capacitor C1 is discharged (through the resistor R2 and the open transistor V1, the resistance of which is low at this time). As soon as the capacitor is discharged, the transistor V2 immediately opens, and the transistor V1 closes. From this point on, the multivibrator again finds itself in the original, stable standby mode. Thus, the standby multivibrator has one stable and one unstable state . During an unstable state, it generates one square wave current (voltage), the duration of which depends on the capacitance of the capacitor C1. The larger the capacitance of this capacitor, the longer the pulse duration. So, for example, with a capacitor capacitance of 50 μF, the multivibrator generates a current pulse with a duration of about 1.5 s, and with a capacitor with a capacity of 150 μF - three times more. Through additional capacitors - positive voltage pulses can be taken from output 1, and negative ones from output 2. Can the multivibrator be brought out of standby mode only by a negative voltage pulse applied to the base of transistor V1? No, not only. This can also be done by applying a voltage pulse of positive polarity, but to the base of the transistor V2. So, it remains for you to experimentally check how the capacitance of the capacitor C1 affects the duration of the pulses and the ability to control the waiting multivibrator with positive voltage pulses. How can a standby multivibrator be used practically? Differently. For example, to convert a sinusoidal voltage into rectangular voltage (or current) pulses of the same frequency, or to turn on another device for some time by applying a short-term electrical signal to the input of a waiting multivibrator. How else? Think!
Multivibrator in generators and electronic switches
Electronic call. A multivibrator can be used for a house call, replacing a conventional electric one with it. It can be assembled according to the scheme shown in (Fig. 4). Transistors V1 and V2 operate in a symmetrical multivibrator that generates oscillations with a frequency of about 1000 Hz, and transistor V3 operates in a power amplifier of these oscillations. Amplified vibrations are converted by the dynamic head B1 into sound vibrations. If you use a subscriber loudspeaker for a call, by including the primary winding of its transition transformer in the collector circuit of transistor V3, all the call electronics mounted on the board will be placed in its case. The battery will also be located there.
An electronic bell can be installed in the corridor by connecting it with two wires to the S1 button. When you press the - button, sound will appear in the dynamic head. Since power is supplied to the device only during ringing signals, two 3336L batteries connected in series or "Krona" will last for several months of ringing. Set the desired sound tone by replacing capacitors C1 and C2 with capacitors of other capacities. A multivibrator assembled according to the same scheme can be used to study and train in listening to the telegraph alphabet - Morse code. In this case, it is only necessary to replace the button with a telegraph key.
Electronic switch. This device, the circuit of which is shown in (Fig. 5), can be used to switch two Christmas tree garlands powered by an AC mains. The electronic switch itself can be powered by two 3336L batteries connected in series, or from a rectifier that would output a constant voltage of 9–12 V.
The switch circuit is very similar to the electronic bell circuit. But the capacitances of capacitors C1 and C2 of the switch are many times larger than the capacitances of similar bell capacitors. The switch multivibrator, in which transistors V1 and V2 operate, generates oscillations with a frequency of about 0.4 Hz, and the load of its power amplifier (transistor V3) is the winding of the electromagnetic relay K1. The relay has one pair of contact plates for switching. For example, a relay RES - 10 (passport RS4.524.302) or another electromagnetic relay that reliably operates from a voltage of 6 - 8 V at a current of 20 - 50 mA is suitable. When the power is turned on, transistors V1 and V2 of the multivibrator open and close alternately, generating square wave signals. When transistor V2 is turned on, a negative supply voltage is applied through resistor R4 and this transistor is applied to the base of transistor V3, saturating it. In this case, the resistance of the emitter-collector section of transistor V3 decreases to several ohms and almost all the voltage of the power source is applied to the winding of relay K1 - the relay is activated and connects one of the garlands to the network with its contacts. When transistor V2 is closed, the power supply circuit of the base of transistor V3 is broken, and it is also closed, no current flows through the relay coil. At this time, the relay releases the anchor and its contacts, switching, connect the second Christmas tree garland to the network. If you want to change the switching time of the garlands, then replace the capacitors C1 and C2 with capacitors of other capacities. Leave the data of resistors R2 and R3 the same, otherwise the operation mode of the transistors in direct current will be violated. A power amplifier, similar to the amplifier on the transistor V3, can also be included in the emitter circuit of the transistor V1 of the multivibrator. In this case, electromagnetic relays (including self-made ones) may not have switching groups of contacts, but normally open or normally closed. The relay contacts of one of the multivibrator arms will periodically close and open the power supply circuit of one garland, and the relay contacts of the other multivibrator arm will periodically close the power supply circuit of the second garland. The electronic switch can be mounted on a board made of getinax or other insulating material and, together with the battery, placed in a plywood box. During operation, the switch consumes a current of no more than 30 mA, so the energy of two 3336L or Krona batteries will be enough for all the New Year holidays. A similar switch can be used for other purposes as well. For example, for the illumination of masks, attractions. Imagine a figurine of the hero of the fairy tale "Puss in Boots" sawn out of plywood and painted. Behind the transparent eyes are light bulbs from a flashlight, switched by an electronic switch, and on the figure itself there is a button. As soon as you press the button, the cat will immediately start winking at you. Isn't it possible to use a switch to electrify some models, such as a lighthouse model? In this case, instead of an electromagnetic relay, a small-sized incandescent bulb, designed for a small glow current, can be included in the collector circuit of the power amplifier transistor, which will imitate beacon flashes. If such a switch is supplemented with a toggle switch, with which two such bulbs can be turned on alternately in the collector circuit of the output transistor, then it can become a direction indicator for your bicycle.
Metronome- this is a kind of clock that allows you to count equal periods of time with an accuracy of fractions of a second by sound signals. Such devices are used, for example, to develop a sense of tact when teaching musical literacy, during the first training in signaling the telegraphic alphabet. You see a diagram of one of these devices in (Fig. 6).
This is also a multivibrator, but asymmetrical. Such a multivibrator uses transistors of different structures: Vl - n - p - n (MP35 - MP38), V2 - p - n - p (MP39 - MP42). This made it possible to reduce the total number of parts of the multivibrator. The principle of its operation remains the same - generation occurs due to positive feedback between the output and input of a two-stage 3H amplifier; the connection is carried out by an electrolytic capacitor C1. The load of the multivibrator is a small-sized dynamic head B1 with a voice coil with a resistance of 4 - 10 ohms, for example 0.1GD - 6, 1GD - 8 (or a telephone capsule), which creates sounds similar to clicks with short-term current pulses. The pulse repetition rate can be adjusted with a variable resistor R1 from about 20 to 300 pulses per minute. Resistor R2 limits the base current of the first transistor when the slider of resistor R1 is in its lowest (according to the circuit) position, corresponding to the highest frequency of the generated oscillations. The metronome can be powered by a single 3336L battery or three 332 cells connected in series. The current consumed by it from the battery does not exceed 10 mA. The variable resistor R1 must have a scale calibrated according to a mechanical metronome. Using it, by simply turning the resistor knob, you can set the desired frequency of the metronome audio signals.
Practical work
As a practical work, I advise you to collect the multivibrator circuits presented in the drawings of the lesson, which will help to comprehend the principle of the multivibrator. Further, I propose to assemble a very interesting and useful in the household "Electronic Nightingale Simulator", based on multivibrators, which can be used as a doorbell. The circuit is very simple, reliable, it works immediately if there are no errors in the installation and the use of serviceable radio elements. I have been using it as a doorbell for 18 years, to this day. It is easy to guess that I collected it - when, like you, I was a novice radio amateur.
is a pulse generator of almost rectangular shape, created in the form of an amplifying element with a positive-feedback circuit. There are two types of multivibrators.
The first type are self-oscillating multivibrators, which do not have a steady state. There are two types: symmetrical - its transistors are the same and the parameters of the symmetrical elements are also the same. As a result of this, the two parts of the oscillation period are equal to each other, and the duty cycle is equal to two. If the parameters of the elements are not equal, then this will already be an asymmetric multivibrator.
The second type is waiting multivibrators, which have a state of stable equilibrium and are often referred to as a single vibrator. The use of a multivibrator in various amateur radio devices is quite common.
Description of the operation of a multivibrator on transistors
We will analyze the principle of operation using the example of the following scheme.
It is easy to see that it practically copies the circuit diagram of a symmetrical flip-flop. The only difference is that the connections between the switching units, both direct and reverse, are carried out by alternating current, and not by direct current. This radically changes the features of the device, since, in comparison with a symmetrical trigger, the multivibrator circuit does not have stable equilibrium states in which it could be for a long time.
Instead, there are two states of quasi-stable equilibrium, due to which the device is in each of them for a strictly defined time. Each such period of time is determined by transient processes occurring in the circuit. The operation of the device consists in a constant change of these states, which is accompanied by the appearance of a voltage at the output, which is very reminiscent of a rectangular shape.
In essence, a symmetrical multivibrator is a two-stage amplifier, and the circuit is built so that the output of the first stage is connected to the input of the second. As a result, after power is applied to the circuit, it necessarily turns out that one of them is open and the other is in the closed state.
Assume that the transistor VT1 is open and is in a state of saturation with the current flowing through the resistor R3. Transistor VT2, as mentioned above, is closed. Now in the circuit there are processes associated with the recharging of capacitors C1 and C2. Initially, the capacitor C2 is absolutely discharged, and after the saturation of VT1, it is gradually charged through the resistor R4.
Since capacitor C2 shunts the collector-emitter junction of transistor VT2 through the emitter junction of transistor VT1, the rate of its charge determines the rate of change of voltage at the collector VT2. After C2 is charged, transistor VT2 closes. The duration of this process (collector voltage rise time) can be calculated using the formula:
t1a = 2.3*R1*C1
Also in the operation of the circuit, the second process occurs, associated with the discharge of the previously charged capacitor C1. Its discharge occurs through the transistor VT1, the resistor R2 and the power supply. As the capacitor discharges at the base of VT1, a positive potential appears, and it begins to open. This process ends after the full discharge of C1. The duration of this process (impulse) is equal to:
t2a = 0.7*R2*C1
After the time t2a, the transistor VT1 will be closed, and the transistor VT2 will be in saturation. After that, the process will be repeated according to a similar scheme, and the duration of the intervals of the following processes can also be calculated using the formulas:
t1b = 2.3*R4*C2 And t2b = 0.7*R3*C2
To determine the oscillation frequency of a multivibrator, the following expression is true:
f = 1/(t2a+t2b)
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