Transistor multivibrator with adjustable frequency. LED flasher or how to assemble a symmetrical multivibrator. Minimum parts required for assembly

This lesson will be devoted to a rather important and popular topic: multivibrators and their applications. If I just tried to list where and how self-oscillating symmetrical and asymmetrical multivibrators are used, it would require a decent number of pages of the book. There is, perhaps, no branch of radio engineering, electronics, automation, pulse or computer technology where such generators are not used. This lesson will provide theoretical information about these devices, and at the end, I will give several examples of their practical use in relation to your creativity

Self-oscillating multivibrator

Multivibrators are electronic devices that generate electrical oscillations close to rectangular in shape. The spectrum of oscillations generated by a multivibrator contains many harmonics - also electrical oscillations, but multiples of the oscillations of the fundamental frequency, which is reflected in its name: “multi - many”, “vibration - oscillation”.

Let's consider the circuit shown in (Fig. 1, a). Do you recognize? Yes, this is a circuit of a two-stage transistor amplifier 3H with output to headphones. What happens if the output of such an amplifier is connected to its input, as shown by the dashed line in the diagram? Positive feedback arises between them and the amplifier will self-excite and become a generator of audio frequency oscillations, and in telephones we will hear a low-pitched sound. This phenomenon is vigorously fought in receivers and amplifiers, but for automatically operating devices it turns out to be useful.

Rice. 1 A two-stage amplifier covered by positive feedback becomes a multivibrator

Now look at (Fig. 1, b). On it you see a diagram of the same amplifier covered positive feedback , as in (Fig. 1, a), only its outline is slightly changed. This is exactly how circuits of self-oscillating, i.e., self-exciting multivibrators are usually drawn. Experience is, perhaps, the best method of knowing the essence of the action of one or another electronic device. You have been convinced of this more than once. And now, in order to better understand the operation of this universal device - an automatic machine, I propose to conduct an experiment with it. Schematic diagram You can see a self-oscillating multivibrator with all the data on 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 low breakdown voltage of the emitter junction. Electrolytic capacitors C1 and C2 - type K50 - 6, K50 - 3 or their imported analogues for a rated voltage of 10 - 12 V. The resistor resistances may differ from those indicated in the diagram by up to 50%. It is only important that the values ​​of the load resistors Rl, R4 and the base resistors R2, R3 be as similar as possible. For power use a Krona battery or power supply. Connect a milliammeter (PA) to the collector circuit of any of the transistors for a current of 10 - 15 mA, and connect a high-resistance DC voltmeter (PU) to the emitter-collector section of the same transistor for a voltage of up to 10 V. Having checked the installation and especially carefully the polarity of the electrolytic switching capacitors, connect a power source to the multivibrator. What do the measuring instruments show? Milliammeter - the current of the transistor collector circuit sharply increases to 8 - 10 mA, and then also sharply decreases almost to zero. The voltmeter, on the contrary, either decreases to almost zero or increases to the voltage of the power source, the collector voltage. What do these measurements indicate? The fact that the transistor of this arm of the multivibrator operates in switching mode. Largest collector current and at the same time, the lowest voltage on the collector corresponds to the open state, and the lowest current and the highest 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 one is closed. It is easy to verify this by connecting the same milliammeter to 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 scale marks. Now, using a clock with a second hand, count how many times per minute the transistors switch from open to closed. About 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. While continuing to monitor the milliammeter needle, try to depict these fluctuations graphically. On the horizontal ordinate axis, plot, on a certain scale, the time intervals when the transistor is in the open and closed states, and on the vertical axis, plot the collector current corresponding to these states. You will get approximately the same graph as the one shown in Fig. 2, b.

Rice. 2 Diagram of a symmetrical multivibrator (a) and the current pulses generated by it (b, c, d).

This means that we can assume that The multivibrator generates rectangular electrical oscillations. In the multivibrator signal, regardless of which output it is taken from, it is possible to distinguish current pulses and pauses between them. The time interval from the moment of the appearance of one current (or voltage) pulse until the moment of the appearance of the next pulse of the same polarity 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 assembled is symmetric. Replace capacitors C1 and C2 with other capacitors with a capacity of 10 - 15 µF. The multivibrator remained symmetrical, but the frequency of the oscillations it generated increased by 3 - 4 times - to 60 - 80 per minute or, which is the same, to approximately 1 Hz. The arrows of measuring instruments barely have time to follow changes in currents and voltages in transistor circuits. And if capacitors C1 and C2 are replaced with paper capacitances of 0.01 - 0.05 μF? How will the arrows of measuring instruments behave now? Having deviated from the zero marks of the scales, they stand still. Maybe generation was disrupted? No! It’s just that the oscillation frequency of the multivibrator has increased to several hundred hertz. These are vibrations in the audio frequency range that DC devices can no longer detect. They can be detected using a frequency meter or headphones connected through a capacitor with a capacity of 0.01 - 0.05 μF to any of the multivibrator outputs or by connecting them directly to the collector circuit of any of the transistors instead of a load resistor. You will hear a low pitch sound on phones. What is the operating 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 transistor V2 and resistor R1; C2 - through the emitter junction of transistor V1 and resistor R4. These capacitor charging circuits, being voltage dividers of the power source, create increasingly negative voltages at the bases of the transistors (relative to the emitters), tending to open the transistors more and more. Turning on a transistor causes the negative voltage at its collector to decrease, which causes the negative voltage at the base of the other transistor to decrease, turning it off. This process occurs in both transistors at once, but only one of them closes, on the basis of which there is a higher positive voltage, for example, due to the difference in current transfer coefficients h21e ratings of resistors and capacitors. The second transistor remains open. But these states of transistors are unstable, because electrical processes in their circuits continue. Let's assume that some time after turning on the power, transistor V2 turned out to be closed, and transistor V1 turned out to be open. From this moment, 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 resistor R2. As 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 transistor V2 becomes close to zero, a current appears in the collector circuit of this now opening transistor, which acts through capacitor C2 on the base of transistor V1 and lowers the negative voltage on it. As a result, the current flowing through transistor V1 begins to decrease, and through transistor V2, on the contrary, increases. This causes transistor V1 to turn off and transistor V2 to open. Now capacitor C2 will begin to discharge, but through the open transistor V2 and resistor R3, which ultimately leads to the opening of the first and closing of the second transistors, etc. The transistors interact all the time, causing the multivibrator to generate 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, which you can verify right now. Try, for example, replacing the basic resistors R2 and R3 with resistors of high resistance. The oscillation frequency of the multivibrator will decrease. Conversely, if their resistance is lower, the oscillation frequency will increase. Another experiment: disconnect the upper (according to the diagram) terminals of 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 as 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 kilo-ohms, C is the capacitance of the coupling capacitors in microfarads. Using this simplified formula, calculate which frequency oscillations your multivibrator generated. Let's return to the initial data of 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, connect a milliammeter to the collector circuit of transistor V2, follow its arrow, and 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 Th pulses will be approximately the same number of times less than the pauses between Th pulses as the capacitance of capacitor C2 has decreased compared to its previous capacity. Now connect the same (or similar) milliammeter to the collector circuit of transistor V1. What does it show meter? 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 have broken the symmetry of the arms of the multivibrator - it has become asymmetrical . Therefore, the vibrations generated by it became asymmetrical : in the collector circuit of transistor V1, the current appears in relatively long pulses, in the collector circuit of transistor V2 - in short ones. Short voltage pulses can be removed from Output 1 of such a multivibrator, and long voltage pulses can be removed from Output 2. Temporarily swap capacitors C1 and C2. Now short voltage pulses will be at Output 1, and long ones at Output 2. Count (on a clock with a second hand) how many electrical pulses per minute this version of the multivibrator generates. About 80. Increase the capacity of capacitor C1 by connecting a second electrolytic capacitor with a capacity of 20 - 30 μF in parallel to it. The pulse repetition rate will decrease. What 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 resistor R2: with a decrease in the resistance of this resistor (but not less than 3 - 5 kOhm, otherwise 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, it decreases. Check it out empirically - is this true? Select a resistor of such a value that the number of pulses per 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, electronic mechanism a clock counting down the 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. To turn the self-oscillating multivibrator, which you have already carried out experiments with in this lesson (according to the diagram in Fig. 2a), into a waiting multivibrator, you need to do the following: remove capacitor C2, and instead connect a resistor between the collector of transistor V2 and the base of transistor V1 (in Fig. 3 - R3) with a resistance of 10 - 15 kOhm; between the base of 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 it is connected to the base (via R5) positive pole element; 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 transistor V1 of such a multivibrator is closed, transistor V2 is open. Check - is this true? The voltage on the collector of the closed transistor should be close to the voltage of the power source, and on the collector of the open transistor should not exceed 0.2 - 0.3 V. Then, turn on a milliammeter with a current of 10 - 15 mA into the collector circuit of transistor V1 and, observing its arrow , connect 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 don’t confuse it: the negative pole of this external electrical signal must be connected to the Uin contact. In this case, the milliammeter needle should immediately deviate to the value of the highest current of the transistor collector circuit, freeze for a while, and then return to starting position to wait for the next signal. Repeat this experiment several times. With each signal, the milliammeter will show the collector current of transistor V1 instantly increasing to 8 - 10 mA and after some time also instantly decreasing to almost zero. These are single current pulses generated by a multivibrator. And if you keep the GB1 battery connected to the Uin terminal for a longer time. The same thing will happen as in previous experiments - only one pulse will appear at the output of the multivibrator. Try it!

Rice. 3 Experienced waiting multivibrator.

And one more experiment: touch the base terminal of 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 connecting the milliammeter to the collector circuit of 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 open transistor current. 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 that opens it is supplied to the base of transistor V2 through resistor R2. Transistor V1 is reliably closed by the positive voltage of element G1 at its base. This state of transistors is very stable. They can remain in this state for any amount of time. But at the base of transistor V1 a voltage pulse of negative polarity appeared. From this moment on, the transistors go into an unstable state. Under the influence of the input signal, transistor V1 opens, and the changing voltage on its collector through capacitor C1 closes transistor V2. The transistors remain in this state until capacitor C1 is discharged (through resistor R2 and open transistor V1, the resistance of which is low at this time). As soon as the capacitor is discharged, transistor V2 will immediately open, and transistor V1 will close. From this moment on, the multivibrator is again in its original, stable standby mode. Thus, a waiting multivibrator has one stable and one unstable state . During an unstable state it generates one square pulse current (voltage), the duration of which depends on the capacitance of capacitor C1. The larger the capacitance of this capacitor, the longer the pulse duration. So, for example, with a capacitor capacity of 50 µF, the multivibrator generates a current pulse lasting about 1.5 s, and with a capacitor with a capacity of 150 µF - three times more. Through additional capacitors - positive impulses voltages can be removed from output 1, and negative ones from output 2. Is it only with a negative voltage pulse applied to the base of transistor V1 that the multivibrator can be brought out of standby mode? No, not only that. This can also be done by applying a voltage pulse of positive polarity, but to the base of transistor V2. So, all you have to do is experimentally check how the capacitance of capacitor C1 affects the duration of the pulses and the ability to control the standby multivibrator with positive voltage pulses. How can you practically use a standby multivibrator? Differently. For example, to convert 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 an apartment bell, replacing a regular electric one. It can be assembled according to the diagram shown in (Fig. 4). Transistors V1 and V2 operate in a symmetrical multivibrator, generating oscillations with a frequency of about 1000 Hz, and transistor V3 operates in a power amplifier for these oscillations. The amplified vibrations are converted by the dynamic head B1 into sound vibrations. If you use the speakerphone to make a call, turning on primary winding its transition transformer into the collector circuit of transistor V3, its case will house all the bell electronics mounted on the board. The battery will also be located there.

Rice. 4. Electronic call based on a multivibrator.

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 ring operation. Set the desired sound tone by replacing capacitors C1 and C2 with capacitors of other capacities. A multivibrator assembled according to the same circuit can be used to study and train in listening to the telegraph alphabet - Morse code. In this case, you only need to replace the button with a telegraph key.

Electronic switch. This device, the diagram of which is shown in (Fig. 5), can be used to switch two Christmas tree garlands powered from the network AC. The electronic switch itself can be powered from two 3336L batteries connected in series, or from a rectifier that would output constant voltage 9 - 12 V.

Rice. 5. Electronic switch based on a multivibrator.

The switch circuit is very similar to the electronic bell circuit. But the capacitance of capacitors C1 and C2 of the switch is many times more capacities 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 electromagnetic relay K1. The relay has one pair of contact plates that operate for switching. Suitable, for example, is a RES-10 relay (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. When the power is turned on, transistors V1 and V2 of the multivibrator alternately open and close, generating square wave signals. When transistor V2 is turned on, a negative supply voltage is applied through resistor R4 and this transistor to the base of transistor V3, driving it into saturation. In this case, the resistance of the emitter-collector section of transistor V3 decreases to several ohms and almost the entire voltage of the power source is applied to the winding of relay K1 - the relay is triggered and with its contacts connects one of the garlands to the network. When transistor V2 is closed, the power supply circuit to the base of transistor V3 is broken, and it is also closed; no current flows through the relay winding. 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 capacitors C1 and C2 with capacitors of other capacities. Leave the data of resistors R2 and R3 the same, otherwise the operating mode of the transistors will be disrupted DC. A power amplifier similar to the amplifier on transistor V3 can also be included in the emitter circuit of transistor V1 of the multivibrator. In this case, electromagnetic relays (including homemade ones) may not have switching groups of contacts, but normally open or normally closed. The relay contacts of one of the arms of the multivibrator will periodically close and open the power circuit of one garland, and the relay contacts of the other arm of the multivibrator will periodically open the power 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 is quite enough for the entire New Year holidays. A similar switch can be used for other purposes. For example, for illuminating masks and attractions. Imagine a figurine of the hero of the fairy tale “Puss in Boots” cut out of plywood and painted. Behind the transparent eyes are light bulbs from a flashlight, switchable 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 the lighthouse model? In this case, in the collector circuit of the power amplifier transistor, instead of an electromagnetic relay, you can include a small-sized incandescent light bulb, designed for a small filament current, which will imitate the flashes of a beacon. If such a switch is supplemented with a toggle switch, with the help of which two such bulbs can be switched 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 using sound signals with an accuracy of fractions of a second. Such devices are used, for example, to develop a sense of tact when teaching musical literacy, during the first training in transmitting signals using the telegraph alphabet. You can see a diagram of one of these devices in (Fig. 6).

Rice. 6. Metronome based on a multivibrator.

This is also a multivibrator, but asymmetrical. This 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 exit and entrance two-stage amplifier 3H; communication is in progress 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 during short-term current pulses. The pulse repetition rate can be adjusted by variable resistor R1 from approximately 20 to 300 pulses per minute. Resistor R2 limits the base current of the first transistor when the slider of resistor R1 is in the lowest (according to the circuit) position, corresponding to the highest frequency of generated oscillations. The metronome can be powered by one 3336L battery or three 332 cells connected in series. The current it consumes from the battery does not exceed 10 mA. 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 sound signals metronome.

Practical work

For practical work, I advise you to assemble the multivibrator circuits presented in the lesson pictures, which will help you understand the principle of operation of the multivibrator. Next, I propose to assemble a very interesting and useful “Electronic Nightingale Simulator” based on multivibrators, which can be used as a doorbell. The circuit is very simple, reliable, and works immediately if there are no errors in installation and the use of serviceable radio elements. I have been using it as a doorbell for 18 years, to this day. It’s not hard to guess that I collected it when, like you, I was a beginner radio amateur.

Electronic call based on multivibrators

A multivibrator is the simplest pulse generator that operates in the self-oscillation mode, that is, when voltage is applied to the circuit, it begins to generate pulses.

The simplest diagram is shown in the figure below:

Moreover, the capacitances of capacitors C1, C2 are always selected as identical as possible, and the nominal value of the base resistances R2, R3 should be higher than the collector ones. This is an important condition for proper operation MV

How does a transistor-based multivibrator work? So: when the power is turned on, capacitors C1 and C2 begin to charge.

The first capacitor in the chain R1-C1-transition BE of the second body.

The second capacitance will be charged through the circuit R4 - C2 - transition BE of the first transistor - housing.

Since there is a base current on the transistors, they almost open. But since there are no two identical transistors, one of them will open a little earlier than its colleague.

Let's assume that our first transistor opens earlier. When it opens, it will discharge capacity C1. Moreover, it will discharge in reverse polarity, closing the second transistor. But the first one is in the open state only for the moment until capacitor C2 is charged to the supply voltage level. At the end of the charging process C2, Q1 is locked.

But by this time C1 is almost discharged. This means that a current will flow through it, opening the second transistor, which will discharge capacitor C2 and will remain open until the first capacitor is recharged. And so on from cycle to cycle until we turn off the power from the circuit.

As is easy to see, the switching time here is determined by the capacitance rating of the capacitors. By the way, the resistance of the basic resistances R1, R3 also contributes a certain factor here.

Let's return to the original state, when the first transistor is open. At this moment, capacitance C1 will not only have time to discharge, but will also begin to charge in reverse polarity along the circuit R2-C1-collector-emitter of open Q1.

But the resistance of R2 is quite large and C1 does not have time to charge to the level of the power source, but when Q1 is locked, it will discharge through the base chain of Q2, helping it to open faster. The same resistance also increases the charging time of the first capacitor C1. But the collector resistances R1, R4 are a load and do not have much effect on the frequency of pulse generation.

As a practical introduction, I propose to assemble, in the same article the design with three transistors is also discussed.

Let's sort out the work single-ended multivibrator on two transistors using a simple circuit as an example homemade amateur radio making the sound of a bouncing metal ball. The circuit works as follows: as capacitance C1 discharges, the volume of the blows decreases. The total duration of the sound depends on the value of C1, and capacitor C2 sets the duration of pauses. Transistors can be absolutely any p-n-p type.

There are two types of domestic micro multivibrators - self-oscillating (GG) and standby (AG).

Self-oscillating ones generate a periodic sequence of rectangular pulses. Their duration and repetition period are set by the parameters of external elements of resistance and capacitance or the level of control voltage.

Domestic microcircuits of self-oscillating MVs, for example, are 530GG1, K531GG1, KM555GG2 You will find more detailed information on them and many others in, for example, Yakubovsky S.V. Digital and analog integrated circuits or ICs and their foreign analogues. Directory in 12 volumes edited by Nefedov

For waiting MVs, the duration of the generated pulse is also set by the characteristics of the attached radio components, and the pulse repetition period is set by the repetition period of the trigger pulses arriving at a separate input.

Examples: K155AG1 contains one standby multivibrator that generates single rectangular pulses with good duration stability; 133AG3, K155AG3, 533AG3, KM555AG3, KR1533AG3 contains two standby MVs that generate single rectangular voltage pulses with good stability; 533AG4, KM555AG4 two waiting MVs that form single rectangular voltage pulses.

Very often in amateur radio practice they prefer not to specialized chips, but assemble it using logical elements.

The simplest multivibrator circuit using NAND gates is shown in the figure below. It has two states: in one state DD1.1 is locked and DD1.2 is open, in the other - everything is the opposite.

For example, if DD1.1 is closed, DD1.2 is open, then capacitance C2 is charged by the output current of DD1.1 passing through resistance R2. The voltage at the DD1.2 input is positive. It keeps DD1.2 open. As capacitor C2 charges, the charging current decreases and the voltage across R2 drops. At the moment the threshold level is reached, DD1.2 begins to close and its output potential increases. The increase in this voltage is transmitted through C1 to output DD1.1, the latter opens, and the reverse process develops, ending with complete locking of DD1.2 and unlocking of DD1.1 - the transition of the device to the second unstable state. Now C1 will be charged through R1 and the output resistance of the microcircuit component DD1.2, and C2 through DD1.1. Thus, we observe a typical self-oscillatory process.

Another one of simple circuits, which can be assembled using logic elements, is a rectangular pulse generator. Moreover, such a generator will operate in self-generation mode, similar to a transistor one. The figure below shows a generator built on one logical digital domestic microassembly K155LA3

multivibrator circuit on K155LA3

A practical example of such an implementation can be found on the electronics page in the design of the calling device.

A practical example of the implementation of the operation of a waiting MV on a trigger in the design of an optical lighting switch using IR rays is considered.

The multivibrator circuit shown in Figure 1 is a cascade connection transistor amplifiers where the output of the first stage is connected to the input of the second through a circuit containing a capacitor and the output of the second stage is connected to the input of the first through a circuit containing a capacitor. Multivibrator amplifiers are transistor switches that can be in two states. The multivibrator circuit in Figure 1 differs from the trigger circuit discussed in the article "". Because it has reactive elements in the feedback circuits, therefore the circuit can generate non-sinusoidal oscillations. You can find the resistance of resistors R1 and R4 from relations 1 and 2:

Where I KBO = 0.5 μA is the maximum reverse collector current of the KT315a transistor,

Iкmax=0.1A - maximum current collector of transistor KT315A, Up=3V - supply voltage. Let's choose R1=R4=100Ohm. Capacitors C1 and C2 are selected depending on the required oscillation frequency of the multivibrator.

Figure 1 - Multivibrator based on KT315A transistors

You can relieve the voltage between points 2 and 3 or between points 2 and 1. The graphs below show how approximately the voltage will change between points 2 and 3 and between points 2 and 1.

T - oscillation period, t1 - time constant of the left arm of the multivibrator, t2 - time constant of the right arm of the multivibrator can be calculated using the formulas:

You can set the frequency and duty cycle of the pulses generated by the multivibrator by changing the resistance of trimming resistors R2 and R3. You can also replace capacitors C1 and C2 with variable (or trimmer) capacitors and, by changing their capacitance, set the frequency and duty cycle of the pulses generated by the multivibrator, this method is even more preferable, so if there are trimmer (or better variable) capacitors, then it is better to use them, and in place set variable resistors R2 and R3 to constant ones. The photo below shows the assembled multivibrator:

In order to make sure that the assembled multivibrator works, a piezodynamic speaker was connected to it (between points 2 and 3). After applying power to the circuit, the piezo speaker began to crackle. Changes in the resistance of the tuning resistors led either to an increase in the frequency of the sound emitted by the piezodynamics, or to its decrease, or to the fact that the multivibrator stopped generating.
A program for calculating the frequency, period and time constants, duty cycle of pulses taken from a multivibrator:

If the program does not work, then copy its html code into notepad and save it in html format.
If you are using the Internet Explorer browser and it is blocking the program, you must allow the blocked content.

A multivibrator (from the Latin I oscillate a lot) is a nonlinear device that converts a constant supply voltage into the energy of almost rectangular pulses. The multivibrator is based on an amplifier with positive feedback.

There are self-oscillating and standby multivibrators. Let's consider the first type.

In Fig. Figure 1 shows a generalized circuit of an amplifier with feedback.

The circuit contains an amplifier with a complex gain coefficient k=Ke-ik, an OOS circuit with a transmission coefficient m, and a PIC circuit with a complex transmission coefficient B=e-i. From the theory of generators it is known that for oscillations to occur 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 lecture 1). That. To generate pulses, it is necessary to fulfill the condition Bk>1 not at one frequency, but over a wide frequency band. Moreover, the shorter the pulse and with shorter edges the signal is required to be obtained, for a wider frequency band it is necessary to fulfill the condition Bk>1. The above condition breaks down into two:

amplitude balance condition - the modulus of the overall generator transmission coefficient must exceed 1 V wide range frequencies - K>1;

phase balance condition - the total phase shift of oscillations in a closed circuit of the generator in the same frequency range must be a multiple of 2 - k + = 2n.

Qualitatively, the process of sudden increase in voltage occurs as follows. Suppose that at some point in time, as a result of fluctuations, the voltage at the generator input increases by a small value u. As a result of fulfilling both generation conditions, a voltage increment will appear at the output of the device: uout = Vkuin > 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. An avalanche-like process of voltage growth occurs over a wide frequency range.

Construction task practical scheme pulse generator is reduced to applying to the input broadband amplifier parts of the output signal with phase difference =2. Since one resistive amplifier shifts the phase of the input voltage by 1800, using two series-connected amplifiers can satisfy the phase balance condition. 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 amplifier stage. 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.

We will qualitatively consider the operation of the multivibrator using voltage time diagrams (diagrams) shown in Fig. 3.

Let the multivibrator switch at time t=t1. Transistor VT1 is in saturation mode, and VT2 is in cutoff mode. From this moment, the processes of recharging capacitors C1 and C2 begin. Until moment t1, capacitor C2 was completely discharged, and C1 was charged to the supply voltage Ep (the polarity of the charged capacitors is indicated in Fig. 2). After unlocking VT1, it begins charging from the source Ep through resistor Rk2 and the base of the unlocked transistor VT1. The capacitor is charged almost to the supply voltage Ep with a charge constant

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 = 0.9 Up, it is easy to obtain the duration

t2-t1= С2Rк2ln102,3С2Rк2

Simultaneously with charging C2 (starting from moment t1), capacitor C1 is recharged. Its negative voltage applied to the base of VT2 maintains the off state of this transistor. Capacitor C1 is recharged through the circuit: Ep, resistor Rb2, C1, E-K open transistor VT1. case with time constant

razr1 = C1Rb2

Since Rb >>Rk, then 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 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 incremental voltage is transmitted through C1 to the base of VT2, which entails an additional opening of VT2. The transistors switch to 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 turning over is much less than all other transient processes and can be considered equal to zero.

From moment t3, the processes in the multivibrator will proceed similarly to those described; you just need to swap the indices of the circuit elements.

Thus, the duration of the pulse front is determined by the charging processes of the coupling capacitor and is numerically equal to:

The duration of the multivibrator being in a quasi-stable state (pulse and pause duration) 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 = u + n =1.4CRb

When comparing the pulse and front durations, it is necessary to take into account that Rb/Rk = h21e/s (h21e for modern transistors is 100, and s2). Consequently, 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 limited: the limits of change in Rb are limited on the larger side by the need to maintain an open transistor, on the smaller side by 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. From the figure it can be seen 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 source, but to an additional voltage source ECM (see Fig. 4). Then the capacitor tends to recharge not to Ep, but to Ecm, and the slope of the exponential will change with a change in Ecm.

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, while 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. A self-oscillating multivibrator based on an op-amp is shown in Fig. 6.

The op-amp 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. To be specific, let's take the positive. Due to the PIC, the maximum possible voltage will be established 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 equal to zero. Starting from moment t0, capacitor C will be charged with a time constant =RC. Until time t1 Ud = U+ - U- >0, and the op-amp output maintains a positive Uoutm. At t=t1, when Ud = U+ - U- = 0, the output voltage of the amplifier will change its polarity to - Uout m. After moment t1, capacitance C is recharged, tending to the level - Uout m. Until 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:

Т=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 asymmetrical multivibrator, the resistor in the OOS should be replaced with a circuit, as shown in Fig. 7. Different durations of positive and negative pulses are ensured by different time constants for recharging the containers:

R"C, - = R"C.

An op-amp multivibrator can be easily converted into a one-shot or 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, because Uout = - Uout m, then the diode is open and the voltage at the inverting input is approximately 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 consisting of diode VD2, C1 and R3 should be added to the circuit. Diode VD2 is maintained in a closed state and can only be opened by a positive input pulse arriving at the input at time t0. When the diode opens, the sign changes and the circuit goes into a state with a positive voltage at the output. Uout = Uout m. After this, capacitor C1 begins to charge with a time constant =RC. At time t1, the input voltages are compared. U- = U+ = Uout m R2/(R1+R2) and =0. At the next moment, the differential signal becomes negative and the circuit returns to a stable state. The diagrams are shown in Fig. 9.

Circuits of waiting multivibrators using discrete and logical elements are used.

The circuit of the multivibrator in question is similar to that discussed earlier.

This lesson will be devoted to a rather important and popular topic: multivibrators and their applications. If I just tried to list where and how self-oscillating symmetrical and asymmetrical multivibrators are used, it would require a decent number of pages of the book. There is, perhaps, no branch of radio engineering, electronics, automation, pulse or computer technology where such generators are not used. This lesson will provide theoretical information about these devices, and at the end, I will give several examples of their practical use in relation to your creativity.

Self-oscillating multivibrator

Multivibrators are electronic devices that generate electrical oscillations close to rectangular in shape. The spectrum of oscillations generated by a multivibrator contains many harmonics - also electrical oscillations, but multiples of the oscillations of the fundamental frequency, which is reflected in its name: “multi - many”, “vibration - oscillation”.

Let's consider the circuit shown in (Fig. 1, a). Do you recognize? Yes, this is a circuit of a two-stage transistor amplifier 3H with output to headphones. What happens if the output of such an amplifier is connected to its input, as shown by the dashed line in the diagram? Positive feedback arises between them and the amplifier will self-excite and become a generator of audio frequency oscillations, and in telephones we will hear a low-pitched sound. This phenomenon is vigorously fought in receivers and amplifiers, but for automatically operating devices it turns out to be useful.

Now look at (Fig. 1, b). On it you see a diagram of the same amplifier covered positive feedback , as in (Fig. 1, a), only its outline is slightly changed. This is exactly how circuits of self-oscillating, i.e., self-exciting multivibrators are usually drawn. Experience is perhaps the best method of understanding the essence of the action of a particular electronic device. You have been convinced of this more than once. 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 the schematic diagram of a self-oscillating multivibrator with all the data on 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 low breakdown voltage of the emitter junction. Electrolytic capacitors C1 and C2 - type K50 - 6, K50 - 3 or their imported analogues for a rated voltage of 10 - 12 V. The resistor resistances may differ from those indicated in the diagram by up to 50%. It is only important that the values ​​of the load resistors Rl, R4 and the base resistors R2, R3 be as similar as possible. For power use a Krona battery or power supply. Connect a milliammeter (PA) to the collector circuit of any of the transistors for a current of 10 - 15 mA, and connect a high-resistance DC voltmeter (PU) to the emitter-collector section of the same transistor for a voltage of up to 10 V. Having checked the installation and especially carefully the polarity of the electrolytic switching capacitors, connect a power source to the multivibrator. What do the measuring instruments show? Milliammeter - the current of the transistor collector circuit sharply increases to 8 - 10 mA, and then also sharply decreases almost to zero. The voltmeter, on the contrary, either decreases to almost zero or increases to the voltage of the power source, the collector voltage. What do these measurements indicate? The fact that the transistor of this arm of the multivibrator operates in switching mode. The highest collector current and at the same time the lowest voltage on the collector correspond to the open state, and the lowest current and the highest 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 one is closed. It is easy to verify this by connecting the same milliammeter to 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 scale marks. Now, using a clock with a second hand, count how many times per minute the transistors switch from open to closed. About 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. While continuing to monitor the milliammeter needle, try to depict these fluctuations graphically. On the horizontal ordinate axis, plot, on a certain scale, the time intervals when the transistor is in the open and closed states, and on the vertical axis, plot the collector current corresponding to these states. You will get approximately the same graph as the one shown in Fig. 2, b.

This means that we can assume that The multivibrator generates rectangular electrical oscillations. In the multivibrator signal, regardless of which output it is taken from, it is possible to distinguish current pulses and pauses between them. The time interval from the moment of the appearance of one current (or voltage) pulse until the moment of the appearance of the next pulse of the same polarity 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 assembled is symmetric. Replace capacitors C1 and C2 with other capacitors with a capacity of 10 - 15 µF. The multivibrator remained symmetrical, but the frequency of the oscillations it generated increased by 3 - 4 times - to 60 - 80 per minute or, which is the same, to approximately 1 Hz. The arrows of measuring instruments barely have time to follow changes in currents and voltages in transistor circuits. And if capacitors C1 and C2 are replaced with paper capacitances of 0.01 - 0.05 μF? How will the arrows of measuring instruments behave now? Having deviated from the zero marks of the scales, they stand still. Maybe generation was disrupted? No! It’s just that the oscillation frequency of the multivibrator has increased to several hundred hertz. These are vibrations in the audio frequency range that DC devices can no longer detect. They can be detected using a frequency meter or headphones connected through a capacitor with a capacity of 0.01 - 0.05 μF to any of the multivibrator outputs or by connecting them directly to the collector circuit of any of the transistors instead of a load resistor. You will hear a low pitch sound on phones. What is the operating 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 transistor V2 and resistor R1; C2 - through the emitter junction of transistor V1 and resistor R4. These capacitor charging circuits, being voltage dividers of the power source, create increasingly negative voltages at the bases of the transistors (relative to the emitters), tending to open the transistors more and more. Turning on a transistor causes the negative voltage at its collector to decrease, which causes the negative voltage at the base of the other transistor to decrease, turning it off. This process occurs in both transistors at once, but only one of them closes, on the basis of which there is a higher positive voltage, for example, due to the difference in current transfer coefficients h21e ratings of resistors and capacitors. The second transistor remains open. But these states of transistors are unstable, because electrical processes in their circuits continue. Let's assume that some time after turning on the power, transistor V2 turned out to be closed, and transistor V1 turned out to be open. From this moment, 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 resistor R2. As 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 transistor V2 becomes close to zero, a current appears in the collector circuit of this now opening transistor, which acts through capacitor C2 on the base of transistor V1 and lowers the negative voltage on it. As a result, the current flowing through transistor V1 begins to decrease, and through transistor V2, on the contrary, increases. This causes transistor V1 to turn off and transistor V2 to open. Now capacitor C2 will begin to discharge, but through the open transistor V2 and resistor R3, which ultimately leads to the opening of the first and closing of the second transistors, etc. The transistors interact all the time, causing the multivibrator to generate 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, which you can verify right now. Try, for example, replacing the basic resistors R2 and R3 with resistors of high resistance. The oscillation frequency of the multivibrator will decrease. Conversely, if their resistance is lower, the oscillation frequency will increase. Another experiment: disconnect the upper (according to the diagram) terminals of 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 as 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 kilo-ohms, C is the capacitance of the coupling capacitors in microfarads. Using this simplified formula, calculate which frequency oscillations your multivibrator generated. Let's return to the initial data of 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, connect a milliammeter to the collector circuit of transistor V2, follow its arrow, and 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 Th pulses will be approximately the same number of times less than the pauses between Th pulses as the capacitance of capacitor C2 has decreased compared to its previous capacity. Now connect the same (or similar) milliammeter to the collector circuit of transistor V1. What does the measuring device 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 have broken the symmetry of the arms of the multivibrator - it has become asymmetrical . Therefore, the vibrations generated by it became asymmetrical : in the collector circuit of transistor V1, the current appears in relatively long pulses, in the collector circuit of transistor V2 - in short ones. Short voltage pulses can be removed from Output 1 of such a multivibrator, and long voltage pulses can be removed from Output 2. Temporarily swap capacitors C1 and C2. Now short voltage pulses will be at Output 1, and long ones at Output 2. Count (on a clock with a second hand) how many electrical pulses per minute this version of the multivibrator generates. About 80. Increase the capacity of capacitor C1 by connecting a second electrolytic capacitor with a capacity of 20 - 30 μF in parallel to it. The pulse repetition rate will decrease. What 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 resistor R2: with a decrease in the resistance of this resistor (but not less than 3 - 5 kOhm, otherwise 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, it decreases. Check it out empirically - is this true? Select a resistor of such a value that the number of pulses per minute is exactly 60. The milliammeter needle will oscillate at a frequency of 1 Hz. The multivibrator in this case will become like an electronic clock mechanism that counts the 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. To turn the self-oscillating multivibrator, which you have already carried out experiments with in this lesson (according to the diagram in Fig. 2a), into a waiting multivibrator, you need to do the following: remove capacitor C2, and instead connect a resistor between the collector of transistor V2 and the base of transistor V1 (in Fig. 3 - R3) with a resistance of 10 - 15 kOhm; between the base of transistor V1 and the grounded conductor, connect a series-connected element 332 (G1 or other 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 (via 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 transistor V1 of such a multivibrator is closed, transistor V2 is open. Check - is this true? The voltage on the collector of the closed transistor should be close to the voltage of the power source, and on the collector of the open transistor should not exceed 0.2 - 0.3 V. Then, turn on a milliammeter with a current of 10 - 15 mA into the collector circuit of transistor V1 and, observing its arrow , connect 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 don’t confuse it: the negative pole of this external electrical signal must be connected to the Uin contact. In this case, the milliammeter needle should immediately deviate to the value of the highest current in the collector circuit of the transistor, freeze for a while, and then return to its original position to wait for the next signal. Repeat this experiment several times. With each signal, the milliammeter will show the collector current of transistor V1 instantly increasing to 8 - 10 mA and after some time also instantly decreasing to almost zero. These are single current pulses generated by a multivibrator. And if you keep the GB1 battery connected to the Uin terminal for a longer time. The same thing will happen as in previous experiments - only one pulse will appear at the output of the multivibrator. Try it!

And one more experiment: touch the base terminal of 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 connecting the milliammeter to the collector circuit of 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 open transistor current. 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 that opens it is supplied to the base of transistor V2 through resistor R2. Transistor V1 is reliably closed by the positive voltage of element G1 at its base. This state of transistors is very stable. They can remain in this state for any amount of time. But at the base of transistor V1 a voltage pulse of negative polarity appeared. From this moment on, the transistors go into an unstable state. Under the influence of the input signal, transistor V1 opens, and the changing voltage on its collector through capacitor C1 closes transistor V2. The transistors remain in this state until capacitor C1 is discharged (through resistor R2 and open transistor V1, the resistance of which is low at this time). As soon as the capacitor is discharged, transistor V2 will immediately open, and transistor V1 will close. From this moment on, the multivibrator is again in its original, stable standby mode. Thus, a waiting multivibrator has one stable and one unstable state . During an unstable state it generates one square pulse current (voltage), the duration of which depends on the capacitance of capacitor C1. The larger the capacitance of this capacitor, the longer the pulse duration. So, for example, with a capacitor capacity of 50 µF, the multivibrator generates a current pulse lasting 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 removed from output 1, and negative ones from output 2. Is it only with a negative voltage pulse applied to the base of transistor V1 that the multivibrator can be brought out of standby mode? No, not only that. This can also be done by applying a voltage pulse of positive polarity, but to the base of transistor V2. So, all you have to do is experimentally check how the capacitance of capacitor C1 affects the duration of the pulses and the ability to control the standby multivibrator with positive voltage pulses. How can you practically use a standby multivibrator? Differently. For example, to convert 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 an apartment bell, replacing a regular electric one. It can be assembled according to the diagram shown in (Fig. 4). Transistors V1 and V2 operate in a symmetrical multivibrator, generating oscillations with a frequency of about 1000 Hz, and transistor V3 operates in a power amplifier for these oscillations. The amplified vibrations are converted by the dynamic head B1 into sound vibrations. If you use a subscriber loudspeaker to make a call, connecting the primary winding of its transition transformer to the collector circuit of transistor V3, its case will house all the bell electronics mounted on the board. 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 ring operation. Set the desired sound tone by replacing capacitors C1 and C2 with capacitors of other capacities. A multivibrator assembled according to the same circuit can be used to study and train in listening to the telegraph alphabet - Morse code. In this case, you only need to replace the button with a telegraph key.

Electronic switch. This device, the diagram of which is shown in (Fig. 5), can be used to switch two Christmas tree garlands powered by an alternating current network. The electronic switch itself can be powered from two 3336L batteries connected in series, or from a rectifier that would provide a constant voltage of 9 - 12 V at the output.

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 greater 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 that operate for switching. Suitable, for example, is a RES-10 relay (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. When the power is turned on, transistors V1 and V2 of the multivibrator alternately open and close, generating square wave signals. When transistor V2 is turned on, a negative supply voltage is applied through resistor R4 and this transistor to the base of transistor V3, driving it into saturation. In this case, the resistance of the emitter-collector section of transistor V3 decreases to several ohms and almost the entire voltage of the power source is applied to the winding of relay K1 - the relay is triggered and with its contacts connects one of the garlands to the network. When transistor V2 is closed, the power supply circuit to the base of transistor V3 is broken, and it is also closed; no current flows through the relay winding. 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 capacitors C1 and C2 with capacitors of other capacities. Leave the data for resistors R2 and R3 the same, otherwise the DC operation mode of the transistors will be disrupted. A power amplifier similar to the amplifier on transistor V3 can also be included in the emitter circuit of transistor V1 of the multivibrator. In this case, electromagnetic relays (including homemade ones) may not have switching groups of contacts, but normally open or normally closed. The relay contacts of one of the arms of the multivibrator will periodically close and open the power circuit of one garland, and the relay contacts of the other arm of the multivibrator will periodically open the power 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 is quite enough for the entire New Year holidays. A similar switch can be used for other purposes. For example, for illuminating masks and attractions. Imagine a figurine of the hero of the fairy tale “Puss in Boots” cut out of plywood and painted. Behind the transparent eyes there 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 the lighthouse model? In this case, in the collector circuit of the power amplifier transistor, instead of an electromagnetic relay, you can include a small-sized incandescent light bulb, designed for a small filament current, which will imitate the flashes of a beacon. If such a switch is supplemented with a toggle switch, with the help of which two such bulbs can be switched 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 using sound signals with an accuracy of fractions of a second. Such devices are used, for example, to develop a sense of tact when teaching musical literacy, during the first training in transmitting signals using the telegraph alphabet. You can see a diagram of one of these devices in (Fig. 6).

This is also a multivibrator, but asymmetrical. This 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 3CH amplifier; communication is carried out by 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 during short-term current pulses. The pulse repetition rate can be adjusted by variable resistor R1 from approximately 20 to 300 pulses per minute. Resistor R2 limits the base current of the first transistor when the slider of resistor R1 is in the lowest (according to the circuit) position, corresponding to the highest frequency of generated oscillations. The metronome can be powered by one 3336L battery or three 332 cells connected in series. The current it consumes from the battery does not exceed 10 mA. 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 sound signals.

Practical work

For practical work, I advise you to assemble the multivibrator circuits presented in the lesson pictures, which will help you understand the principle of operation of the multivibrator. Next, I propose to assemble a very interesting and useful “Electronic Nightingale Simulator” based on multivibrators, which can be used as a doorbell. The circuit is very simple, reliable, and works immediately if there are no errors in installation and the use of serviceable radio elements. I have been using it as a doorbell for 18 years, to this day. It’s not hard to guess that I collected it when, like you, I was a beginner radio amateur.