Multivibrators are another form of oscillators. An oscillator is an electronic circuit that is capable of maintaining an alternating current signal at its output. It can generate square, linear or pulse signals. To oscillate, the generator must satisfy two Barkhausen conditions:
T loop gain should be slightly greater than unity.
The cycle phase shift must be 0 degrees or 360 degrees.
To satisfy both conditions, the oscillator must have some form of amplifier, and part of its output must be regenerated into the input. If the gain of the amplifier is less than one, the circuit will not oscillate, and if it is greater than one, the circuit will be overloaded and produce a distorted waveform. A simple generator can generate a sine wave, but cannot generate a square wave. A square wave can be generated using a multivibrator.
A multivibrator is a form of generator that has two stages, thanks to which we can get a way out of any of the states. These are basically two amplifier circuits arranged with regenerative feedback. In this case, none of the transistors conducts at the same time. Only one transistor is conducting at a time, while the other is in the off state. Some circuits have certain states; the state with fast transition is called switching processes, where there is a rapid change in current and voltage. This switching is called triggering. Therefore, we can run the circuit internally or externally.
Circuits have two states.
One is the steady state, in which the circuit remains forever without any triggering.
The other state is unstable: in this state, the circuit remains for a limited period of time without any external triggering and switches to another state. Hence, the use of multivibartors is done in two state circuits such as timers and flip-flops.
Astable multivibrator using a transistor
It is a free-running generator that continuously switches between two unstable states. In the absence of an external signal, the transistors alternately switch from the off state to the saturation state at a frequency determined by the RC time constants of the communication circuits. If these time constants are equal (R and C are equal), then a square wave with a frequency of 1/1.4 RC will be generated. Hence, an astable multivibrator is called a pulse generator or square wave generator. The greater the value of the base load R2 and R3 relative to the collector load R1 and R4, the greater the current gain and the sharper the signal edge will be.
The basic principle of operation of an astable multivibrator is a slight change in the electrical properties or characteristics of the transistor. This difference causes one transistor to turn on faster than the other when power is first applied, causing oscillation.
Diagram Explanation
An astable multivibrator consists of two cross-coupled RC amplifiers.
The circuit has two unstable states
When V1 = LOW and V2 = HIGH then Q1 ON and Q2 OFF
When V1 = HIGH and V2 = LOW, Q1 is OFF. and Q2 ON.
In this case, R1 = R4, R2 = R3, R1 must be greater than R2
C1 = C2
When the circuit is first turned on, none of the transistors are turned on.
The base voltage of both transistors begins to increase. Either transistor turns on first due to the difference in doping and electrical characteristics of the transistor.
Rice. 1: Schematic diagram of the operation of a transistor astable multivibrator
We can't tell which transistor conducts first, so we assume Q1 conducts first and Q2 is off (C2 is fully charged).
Q1 is conducting and Q2 is off, hence VC1 = 0V since all current to ground is due to Q1 short circuit, and VC2 = Vcc since all voltage across VC2 drops due to TR2 open circuit (equal to supply voltage) .
Due to the high voltage of VC2, capacitor C2 starts charging through Q1 through R4 and C1 starts charging through R2 through Q1. The time required to charge C1 (T1 = R2C1) is longer than the time required to charge C2 (T2 = R4C2).
Since the right plate C1 is connected to the base of Q2 and is charging, then this plate has a high potential and when it exceeds the voltage of 0.65V, it turns on Q2.
Since C2 is fully charged, its left plate has a voltage of -Vcc or -5V and is connected to the base of Q1. Therefore it turns off Q2
TR Now TR1 is off and Q2 is conducting, hence VC1 = 5 V and VC2 = 0 V. The left plate of C1 was previously at -0.65 V, which begins to rise to 5 V and connects to the collector of Q1. C1 first discharges from 0 to 0.65V and then begins to charge through R1 through Q2. During charging, the right plate C1 is at low potential, which turns off Q2.
The right plate of C2 is connected to the collector of Q2 and is pre-positioned at +5V. So C2 first discharges from 5V to 0V and then starts charging through resistance R3. The left plate C2 is at high potential during charging, which turns on Q1 when it reaches 0.65V.
Rice. 2: Schematic diagram of the operation of a transistor astable multivibrator
Now Q1 is conducting and Q2 is off. The above sequence is repeated and we get a signal at both the collectors of the transistor which is out of phase with each other. To obtain a perfect square wave by any collector of the transistor, we take both the collector resistance of the transistor, the base resistance, i.e. (R1 = R4), (R2 = R3), and also the same value of the capacitor, which makes our circuit symmetrical. Therefore, the duty cycle for low and high output is the same that generates a square wave
Constant The time constant of the waveform depends on the base resistance and collector of the transistor. We can calculate its time period by: Time constant = 0.693RC
The principle of operation of a multivibrator on video with explanation
In this video tutorial from the Soldering Iron TV channel, we will show how the elements of an electrical circuit are interconnected and get acquainted with the processes occurring in it. The first circuit on the basis of which the operating principle will be considered is a multivibrator circuit using transistors. The circuit can be in one of two states and periodically transitions from one to another.
Analysis of 2 states of the multivibrator.
All we see now are two LEDs blinking alternately. Why is this happening? Let's consider first first state.
The first transistor VT1 is closed, and the second transistor is completely open and does not interfere with the flow of collector current. The transistor is in saturation mode at this moment, which reduces the voltage drop across it. And therefore the right LED lights up at full strength. Capacitor C1 was discharged at the first moment of time, and the current freely passed to the base of transistor VT2, completely opening it. But after a moment, the capacitor begins to quickly charge with the base current of the second transistor through resistor R1. After it is fully charged (and as you know, a fully charged capacitor does not pass current), the transistor VT2 therefore closes and the LED goes out.
The voltage across capacitor C1 is equal to the product of the base current and the resistance of resistor R2. Let's go back in time. While transistor VT2 was open and the right LED was on, capacitor C2, previously charged in the previous state, begins to slowly discharge through the open transistor VT2 and resistor R3. Until it is discharged, the voltage at the base of VT1 will be negative, which completely turns off the transistor. The first LED is not lit. It turns out that by the time the second LED fades out, capacitor C2 has time to discharge and becomes ready to pass current to the base of the first transistor VT1. By the time the second LED stops lighting, the first LED lights up.
A in the second state the same thing happens, but on the contrary, transistor VT1 is open, VT2 is closed. The transition to another state occurs when capacitor C2 is discharged, the voltage across it decreases. Having completely discharged, it begins to charge in the opposite direction. When the voltage at the base-emitter junction of transistor VT1 reaches a voltage sufficient to open it, approximately 0.7 V, this transistor will begin to open and the first LED will light up.
Let's look at the diagram again.
Through resistors R1 and R4, the capacitors are charged, and through R3 and R2, discharge occurs. Resistors R1 and R4 limit the current of the first and second LEDs. Not only the brightness of the LEDs depends on their resistance. They also determine the charging time of the capacitors. The resistance of R1 and R4 is selected much lower than R2 and R3, so that the charging of the capacitors occurs faster than their discharge. A multivibrator is used to produce rectangular pulses, which are removed from the collector of the transistor. In this case, the load is connected in parallel to one of the collector resistors R1 or R4.
The graph shows the rectangular pulses generated by this circuit. One of the regions is called the pulse front. The front has a slope, and the longer the charging time of the capacitors, the greater this slope will be.
If a multivibrator uses identical transistors, capacitors of the same capacity, and if resistors have symmetrical resistances, then such a multivibrator is called symmetrical. It has the same pulse duration and pause duration. And if there are differences in parameters, then the multivibrator will be asymmetrical. When we connect the multivibrator to a power source, at the first moment of time both capacitors are discharged, which means that current will flow to the base of both capacitors and an unsteady operating mode will appear, in which only one of the transistors should open. Since these circuit elements have some errors in ratings and parameters, one of the transistors will open first and the multivibrator will start.
If you want to simulate this circuit in the Multisim program, then you need to set the values of resistors R2 and R3 so that their resistances differ by at least a tenth of an ohm. Do the same with the capacitance of the capacitors, otherwise the multivibrator may not start. In the practical implementation of this circuit, I recommend supplying voltage from 3 to 10 Volts, and now you will find out the parameters of the elements themselves. Provided that the KT315 transistor is used. Resistors R1 and R4 do not affect the pulse frequency. In our case, they limit the LED current. The resistance of resistors R1 and R4 can be taken from 300 Ohms to 1 kOhm. The resistance of resistors R2 and R3 is from 15 kOhm to 200 kOhm. Capacitor capacity is from 10 to 100 µF. Let's present a table with the values of resistances and capacitances, which shows the approximate expected pulse frequency. That is, to get a pulse lasting 7 seconds, that is, the duration of the glow of one LED is equal to 7 seconds, you need to use resistors R2 and R3 with a resistance of 100 kOhm and a capacitor with a capacity of 100 μF.
Conclusion.
The timing elements of this circuit are resistors R2, R3 and capacitors C1 and C2. The lower their ratings, the more often the transistors will switch, and the more often the LEDs will flicker.
A multivibrator can be implemented not only on transistors, but also on microcircuits. Leave your comments, don’t forget to subscribe to the “Soldering Iron TV” channel on YouTube so you don’t miss new interesting videos.
Another interesting thing about the radio transmitter.
Schematic diagram of a powerful transistor multivibrator with control, built on transistors KT972, KT973. Many radio amateurs began their creative journey by assembling simple direct-amplification radio receivers, simple audio power amplifiers and assembling simple multivibrators consisting of a pair of transistors, two or four resistors and two capacitors.
A traditional symmetrical multivibrator has a number of disadvantages, including a relatively high output resistance, long pulse rises, limited supply voltage, and low efficiency when operating with a low-impedance load.
Schematic diagram
In Fig. 1. shows a diagram of a controlled symmetrical two-phase multivibrator operating at audio frequencies, the load to which is connected via a bridge circuit. Due to this, the amplitude swing of the signal across the load is almost twice the supply voltage of the multivibrator, which makes it possible to obtain a significantly higher volume compared to the load would be included in one of the arms of the multivibrator.
In addition, the load is supplied with “real” AC voltage, which significantly improves the operating conditions of the dynamic head connected as a load - there is no effect of indentation or protrusion of the diffuser (depending on the polarity of the speaker). There are also no clicks when turning the multivibrator on or off.
Rice. 1. Schematic diagram of a powerful multivibrator using transistors KT972, KT973.
A symmetrical two-phase multivibrator consists of two push-pull arms, the voltage on which alternately changes from low to high. Let's assume that when the power is turned on, the composite transistor VT2 opens first.
Then the voltage at the terminals of the collectors of transistors VT1, VT2 will become close to zero (VT1 is open, VT2 is closed). A composite pnp transistor VT5 is connected to the connection point of their collectors through the current-limiting resistor R12, which will open. A voltage of about 8 V will be applied to the load when the multivibrator supply voltage is 9 V. With the recharging of capacitors C2, C4, the multivibrator will switch - VT1, VT6 will open, VT2, VT5 will close.
The same voltage will be applied to the load, but in reverse polarity. The switching frequency of the multivibrator depends on the capacitance of capacitors C2, C4, and, to a lesser extent, on the set resistance of the tuning resistor R7. With a supply voltage of 9 V, the frequency can be adjusted from 1.4 to 1.5 kHz.
When the resistance R7 decreases below the conventional value, the generation of sound frequencies is disrupted. It should be noted that after startup, the multivibrator can operate without resistors R5, R11. The voltage shape at the output of the multivibrator is close to rectangular.
Resistors R6, R8 and diodes VD1, VD2 protect the emitter junctions of transistors VT2, VT6 from breakdown, which is especially important when the multivibrator supply voltage is more than 10V. Resistors R1, R13 are necessary for stable generation; in their absence, the multivibrator may “wheeze”. The VD3 diode protects powerful transistors from power supply voltage reversal. If it is absent and the power supply is sufficiently powerful, the built-in protective circuits of the transistors may be damaged when the voltage is reversed.
To expand the functionality of this multivibrator, it has the ability to turn on/off when a positive polarity voltage is applied to the control input. If the control input is not connected anywhere or the voltage on it is no more than 0.5 V, transistors VT3, VT4 are closed, the multivibrator works.
When a high level voltage is applied to the control input, for example, from the TTLSH output. CMOS microcircuits, a sensor of electrical or non-electrical quantities, for example, a humidity sensor, transistors VT3, VT4 open, the multivibrator is inhibited. In this state, the multivibrator consumes a current of less than 200 μA, excluding the current through R2, R3, R9.
Parts and installation
The multivibrator can be mounted on a printed circuit board measuring 70*50 mm, a sketch of which is shown in Fig. 2 Fixed resistors can be used in any small size. Trimmer resistor RP1-63M, SP4-1 or similar imported one. Oxide capacitors K50-29, K50-35 or analogues Capacitors C2, C4 - K73-9, K73-17, K73-24 or any small-sized film.
Rice. 2. Printed circuit board for a powerful transistor multivibrator circuit.
KD522A diodes can be replaced with KD503. KD521. D223 with any letter index or imported 1N914, 1N4148. Instead of diodes KD226A and KD243A, any of the series KD226, KD257, KD258, 1 N5401 ... 1 N5407 is suitable.
Composite transistors KT972A can be replaced by any of this series or from the KT8131 series, and instead of KT973 by any of the KT973, KT8130 series. If necessary, powerful transistors are installed on small heat sinks. In the absence of such transistors, they can be replaced with analogues of two transistors connected according to a Darlington circuit, Fig. 3. Instead of low-power pnp transistors KT315G, any of the KT312, KT315, KT342, KT3102, KT645, SS9014 and similar series are suitable.
Rice. 3. Schematic diagram of equivalent replacement of transistors KT972, KT973.
The load of this multivibrator can be a dynamic head, a telephone capsule, a piezoceramic sound emitter, or a pulse step-up/step-down transformer.
When using a dynamic head with a winding resistance of 8 Ohms, it should be taken into account that with a supply voltage of 9 V, 8 W of AC voltage power will be supplied to the load. Therefore, a two...four-watt dynamic head can be damaged after just 1...2 minutes of operation.
Setting up
The operating frequency of the multivibrator is significantly influenced by the load capacitance and supply voltage. For example, when the supply voltage changes from 5 to 15 V, the frequency changes from 2850 to 1200 Hz when operating on a multivibrator with a load in the form of a telephone capsule with a winding resistance of 56 Ohms. In the region of low supply voltages, the change in operating frequency is more significant
By selecting the resistances of resistors R5, R11, R6, R8, you can set the pulse shape to be almost strictly rectangular when the multivibrator is operating with a specific connected load at a given supply voltage.
This multivibrator can find application in various signaling devices, sound warning devices, when, with a small available voltage of the power source, it is necessary to obtain significant power at the sound emitter. In addition, it is convenient to use in low-to-high voltage converters, including those operating at a low lighting network frequency of 50 Hz.
Butov A. L. RK-2010-04.
RADIO signal:
MULTIVIBRATOR-1
Just a theory or a simple theory
“MULTI” - a lot, “VIBRATO” - vibration, oscillation, therefore, “MULTIVIBRATOR” is a device that creates (generates) many, many vibrations.
Let us first understand how it creates vibrations, or how vibrations arise in it, and only then we will find out why there are many of them.
2. HOW TO CREATE A MULTIVIBRATOR?
Step #1. Let's take the simplest low-frequency amplifier (see my article “Transistor”, item 4 on the “Radio Components” page): 
(Here I do not describe its operating principle.)
Step #2. Let's combine two identical amplifiers so that we get a two-stage ULF: 
Step #3. Let's connect the output of this amplifier to its input: 
A so-called positive feedback (POF) will arise. You've probably heard the whistling sound that speakers made if the person with the microphone got too close to them. The same thing happens with the music center in karaoke mode if you bring the microphone to the speakers. In any such case, the signal from the output of the amplifier arrives at its own input, the amplifier enters the self-excitation mode and turns into a self-oscillator, and sound appears. Sometimes the amplifier can self-excite even at ultrasonic frequencies. In short, when making amplifiers, PIC is harmful and you have to fight it in every possible way, but that’s a slightly different story.
Let's return to our amplifier covered by PIC, i.e. MULTIVIBRATOR! Yes, it's already him! True, to portray exactly multivibrator accepted as in Fig. right. By the way, there are a sufficient number of “perverts” on the Internet who draw this diagram both upside down and lying on its side. Why is this? Probably, as in the joke, “to be different.” Or in s share, or (there is such a Russian word!) in s show off.
The multivibrator can be assembled using n-p-n or p-n-p transistors: 
You can evaluate the operation of the multivibrator by ear or visually. In the first case, the load should be a sound emitter, in the second - a light bulb or LED: 
If low-impedance speakers are used, an output transformer or an additional amplifier stage will be required: 
The load can be included in both arms of the multivibrator: 
In the case of using LEDs, it is advisable to include additional resistors, the role of which is played, in this case, by R1 and R4.
3. HOW DOES A MULTIVIBRATOR WORK?
At the moment the power is turned on, the transistors of both arms of the multivibrator open, since positive (negative - hereinafter in parentheses for p-n-p transistors) 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 VT2 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 at the bases of the transistors (relative to the emitters) positive (negative) voltages that are increasingly increasing in value, tending to open the transistors more and more. Turning on a transistor causes the positive (negative) voltage at its collector to decrease, which causes the positive (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 negative (positive) voltage, for example, due to the difference in current transfer coefficients h21e (see my article “Transistor”, paragraph 4 on page “Radio components”), values of resistors and capacitors, since, even when selecting identical pairs, the parameters of the elements will still be slightly different. 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 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 negative (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 positive (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 operation of the multivibrator is illustrated by graphs of the voltages Ube and Uk of one and the second transistor: 
As you can see, the multivibrator generates practically “rectangular” oscillations. Some violation of the rectangular shape is associated with transient processes at the moments when the transistors are turned on. From here it is clear that the signal can be “removed” from any transistor. It’s just that it’s most common to depict it exactly as shown above.
In practice, the oscillation shape of a multivibrator can be considered “purely rectangular”: 
On the one hand, the multivibrator waveform seems to be quite simple. But this is not entirely true. More precisely, not like that at all. The simplest waveform is a sine wave: 
If the generator creates ideal sinusoidal signal, then it corresponds strictly one a certain oscillation frequency. The more the signal shape differs from a sinusoid, the more frequencies that are multiples of the fundamental frequency are present in the signal spectrum. And the multivibrator signal shape is quite far from a sinusoid. Therefore, if, for example, the frequency of its oscillations is 1000 Hz, then the spectrum will contain frequencies of 2000 Hz, and 3000 Hz, and 4000 Hz... etc. true amplitudes of these harmonics will be significantly less than the main signal. But they will! That's why this generator is called MULTI vibrator.
The oscillation frequency of the multivibrator depends both on the capacitance of the coupling capacitors and on the resistance of the base resistors. If the conditions are met in the multivibrator: R1=R4, R2=R3, R1
The approximate oscillation frequency of a symmetrical multivibrator can be calculated using a simplified formula:
, where f is the frequency in Hz, R is the resistance of the base resistor in kOhm, C is the capacitance of the coupling capacitor in uF.
4. CHANGE OF FREQUENCY and more
As noted above, the frequency of the pulses generated by the multivibrator is determined by the values of the coupling capacitors and base resistors. From the above formula it can be seen that an increase in the capacitance of the capacitors and/or an increase in the resistance of the base resistors leads to a decrease in the frequency of the multivibrator and, accordingly, vice versa. Of course, it is possible to solder capacitors of different capacities or resistors of different resistances, but only at the experimental stage. The frequency is quickly changed using a variable resistor R5 in the base circuits: 
The shape of the oscillation graph of a multivibrator is called a “meander”: 
The time from the beginning of one pulse to the beginning of another - period T - consists of:
tи – pulse duration and tп – pause duration.
The ratio S=T/ti is called duty cycle. For a symmetrical multivibrator S=2.
The reciprocal of the duty cycle is called the duty cycle D=1/S. For a symmetrical multivibrator D=0.5.
The multivibrator, the circuit of which is shown below, produces rectangular pulses. The frequency of their repetition can be varied within wide limits, while the duty cycle of the pulses remains unchanged.
The operation of the multivibrator is different in that at times when transistor VT1 is closed, capacitor C2 is discharged through a chain consisting of diode VD3 and resistor R4, as well as through resistor R3. Similarly, when transistor VT2 is closed, capacitor C1 is discharged through diode VD2 and resistors R4 and R5.
The pulse repetition rate can be adjusted within wide limits by changing only the resistance of resistor R4.
A multivibrator with the details shown in the diagram generates pulses with a repetition frequency from 140 to 1400 Hz.
In the multivibrator, you can use diodes D2V-D2I, D9V-D9L, and any low-power transistors with an n-p-n or p-n-p structure. When using transistors with a pnp structure, the switching polarity of all diodes and the power supply must be reversed.
If you slightly change the connection of resistor R7, then it swells multivibrator with variable duty cycle pulses: 
Depending on the position of the resistor R7 slider, this multivibrator becomes asymmetrical, and the graph of its oscillations can be, for example, like this: 
In one and the other case, the ratio T/ti changes - the duty cycle changes.
It is also clear, I hope, that the duty cycle can be roughly changed by installing capacitors of different capacities.
5. ASSYMMETRICAL MULTIVIBRATOR on transistors of different conductivities:
An asymmetrical multivibrator consists of an amplifier stage on two transistors, the output of which (collector of transistor VT2) is connected to the input (base of transistor VT1) through capacitor C1. The load is resistor R2, from which the signal is removed (an LED, an incandescent light bulb or a speaker can be turned on instead). Direct conduction transistor VT1 (p-n-p type) opens when a potential negative relative to the emitter is applied to the base. Transistor VT2 of reverse conductivity (n-p-n type), opens when a potential positive relative to the emitter is applied to the base. 
When turned on, capacitor C1 is charged through resistors R2 and R1, and the base potential decreases. When a negative potential arises at the base of VT1, transistor VT1 opens and the collector-emitter resistance drops. The base of transistor VT2 turns out to be connected to the positive pole of the source, transistor VT2 also opens, and the collector current increases. As a result, current flows through R2, capacitor C1 is discharged through resistor R1 and transistor VT2. The base potential of VT1 increases, transistor VT1 closes, causing transistor VT2 to close. After this, capacitor C1 is charged again, then discharged, etc. The frequency of the generated pulses is inversely proportional to the charging time of the capacitor T ~ R1×C. As the supply voltage increases, the capacitor charges faster, and the frequency of the generated pulses increases. As the resistance of resistor R1 or the capacitance of capacitor C1 increases, the oscillation frequency decreases.
In reality, the frequency is changed, for example, like this: 
Examples from the site http://lessonradio.narod.ru/Diagram.htm
6. STANDBY 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 a self-oscillating multivibrator into a waiting multivibrator (see the diagram from point 3), you need to do the following: remove capacitor C2, and instead connect resistor R3 between the collector of transistor VT2 and the base of transistor VT1; between the base of transistor VT1 and the grounded conductor, connect a series-connected 1.5 V element and a resistor with resistance R5, but so that the positive pole of the element is connected to the base (via R5); connect capacitor C2 to the base circuit of transistor VT1, the second terminal of which will act as a contact input control signal. The initial state of transistor VT1 of such a multivibrator is closed, transistor VT2 is open. 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. Include a milliammeter (for a current of 10-15 mA) in the collector circuit of transistor V1 and, observing it arrow, switch between contact UPR signal and with a grounded conductor, literally for a moment, one or two AAA elements connected in series (in the GB1 diagram). WARNING: The negative pole of this external electrical signal must be connected to the contact UPR signal. 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. If you repeat this experiment several times, then the milliammeter with each signal will show an instantaneous increase to 8 - 10 mA and after some time, the collector current of transistor VT1 also instantly decreases almost to zero. These are single current pulses generated by a multivibrator. Even if the GB1 battery is kept connected to the clamp longer UPR signal, the same thing will happen - only one pulse will appear at the output of the multivibrator. 
If you touch the terminal of the base of transistor VT1 with any metal object taken in your hand, then perhaps in this case the waiting multivibrator will work - from the electrostatic charge of the body. You can connect a milliammeter to the collector circuit of transistor VT2. 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 negative polarity.
What is the operating principle of a standby multivibrator? In such a multivibrator, the connection between the collector of transistor VT2 and the base of transistor VT1 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 VT2 through resistor R2. Transistor VT1 is reliably closed by the positive voltage of element G1 at its base. This state of transistors is very stable. VT1 can remain in this state for any amount of time. When a voltage pulse of negative polarity appears at the base of transistor VT1, the transistors go into an unstable state. Under the influence of the input signal, transistor VT1 opens, and the changing voltage on its collector through capacitor C1 closes transistor VT2. The transistors remain in this state until capacitor C1 is discharged (through resistor R2 and open transistor VT1, the resistance of which is low at this time). As soon as the capacitor is discharged, transistor VT2 will immediately open, and transistor VT1 will close. From this moment on, the multivibrator is again in its original, stable standby mode. Thus, the 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 VT1 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 VT2.
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.
An example of using a waiting multivibrator is a maximum speed indicator.
When running in a new car, the engine speed should not exceed for a certain time the maximum permissible value recommended by the manufacturer.
To control the engine speed, you can use a device assembled according to the diagram given here. An incandescent lamp is used as an indicator of the maximum engine speed. 
The main parts of the tachometer are a standby multivibrator on transistors T1 and T2 and a Schmitt trigger on transistors T5 and T6. The input signal coming from the breaker is fed to the differentiating chain R4C1 (this is necessary to obtain pulses of the same duration). Further signal formation is performed by the multivibrator. Diode D1 does not transmit negative half-waves of the input signal to the base of transistor T2. The pulses generated by the multivibrator are fed to the Schmitt trigger through an emitter follower made on transistor T3 and an integrating circuit R7C3. Indicator lamp L1, connected to the emitter circuit of transistor T6, lights up only when the engine speed exceeds a preset one (using variable resistor R8).
The finished device can be calibrated using a standard tachometer or a sound generator. So, for example, for a four-stroke four-cylinder engine, 1500 rpm corresponds to a sound generator frequency of 60 Hz, 3000 rpm - 100 Hz, 6000 rpm - 200 Hz, and so on.
When using parts with the data indicated in the diagram, the tachometer allows you to register from 500 to 10,000 rpm. Current consumption - 20 mA.
Transistors BC107 can be replaced with KT315 with any letter index. Any silicon diode can be used as diode D1. The use of germanium transistors and diodes is not recommended due to the severe temperature conditions.
7. MULTI-PHASE MULTIVIBRATORS
are obtained by adding amplification stages and PICs.
Three-phase multivibrator: 
Example from the site http://www.votshema.ru/324-simmetrichnyy-multivibrator.html
A four-phase multivibrator requires special measures to ensure stable operation: 
Example from the site http://www.moyashkola.net/krugok/r_begog.htm
8. MULTIVIBRATORS ON LOGIC ELEMENTS
The multivibrator can be made using logical elements, for example, NAND. A diagram of a possible option, for example, is as follows: 
The function of the active elements here is performed by 2I-NOT logic elements (see my article “CHICROCIRCUIT” on page “RADIO components”), connected by inverters. Thanks to the PIC between the output DD1.2 and the input DD1.1, as well as the output DD1.1 and the input DD1.2, created by capacitors C1 and C2, the device is excited and generates electrical pulses. The pulse repetition rate depends on the values of capacitors and resistors R1 and R2. By reducing the capacitance of the capacitors to 1...5 µF we obtain an audio frequency of 500...1000 Hz. The headphone must be connected to one of the outputs of the multivibrator through a capacitor with a capacity of 0.01...0.015 μF.
Sometimes the same multivibrator is depicted like this: 
The multivibrator can be made on three logical elements: 
All elements are switched on by inverters and connected in series. The timing chain is formed by C1 and R1. An incandescent light bulb can be used as an indicator. To smoothly change the frequency, instead of R1, you should include a 1.5 kOhm variable resistor. 
If the capacitance of the capacitor is 1 µF, then the oscillation frequency will become sound.
How does such a multivibrator work? After switching on, one of the logical elements will be the first to take one of the possible states and thereby affect the state of other elements. Let it be element DD1.2, which turns out to be in a single state. Through elements DD1.1 and DD1.2, the capacitor is instantly charged, and element DD1.1 is in the zero state. The DD1.3 element finds itself in the same state, since its input is logical 1. This state is unstable, because the output of DD1.3 is logical 0, and the capacitor begins to discharge through the resistor and the output stage of the DD1.3 element. As the discharge progresses, the positive voltage at the input of element DD1.1 decreases. As soon as it becomes equal to the threshold, this element will switch to the single state, and the DD1.2 element will switch to the zero state. The capacitor will begin to charge through element DD1.3 (its output is now at logical level 1), a resistor and element DD1.2. Soon the voltage at the input of the first element will exceed the threshold, and all elements will switch to opposite states. This is how electrical pulses are formed at the output of the multivibrator - at the inverse output of element DD1.3.
The “three-element” multivibrator can be simplified by removing DD1.3 from it: 
It works similarly to the previous one. It is this kind of multivibrator that is most often used in various radio-electronic devices.
You can also make a waiting multivibrator using logic elements. Like the previous one, it is built on 2 logical elements. 
The first DD1.1 is used for its intended purpose - as a 2I-NOT element. Button SB1 acts as a trigger signal sensor. To indicate pulses, for example, an LED is used. The pulse duration can be increased by increasing the capacitance C1 and resistance R1. Instead of R1, you can turn on a variable (tuning) resistor with a resistance of about 2 kOhm (but not more than 2.2 kOhm) to change the pulse duration within certain limits. But if the resistance is less than 100 Ohms, the multivibrator will stop working.
Operating principle. At the initial moment, the lower pin of the DD1.1 element is not connected to anything - it has a logical level of 1. And for the 2I-NOT element, this is enough for it to be in the zero state. The DD1.2 input is also at a logic 0 level, since the voltage drop across the resistor created by the input current of the element keeps the input transistor of the element in the closed state. The logic 1 voltage at the output of this element maintains the first element in the zero state. When the button is pressed, a trigger pulse of negative polarity is applied to the input of the first element, which switches element DD1.1 to the single state. The positive voltage jump that occurs at this moment at its output is transmitted through a capacitor to the inputs of the second element and switches it from a single state to a zero state. This state of the elements remains even after the end of the triggering pulse. From the moment a positive pulse appears at the output of the first element, the capacitor begins to charge - through the output stage of this element and a resistor. As charging occurs, the voltage across the resistor drops. As soon as it reaches the threshold, the second element will switch to the one state, and the first to the zero state. The capacitor will quickly discharge through the output stage of the first element and the water stage of the second, and the device will be in standby mode.
It should be borne in mind that for normal operation of the multivibrator, the duration of the triggering pulse must be less than the duration of the generated one.
P.S. The topic "MULTIVIBRATOR" is an example of a creative approach to the study of electrical vibrations in a school physics course. And not only. Creating simple circuits, modeling their operation, observing and measuring electrical quantities is going far beyond the scope of ordinary school physics and computer science. And the creation of real devices completely changes young people’s idea of what and how they can STUDY at school (I hate the word “TEACH”).
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-vibration”.
Let's consider the circuit shown in (Fig. 1, a). Do you recognize? Yes, this is a circuit of a two-stage 3H transistor amplifier 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 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 the same. 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 that generate 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 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-oscillatory 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 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 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 applied 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
As a practical work, I advise you to assemble the multivibrator circuits presented in the drawings of the lesson, 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.
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 generator's overall transmission coefficient must exceed 1 in a wide frequency range - 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 amount u. As a result of both generation conditions being met, a voltage increment will appear at the device output: 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.
The task of constructing a practical pulse generator circuit comes down to feeding a portion of the output signal with a phase difference =2 to the input of a broadband amplifier. 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 amplification 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.
We will qualitatively consider the operation of the multivibrator using voltage timing 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 of 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.
