LED flasher - multivibrator. Symmetrical multivibrator, calculation and circuit of a multivibrator Circuit for monitoring the operation of a multivibrator using transistors
Hello dear friends and all readers of my blog site. Today's post will be about a simple but interesting device. Today we will look at, study and assemble an LED flasher, which is based on a simple generator rectangular pulses- multivibrator.
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This page now bears the name “This is interesting.”
You probably ask: “How can I find it?” And it's very simple!
You may have noticed that there is a kind of peeling corner on the blog with the inscription “Hurry here”.
Moreover, as soon as you move the mouse cursor to this inscription, the corner begins to peel off even more, revealing the inscription - the link “This is interesting”.
It leads to a secret page where a small but pleasant surprise awaits you - a gift prepared by me. Moreover, in the future this page will contain useful materials, amateur radio software and something else - I haven’t thought of it yet. So, periodically look around the corner - in case I hid something there.
Okay, I got a little distracted, now let's continue...
In general, there are many multivibrator circuits, but the most popular and discussed is the astable symmetrical multivibrator circuit. She is usually depicted this way.
For example, I soldered this multivibrator flasher about a year ago from scrap parts and, as you can see, it flashes. It blinks despite the clumsy installation done on the breadboard.
This scheme is working and unpretentious. You just need to decide how it works?
Multivibrator operating principle
If we assemble this circuit on a breadboard and measure the voltage with a multimeter between the emitter and collector, what will we see? We will see that the voltage on the transistor either rises almost to the voltage of the power supply, then drops to zero. This suggests that the transistors in this circuit operate in switch mode. I note that when one transistor is open, the second is necessarily closed.
The transistors are switched as follows.
When one transistor is open, say VT1, capacitor C1 discharges. Capacitor C2, on the contrary, is quietly charged with the base current through R4.
During the discharge process, capacitor C1 keeps the base of transistor VT2 under negative voltage - it locks it. Further discharge brings capacitor C1 to zero and then charges it in the other direction.
Now the voltage at the base of VT2 increases, opening it. Now capacitor C2, once charged, is subject to discharge. Transistor VT1 turns out to be locked with negative voltage at the base.
And all this pandemonium continues non-stop until the power is turned off.
Multivibrator in its design
Having once made a multivibrator flasher on a breadboard, I wanted to refine it a little - make a normal printed circuit board for the multivibrator and at the same time make a scarf for LED indication. I developed them in the Eagle CAD program, which is not much more complicated than Sprintlayout but has a strict connection to the diagram.
Multivibrator printed circuit board on the left. Electrical diagram on the right.
PCB. Electrical diagram.
I printed out the drawings of the printed circuit board on photo paper using a laser printer. Then, in full accordance with the folk tradition, he etched the scarves. As a result, after soldering the parts, we got scarves like this. 
To be honest, after complete installation and connecting the power, a small bug occurred. The plus sign made from LEDs did not blink. It burned simply and evenly as if there was no multivibrator at all.
I had to be pretty nervous. Replacing the four-point indicator with two LEDs corrected the situation, but as soon as everything was returned to its place, the flashing light did not blink.
It turned out that the two LED arms were connected by a jumper; apparently, when I tinned the scarf, I went a little too far with the solder. As a result, the LED “hangers” lit up synchronously rather than at intervals. Well, nothing, a few movements with a soldering iron corrected the situation.
I captured the result of what happened on video:
In my opinion it turned out not bad. 🙂 By the way, I’m leaving links to diagrams and boards - enjoy them for your health.
Multivibrator board and circuit.
Board and circuit of the "Plus" indicator.
In general, the use of multivibrators is varied. They are suitable not only for simple LED flashers. After playing with the values of resistors and capacitors, you can output audio frequency signals to the speaker. Wherever a simple pulse generator may be needed, a multivibrator is definitely suitable.
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LEDs are installed on the multivibrator board
LED flasher - symmetrical multivibrator The application of the symmetrical multivibrator circuit is very wide. Elements of multivibrator circuits can be found in computer technology, radio measuring and medical equipment.
A set of parts for assembling LED flashers can be purchased at the following link http://ali.pub/2bk9qh . If you want to seriously practice soldering simple designs The master recommends purchasing a set of 9 sets, which will greatly save your shipping costs. Here is the link to purchase http://ali.pub/2bkb42 . The master collected all the sets and they started working. Success and growth of skills in soldering.
is a pulse generator of almost rectangular shape, created in the form of an amplifying element with a positive-feedback circuit. There are two types of multivibrators.
The first type is self-oscillating multivibrators, which do not have a stable state. There are two types: symmetrical - its transistors are the same and the parameters of the symmetrical elements are also the same. As a result, the two parts of the oscillation period are equal to each other, and the duty cycle is equal to two. If the parameters of the elements are not equal, then it will already be an asymmetrical multivibrator.
The second type is waiting multivibrators, which have a state of stable equilibrium and are often called a single-vibrator. The use of a multivibrator in various amateur radio devices is quite common.
Description of the operation of a transistor multivibrator
Let us analyze the operating principle using the following diagram as an example.
It's easy to see that she practically copies schematic diagram symmetrical trigger. The only difference is that connections between switching blocks, both direct and reverse, are carried out according alternating current, and not on a constant basis. This radically changes the features of the device, since in comparison with a symmetrical trigger, the multivibrator circuit does not have stable equilibrium states in which it could remain for a long time.
Instead, there are two states of quasi-stable equilibrium, due to which the device remains in each of them for a strictly defined time. Each such period of time is determined by transient processes occurring in the circuit. The operation of the device consists of a constant change in these states, which is accompanied by the appearance at the output of a voltage very similar in shape to a rectangular one.
Essentially, a symmetrical multivibrator is a two-stage amplifier, and the circuit is constructed so that the output of the first stage is connected to the input of the second. As a result, after applying power to the circuit, it is sure that one of them is open and the other is in a closed state.
Let's assume that transistor VT1 is open and is in a state of saturation with current flowing through resistor R3. Transistor VT2, as mentioned above, is closed. Now processes occur in the circuit associated with the recharging of capacitors C1 and C2. Initially, capacitor C2 is completely discharged and, following the saturation of VT1, it is gradually charged through resistor R4.
Since capacitor C2 bypasses the collector-emitter junction of transistor VT2 through the emitter junction of transistor VT1, its charging rate determines the rate of change of voltage at the collector VT2. After charging C2, transistor VT2 closes. The duration of this process (the duration of the collector voltage rise) can be calculated using the formula:
t1a = 2.3*R1*C1
Also in the operation of the circuit, a second process occurs, associated with the discharge of the previously charged capacitor C1. Its discharge occurs through transistor VT1, resistor R2 and the power source. As the capacitor at the base of VT1 discharges, a positive potential appears and it begins to open. This process ends after C1 is completely discharged. The duration of this process (pulse) is equal to:
t2a = 0.7*R2*C1
After time t2a, transistor VT1 will be off, and transistor VT2 will be in saturation. After this, the process will be repeated according to a similar pattern and the duration of the intervals of the following processes can also be calculated using the formulas:
t1b = 2.3*R4*C2 And t2b = 0.7*R3*C2
To determine the oscillation frequency of a multivibrator, the following expression is valid:
f = 1/ (t2a+t2b)
Portable USB oscilloscope, 2 channels, 40 MHz....
Waiting multivibrators after the arrival of a short trigger pulse, one output pulse is generated. They belong to the class monostable devices and have one long-term stable and one quasi-stable equilibrium state. The circuit of the simplest standby multivibrator based on bipolar transistors, having one resistive and one capacitive collector-base connections, is shown in Fig. 8. Thanks to the base connection VT 2 with power supply + E through R b2, an unlocking current flows in the base circuit, sufficient to saturate this transistor. In this case, the output voltage removed from the collector VT 2 is close to zero. Transistor VT 1 is locked by the negative voltage obtained by dividing the voltage of the bias source - E cm divider R b1 R With. Thus, after turning on the power supplies, the state of the circuit is determined. In this state the capacitor WITH 1 charged to source voltage + E(plus on the left, minus on the right cover).
Rice. 8. Waiting transistor multivibrator
The waiting multivibrator can remain in this state for as long as desired - until the triggering pulse arrives. A positive trigger pulse (Fig. 9) unlocks the transistor VT 1, which leads to an increase in the collector current and a decrease in the collector potential of this transistor. Negative potential gain across a capacitor WITH 1 is transmitted to the base VT 2, brings this transistor out of saturation and causes it to go into active mode. The collector current of the transistor decreases, the voltage at the collector receives a positive increment, which from the collector VT 2 via resistor R c is transmitted to the base VT 1, causing it to further unlock. To reduce unlocking time VT 1 in parallel R c include the accelerating capacitor WITH usk. The process of switching transistors occurs like an avalanche and ends with the transition of the multivibrator to the second quasi-stable equilibrium state. In this state, the capacitor discharges WITH 1 via resistor R b2 and saturated transistor VT 1 per power supply +E. Positively charged plate WITH 1 via saturated transistor VT 1 is connected to the common wire, and the negatively charged one is connected to the base VT 2. Thanks to this, the transistor VT 2 is kept locked. After discharge WITH 1 base potential VT 2 becomes non-negative. This leads to an avalanche-like switching of transistors ( VT 2 is unlocked and VT 1 is locked). The formation of the output pulse ends. Thus, the duration of the output pulse is determined by the process of discharging the capacitor WITH 1
.
Output pulse amplitude
.
At the end of the output pulse formation, the recovery stage begins, during which the capacitor is charged WITH 1 from source + E through a resistor R k1 and the emitter junction of the saturated transistor VT 2. Recovery time
.
The minimum repetition period with which trigger pulses can follow is
.
Rice. 9. Voltage timing diagrams in the waiting multivibrator circuit
Operational amplifiers
Operational amplifiers(op amp) are called high quality amplifiers DC(UPT), designed to perform various operations on analog signals when operating in a circuit with negative feedback.
DC amplifiers allow you to amplify slowly changing signals, since they have a zero lower limiting frequency of the amplification band (f n = 0). Accordingly, such amplifiers do not have reactive components (capacitors, transformers) that do not transmit the DC component of the signal.
In Fig. 10a shows the symbol of the op-amp. The amplifier shown has one output terminal (shown on the right) and two input terminals (shown on the left side). The sign Δ or > characterizes the gain. An input whose voltage is shifted in phase by 180 0 relative to the output voltage is called inverting and is indicated by the inversion sign ○, and the input, the voltage at which is in phase with the output, is non-inverting. The op-amp amplifies the differential (difference) voltage between the inputs. The operational amplifier also contains pins for supplying the supply voltage and may contain frequency correction (FC) pins and balancing pins (NC). To facilitate understanding of the purpose of the outputs and increase the information content in the symbol, it is allowed to introduce one or two additional fields on both sides of the main field, in which labels characterizing the output functions are indicated (Fig. 10, b). Currently operational amplifiers are produced in the form of integrated circuits. This allows us to consider them as separate components with certain parameters.
The parameters and characteristics of an op-amp can be divided into input, output and transmission characteristics.
Input parameters.
Rice. 10. Symbol of operational amplifier: a – without additional field; b – with an additional field; NC – balancing terminals; FC – frequency correction outputs; U – supply voltage terminals; 0V – common output
Transmission characteristics.
Voltage Gain TO U (10 3 – 10 6)
,
Where U input1 , U vx2– voltage at the inputs of the op-amp.
Common Mode Ratio TO U sf
.
Common mode rejection ratio TO os sf
.
The unity gain frequency f 1 is the frequency at which the voltage gain is equal to unity (units are tens of MHz).
The rate of rise of the output voltage V U out is the maximum possible rate of change of the output signal.
Output parameters.
Maximum output voltage of the op amp U out max. Typically, this voltage is 2-3 V lower than the power supply voltage.
Output resistance Rout (tens - hundreds of Ohms).
Basic circuits for connecting an operational amplifier.
Op amps are typically used with deep negative feedback because they have significant voltage gain. In this case, the resulting parameters of the amplifier depend on the elements of the feedback circuit.
Depending on which input of the op-amp the input signal source is connected to, there are two main connection schemes (Fig. 11). When the input voltage is applied to the non-inverting input (Fig. 11, a), the voltage gain is determined by the expression
. (1)
This inclusion of an op-amp is used when increased input impedance is required. If in the diagram Fig. 11, and remove resistance R 1 and short-circuit resistance R 2, you get a voltage follower ( TO u=1), which is used to match the high impedance of the signal source and the low impedance of the receiver.
Rice. 11. Op-amp amplifier circuits: a – non-inverting amplifier; b – inverting amplifier
When the input voltage is applied to the inverting input (Fig. 11, b), the gain is equal to
. (2)
As can be seen from expression (2), with this connection, the input voltage is inverted.
In the considered circuits, a resistance R e is connected to one of the inputs. It does not affect gain and is introduced when necessary to reduce output voltage variations caused by temporary or temperature variations in input currents. Resistance R e is chosen such that the equivalent resistances connected to the op-amp inputs are the same. For the diagrams in Fig. 10 
.
By modifying the diagram in Fig. 11, b, you can get a summing device (Fig. 12, a), in which
. (3)
When voltage is simultaneously applied to both inputs of the op-amp, a subtractive device is obtained (Fig. 12, b), for which
. (4)
This expression is valid if the condition is met 
.
Rice. 12. Op-amp switching circuits: a – voltage adder; b – subtracting device
Multivibrator (from Latin I oscillate a lot) is a nonlinear device that converts constant voltage power supply into energy pulses of almost rectangular shape. 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 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 abrupt voltage growth 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 supplying a part 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 possible schemes, implementing 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 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.
