LED flasher or how to assemble a symmetrical multivibrator with your own hands. The circuit of a symmetrical multivibrator must be studied and collected in electronics clubs. The multivibrator circuit is one of the most famous and often used in various electronic designs. A symmetrical multivibrator during operation generates oscillations in shape approaching rectangular. The simplicity of the multivibrator is due to its design - it is only two transistors and several additional elements. The wizard invites you to assemble your first electronic LED flasher circuit. In order not to be disappointed in case of failure, below are detailed step-by-step instructions for assembling a multivibrator LED flasher with photo and video illustrations.
A little theory. A multivibrator is essentially a two-stage amplifier on transistors VT1 and VT2 with a positive feedback circuit through an electrolytic capacitor C2 between the amplification stages on transistors VT2 and VT1. This feedback turns the circuit into a generator. The name symmetrical multivibrator is due to the same values of the pairs of elements R1=R2, R3=R4, C1=C2. With such values of the elements, the multivibrator will generate pulses and pauses between pulses of equal duration. The pulse repetition rate is set to a greater extent by the values of the pairs R1=R2 and C1=C2. The duration of pulses and pauses can be controlled by LED flashes. If the equality of pairs of elements is violated, the multivibrator becomes asymmetrical. The asymmetry will be due primarily to the difference in the duration of the pulse and the duration of the pause.
The multivibrator is assembled on two transistors; in addition, four resistors, two electrolytic capacitors and two LEDs are required to indicate the operation of the multivibrator. The task of purchasing parts and a printed circuit board is easily solved. Here is a link to purchase a ready-made set of parts http://ali.pub/2bk9qh . The kit includes all parts, a good quality 28mm x 30mm printed circuit board, schematic, wiring diagram and specification sheet. There are practically no errors in the location of parts on the printed circuit board drawing.
Composition of the multivibrator parts kit
Let's start assembling the circuit; for work you will need a low-power soldering iron, soldering flux, solder, side cutters and batteries. The circuit is simple, but it must be assembled correctly and without errors.
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 are 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 structures, 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. Good luck and growth of soldering skills.
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 simultaneously. 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.
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
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.
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.
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, 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.
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.
A transistor multivibrator is a square wave generator. Below in the photo is one of the oscillograms of a symmetrical multivibrator.
A symmetrical multivibrator generates rectangular pulses with a duty cycle of two. You can read more about duty cycle in the article frequency generator. We will use the operating principle of a symmetrical multivibrator to alternately turn on the LEDs.
The scheme consists of:
– two KT315B (can be with any other letter)
– two capacitors with a capacity of 10 microFarads
– four, two 300 Ohm and two 27 KiloOhm
– two Chinese 3 Volt LEDs
This is what the device looks like on a breadboard:
And this is how it works:
To change the blinking duration of the LEDs, you can change the values of capacitors C1 and C2, or resistors R2 and R3.
There are also other types of multivibrators. You can read more about them. It also describes the operating principle of a symmetrical multivibrator.
If you are too lazy to assemble such a device, you can buy a ready-made one;-) I even found a ready-made device on Alika. You can look it up at this link.
Here is a video that describes in detail how a multivibrator works:
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 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.
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 under consideration is similar to that discussed earlier.
This article describes a device designed simply so that a novice radio amateur (electrician, electronics engineer, etc.) can better understand the circuit diagrams and gain experience during the assembly of this device. Although it is possible that this simplest multivibrator, which is described below, can also find practical application. Let's look at the diagram:
Figure 1 - The simplest multivibrator on a relay
Connecting resistor R2
Connecting a capacitor
Connecting resistor R1
Connecting the relay contacts to its winding
Connecting wires for power supply
You can buy a relay at a radio parts store or get it from old broken equipment. For example, you can desolder relays from boards from refrigerators:
If the relay has bad contacts, you can clean them a little.
