Powerful asymmetrical transistor multivibrator. A selection of simple and effective schemes
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 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
When power is applied to the circuit, the capacitor begins to charge through resistor R1, the contacts K1.1 are open, when the capacitor is charged to a certain voltage, the relay will operate and the contacts close, when the contacts are closed, the capacitor will begin to discharge through these contacts and resistor R2, when the capacitor is discharged to a certain voltage, the contacts will open and the process will then be repeated cyclically. This multivibrator works because the relay operating current is greater than the holding current. The resistance of the resistors CANNOT be changed within wide limits and this is a disadvantage of this circuit. The resistance of the power supply affects the frequency and because of this, this multivibrator will not work from all power sources. The capacitance of the capacitor can be increased, but the frequency of contact closure will decrease. If the relay has a second group of contacts and large capacitance values are used, then this circuit can be used to periodically automatically turn on/off devices. The assembly process is shown in the photos below:
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.
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 rectangular pulse generator - a 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. Blinks despite clumsy installation done on 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 the 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.
PCB drawings using laser printer I printed it on photo paper. 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 overboard with the solder. As a result, the LED “hangers” were lit not at intervals, but synchronously. 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.
It seems that I told everything that I planned. If you missed something, write in the comments - I’ll add what’s needed, and what’s not needed, I’ll correct it. I'm always happy to receive comments!
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Best regards, Vladimir Vasiliev.
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is a pulse generator of almost rectangular shape, created in the form of an amplifying element with a positive circuit feedback. 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 is easy to see that it practically copies the circuit diagram of a symmetrical trigger. The only difference is that the connections between the switching blocks, both direct and reverse, are carried out using alternating current, and not direct current. This radically changes the features of the device, since in comparison with a symmetrical trigger, the multivibrator circuit does not have stable equilibrium states in which it could 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....
Multivibrator.
The first circuit is the simplest multivibrator. Despite its simplicity, its scope is very wide. None electronic device can't do without it.
The first figure shows its circuit diagram.
LEDs are used as a load. When the multivibrator is working, the LEDs switch.
For assembly you will need a minimum of parts:
1. Resistors 500 Ohm - 2 pieces
2. Resistors 10 kOhm - 2 pieces
3. Electrolytic capacitor 47 uF for 16 volts - 2 pieces
4. Transistor KT972A - 2 pieces
5. LED - 2 pieces
KT972A transistors are composite transistors, that is, their housing contains two transistors, and it is highly sensitive and can withstand significant current without heat sink.
Once you have purchased all the parts, arm yourself with a soldering iron and start assembling. To conduct experiments, you do not need to make a printed circuit board; you can assemble everything using a surface-mounted installation. Solder as shown in the pictures.
Let your imagination tell you how to use the assembled device! For example, instead of LEDs, you can install a relay, and use this relay to switch a more powerful load. If you change the values of resistors or capacitors, the switching frequency will change. By changing the frequency you can achieve very interesting effects, from a squeak in the dynamics to a pause for many seconds..
Photo relay.
And this is a diagram of a simple photo relay. This device can be successfully used wherever you want, to automatically illuminate the DVD tray, to turn on the light, or to alarm against intrusion into a dark closet. Two schematic options are provided. In one embodiment, the circuit is activated by light, and in the other by its absence.
It works like this: when light from the LED hits the photodiode, the transistor will open and LED-2 will start to glow. The sensitivity of the device is adjusted using a trimming resistor. As a photodiode, you can use a photodiode from an old ball mouse. LED - any infrared LED. The use of infrared photodiode and LED will avoid interference from visible light. Any LED or a chain of several LEDs is suitable as LED-2. An incandescent lamp can also be used. And if you install an electromagnetic relay instead of an LED, you can control powerful incandescent lamps or some mechanisms.
The figures show both circuits, the pinout (location of the legs) of the transistor and LED, as well as the wiring diagram.
If there is no photodiode, you can take an old MP39 or MP42 transistor and cut off its housing opposite the collector, like this:
Instead of a photodiode, you will need to include in the circuit p-n junction transistor. You will have to determine experimentally which one will work better.
Power amplifier based on TDA1558Q chip.
This amplifier has an output power of 2 X 22 watts and is simple enough for beginner hams to replicate. This diagram will be useful for you homemade speakers, or for a homemade music center that can be made from an old MP3 player.
To assemble it you will need only five parts:
1. Microcircuit - TDA1558Q
2. Capacitor 0.22 uF
3. Capacitor 0.33 uF – 2 pieces
4. Electrolytic capacitor 6800 uF at 16 volts
The microcircuit has a fairly high output power and will need a radiator to cool it. You can use a heatsink from the processor.
The entire assembly can be done by surface mounting without the use of a printed circuit board. First, you need to remove pins 4, 9 and 15 from the microcircuit. They are not used. The pins are counted from left to right if you hold it with the pins facing you and the markings facing up. Then carefully straighten the leads. Next, bend pins 5, 13 and 14 up, all these pins are connected to the power positive. The next step is to bend pins 3, 7 and 11 down - this is the power supply minus, or “ground”. After these manipulations, screw the chip to the heat sink using thermal conductive paste. The pictures show the installation from different angles, but I will still explain. Pins 1 and 2 are soldered together - this is the input of the right channel, a 0.33 µF capacitor must be soldered to them. The same must be done with pins 16 and 17. The common wire for the input is the minus power supply or ground.
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 schemes have certain conditions; states with fast transitions are called switching processes, where rapid change 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 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 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 the current to ground is due to short circuit Q1, and VC2 = Vcc, since all the voltage across VC2 drops due to TR2 being 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 are interconnected electrical circuit and get acquainted with the processes taking place 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. At practical implementation For this circuit, I recommend powering it with a voltage of 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.
