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 each and two 27 KiloOhm each
– 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:
If you look at it, all electronics consists of a large number of individual bricks. These are transistors, diodes, resistors, capacitors, inductive elements. And from these bricks you can build anything you want.
From a harmless children's toy that makes, for example, the sound of “meow”, to the guidance system of a ballistic missile with a multiple warhead for eight megaton charges.
One of the very well-known and often used circuits in electronics is a symmetrical multivibrator, which is an electronic device that produces (generates) oscillations in shape, approaching rectangular.
The multivibrator is assembled on two transistors or logic circuits with additional elements. Essentially, this is a two-stage amplifier with a positive feedback circuit (POC). This means that the output of the second stage is connected through a capacitor to the input of the first stage. As a result, the amplifier turns into a generator due to positive feedback.
In order for the multivibrator to start generating pulses, it is enough to connect the supply voltage. Multivibrators can be symmetrical And asymmetrical.
The figure shows a circuit of a symmetrical multivibrator.
In a symmetrical multivibrator, the values of the elements of each of the two arms are absolutely the same: R1=R4, R2=R3, C1=C2. If you look at the oscillogram of the output signal of a symmetrical multivibrator, it is easy to notice that the rectangular pulses and pauses between them are the same in time. t pulse ( t and) = t pause ( t p). Resistors in the collector circuits of transistors do not affect the pulse parameters, and their value is selected depending on the type of transistor used.
The pulse repetition rate of such a multivibrator is easily calculated using a simple formula:
Where f is the frequency in hertz (Hz), C is the capacitance in microfarads (µF) and R is the resistance in kilo-ohms (kOhm). For example: C = 0.02 µF, R = 39 kOhm. We substitute it into the formula, perform the actions and get a frequency in the audio range approximately equal to 1000 Hz, or more precisely 897.4 Hz.
In itself, such a multivibrator is uninteresting, since it produces one unmodulated “squeak”, but if the elements select a frequency of 440 Hz, and this is the A note of the first octave, then we will get a miniature tuning fork, with which you can, for example, tune a guitar on a hike. The only thing you need to do is add a single transistor amplifier stage and a miniature speaker.
The following parameters are considered to be the main characteristics of a pulse signal:
Frequency. Unit of measurement (Hz) Hertz. 1 Hz – one oscillation per second. Frequencies perceived by the human ear are in the range of 20 Hz – 20 kHz.
Pulse duration. It is measured in fractions of a second: miles, micro, nano, pico and so on.
Amplitude. In the multivibrator under consideration, amplitude adjustment is not provided. Professional devices use both step and smooth amplitude adjustment.
Duty factor. The ratio of the period (T) to the pulse duration ( t). If the pulse length is 0.5 periods, then the duty cycle is two.
Based on the above formula, it is easy to calculate a multivibrator for almost any frequency with the exception of high and ultra-high frequencies. There are slightly different physical principles at work there.
In order for the multivibrator to produce several discrete frequencies, it is enough to install a two-section switch and five or six capacitors of different capacities, naturally identical in each arm, and use the switch to select the required frequency. Resistors R2, R3 also affect the frequency and duty cycle and can be made variable. Here is another multivibrator circuit with adjustable switching frequency.
Reducing the resistance of resistors R2 and R4 to less than a certain value, depending on the type of transistors used, can cause generation failure and the multivibrator will not work, therefore, in series with resistors R2 and R4, you can connect a variable resistor R3, which can be used to select the switching frequency of the multivibrator.
The practical applications of a symmetrical multivibrator are very extensive. Pulse computing technology, radio measuring equipment in the production of household appliances. A lot of unique medical equipment is built on circuits based on the same multivibrator.
Due to its exceptional simplicity and low cost, the multivibrator has found wide application in children's toys. Here is an example of a regular LED flasher.
With the values of electrolytic capacitors C1, C2 and resistors R2, R3 indicated in the diagram, the pulse frequency will be 2.5 Hz, which means the LEDs will flash approximately twice per second. You can use the circuit proposed above and include a variable resistor together with resistors R2, R3. Thanks to this, it will be possible to see how the flash frequency of the LEDs will change when the resistance of the variable resistor changes. You can install capacitors of different ratings and observe the result.
While still a schoolboy, I assembled a Christmas tree garland switch using a multivibrator. Everything worked out, but when I connected the garlands, my device began to switch them with a very high frequency. Because of this, the TV in the next room began to show wild interference, and the electromagnetic relay in the circuit crackled like a machine gun. It was both joyful (it works!) and a little scary. The parents were quite alarmed.
Such an annoying mistake with too frequent switching did not give me peace. And I checked the circuit, and the capacitors were at their nominal value. I didn't take into account only one thing.
The electrolytic capacitors were very old and dried out. Their capacity was small and did not at all correspond to what was indicated on their body. Due to the low capacitance, the multivibrator operated at a higher frequency and switched the garlands too often.
At that time I did not have instruments that could measure the capacitance of capacitors. Yes, and the tester used a pointer, and not a modern digital multimeter.
Therefore, if your multivibrator produces an excessive frequency, then first check the electrolytic capacitors. Fortunately, now you can buy a universal radio component tester for little money, which can measure the capacitance of a capacitor.
A multivibrator is a device for creating non-sinusoidal oscillations. The output produces a signal of any shape other than a sine wave. The signal frequency in a multivibrator is determined by resistance and capacitance, rather than inductance and capacitance. The multivibrator consists of two amplifier stages, the output of each stage is fed to the input of the other stage.
Multivibrator operating principle
A multivibrator can create almost any waveform, depending on two factors: the resistance and capacitance of each of the two amplifier stages and where the output is taken from in the circuit.
For example, if the resistance and capacitance of two stages are equal, one stage conducts 50% of the time and the other stage conducts 50% of the time. For the discussion of multivibrators in this section, it is assumed that the resistance and capacitance of both stages are equal. When these conditions exist, the output signal is a square wave.
Bistable multivibrators (or “flip-flops”) have two stable states. At steady state, one of the two amplifier stages is conducting and the other stage is not conducting. In order to move from one stable state to another, a bistable multivibrator must receive an external signal.
This external signal is called an external trigger pulse. It initiates the transition of the multivibrator from one state to another. Another trigger pulse is needed to force the circuit back to its original state. These trigger pulses are called "start" and "reset".
Apart from the bistable multivibrator, there are also a monostable multivibrator, which has only one stable state, and an astable multivibrator, which has no stable state.
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 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 in 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....
Simple circuits of homemade LED flashers based on transistor multivibrators. Figure 1 shows a multivibrator circuit that switches two LEDs. The LEDs blink alternately, that is, when HL1 is on, the HL2 LED is not on, but vice versa.
You can mount the diagram into a Christmas tree toy. When the power is turned on, the toy will flash. If the LEDs are of different colors, then the toy will simultaneously blink and change the color of the glow.
The blinking frequency can be changed by selecting the resistances of resistors R2 and R3; by the way, if these resistors are not of the same resistance, you can ensure that one LED glows longer than the other.
But two LEDs are somehow not enough for even the smallest tabletop Christmas tree. Figure 2 shows a circuit that switches two strings of three LEDs. There are more LEDs, and so is the voltage required to power them. Therefore, now the source is not 5-volt, but 9-volt (or 12-volt).
Fig.1. Circuit of the simplest flasher using LEDs and transistors.
Fig.2. Circuit of a simple flasher with six LEDs and two transistors.
Rice. 3. LED flasher circuit with powerful outputs for load.
As a power source, you can use a power supply from an old television game console like “Dandy” or buy an inexpensive “mains adapter” with an output voltage of 9V or 12V in the store.
And yet, even six LEDs are not enough for a home Christmas tree. It would be nice to triple the number of LEDs. Yes, and use not simple LEDs, but extremely bright ones. But, if each garland already has nine LEDs connected in series, and even super bright ones, then the total voltage required for their glow will already be 2.3Vx9=20.7V.
Plus, a few more volts are needed for the multivibrator to function. Moreover, the “network adapters” on sale are usually inexpensive ones, no more than 12V.
You can get out of this situation if you divide the LEDs into three groups of three. And turn on the groups in parallel. But this will lead to an increase in current through the transistors and disrupt the operation of the multivibrator. However, it is possible to make additional amplification stages using two more transistors (Fig. 3).
Two garlands are good, but they just blink alternately. If only there were at least three! For such a case, there is a so-called “three-phase multivibrator” circuit. It is shown in Figure 4.
Fig.4. Multivibrator circuit with three transistors.
If you turn on LED garlands in the collector circuits of transistors (Fig. 5), you will get a kind of running fire effect. The speed of reproduction of the light effect can be adjusted by replacing capacitors C1, C2 and C3 with capacitors of other capacities. And also replacing resistors R2, R4, R6 with resistors of a different resistance. As capacitance or resistance increases, the LED switching speed decreases.
Rice. 5. Multivibrator circuit to obtain the effect of running fire.
And in Figure 6 there is a more powerful version with 27 LEDs. In the “flashing lights” according to the diagrams in Figures 3 and 6, you can use almost any LEDs, but it is still desirable to be super bright or super bright.
Rice. 6. Diagram of a more powerful flasher with 27 LEDs.
Installation can be done on prototype printed circuit boards, which are sold in radio parts stores. Or without boards at all, soldering the parts together.
