Perfection is not achieved when there is nothing left to add,
and then when there is nothing to remove.
Antoine de Saint-Exupery
Some people consider surface mounting difficult to implement at home due to the small size of SMD elements and... the lack of holes for parts leads.
This is partly true, but upon careful examination it turns out that the small size of the elements simply requires careful installation, of course, provided that we are talking about simple SMD components that do not require special equipment for installation. The absence of reference points, which are holes for the pins of parts, only creates the illusion of difficulty in making a printed circuit board design.
You need practice in creating simple designs on SMD elements in order to acquire skills, self-confidence, and be convinced of the prospects of surface mounting for yourself personally. After all, the process of manufacturing a printed circuit board is simplified (there is no need to drill holes or mold parts leads), and the resulting gain in installation density is noticeable to the naked eye.
The basis of our designs is an asymmetrical multivibrator circuit using transistors of various structures.
We will assemble a “flashing light” on an LED, which will serve as a talisman, and we will also create a foundation for future designs by making a prototype of a microcircuit that is popular among radio amateurs, but not entirely accessible.
Rice. 1. Single-ended multivibrator circuit
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Thank you for your attention!
Igor Kotov, editor-in-chief of Datagor magazine
Author of the books “For a young radio amateur to read with a soldering iron”, “Secrets of amateur radio craftsmanship”, co-author of the series of books “To be read with a soldering iron” in the publishing house “SOLON-Press”, I have publications in the magazines “Radio”, “Instruments and Experimental Techniques”, etc. .
The multivibrator circuit shown in Figure 1 is a cascade connection of transistor amplifiers where the output of the first stage is connected to the input of the second through a circuit containing a capacitor and the output of the second stage is connected to the input of the first through a circuit containing a capacitor. Multivibrator amplifiers are transistor switches that can be in two states. The multivibrator circuit in Figure 1 differs from the trigger circuit discussed in the article "". Because it has reactive elements in the feedback circuits, the circuit can therefore generate non-sinusoidal oscillations. You can find the resistance of resistors R1 and R4 from relations 1 and 2:
Where I KBO = 0.5 μA is the maximum reverse collector current of the KT315a transistor,
Ikmax=0.1A is the maximum collector current of the KT315a transistor, Up=3V is the supply voltage. Let's choose R1=R4=100Ohm. Capacitors C1 and C2 are selected depending on the required oscillation frequency of the multivibrator.
Figure 1 - Multivibrator based on KT315A transistors
You can relieve the voltage between points 2 and 3 or between points 2 and 1. The graphs below show how approximately the voltage will change between points 2 and 3 and between points 2 and 1.
T - oscillation period, t1 - time constant of the left arm of the multivibrator, t2 - time constant of the right arm of the multivibrator can be calculated using the formulas:
You can set the frequency and duty cycle of the pulses generated by the multivibrator by changing the resistance of trimming resistors R2 and R3. You can also replace capacitors C1 and C2 with variable (or trimmer) capacitors and, by changing their capacitance, set the frequency and duty cycle of the pulses generated by the multivibrator, this method is even more preferable, so if there are trimmer (or better variable) capacitors, then it is better to use them, and in place set variable resistors R2 and R3 to constant ones. The photo below shows the assembled multivibrator:
In order to make sure that the assembled multivibrator works, a piezodynamic speaker was connected to it (between points 2 and 3). After applying power to the circuit, the piezo speaker began to crackle. Changes in the resistance of the tuning resistors led to either an increase in the frequency of the sound emitted by the piezodynamics, or to its decrease, or to the fact that the multivibrator stopped generating.
A program for calculating the frequency, period and time constants, duty cycle of pulses taken from a multivibrator:
If the program does not work, then copy its html code into notepad and save it in html format.
If you are using the Internet Explorer browser and it is blocking the program, you must allow the blocked content.
Electronic computing is a relatively young scientific and technical area, but it has a most revolutionary impact on all areas of science and technology, on all aspects of social life. Characteristic is the constant development of the computer element base. The element base is developing very quickly; New types of logical circuits appear, existing ones are modified. There are many different electronic devices: logic elements, registers, adders, decoders, multiplexers, counters, frequency dividers, flip-flops, generators, etc.
Generators convert the energy of the power source into the energy of periodic or quasi-periodic electrical oscillations. The main purpose of generators in electronics is the generation of initial setting and synchronization pulses, control signals of various shapes and durations.
The entire variety of generators can be divided into the following types:
Rectangular pulse generators;
Linear voltage generators (LIN);
Step-variable voltage generators;
Sine Wave Generators
Typical square wave shapes are shown in Fig. 1
Rectangular pulse generators that have energy-storing elements in the feedback loop are called multivibrators.
Multivibrators are divided into two groups:
Self-oscillating multivibrators;
Waiting multivibrators or monovibrators.
The main difference between these multivibrators is that self-oscillating multivibrators form a pulse sequence when supply voltage is applied to the circuit, since they have two feedback circuits with energy storage devices, and standby multivibrators form a single pulse with specified parameters for external triggering, since one the feedback loop has no energy storage. A monovibrator is something between a multivibrator and a flip-flop.
There are soft and hard excitation modes of multivibrators. In soft mode, any changes in voltage in the feedback circuit at the moment the power is turned on lead to the occurrence of a generation mode; in hard mode, generation occurs when the voltage in the feedback circuit reaches a certain threshold.
Multivibrators are divided into restartable and non-restartable. In the first case, when a trigger pulse is applied, the generation of output signals begins anew from the initial state. Restarts allow you to unlimitedly increase the duration of the output pulse, regardless of the parameters of the multivibrator circuit. Non-restartable multivibrators do not respond to external trigger pulses
The high input resistance of field-effect transistors (FETs) makes it possible to design multivibrators for very low pulse repetition frequencies with small capacitances of timing capacitors. Thanks to this, the shape of the output pulses is less distorted, and the duty cycle is greater than that of multivibrators based on bipolar transistors.
For self-oscillating multivibrators, PTs with a control p-n junction are most suitable, since during the charging of capacitors the voltage in the gate-source section is applied in the forward direction and therefore the resistance of this section is low and the charging time of the capacitors becomes short.
The circuit of PT multivibrators with a control p-n junction and a p-type channel is shown in Fig. 2. In this multivibrator, through resistors, a small negative voltage is applied to the gate relative to the source, which increases the stability of the oscillation period and the duration of the output pulses. Unlike a multivibrator on power supply transistors, the operation of the device is not disrupted if the resistors are connected between the gate and the common point (circuit with a “zero” gate ).
The timing diagrams of the operation of an asymmetrical multivibrator are shown in Fig. 3. In basic terms, the operating principle of this multivibrator is the same as that of a tube multivibrator. What distinguishes it from a BT multivibrator is that in temporarily stable equilibrium states, the discharge of capacitors occurs almost exclusively through resistors and not to zero voltage, but to a value at which the gate voltage becomes equal to the cutoff voltage (usually 1-6 V)
To generate rectangular pulses with frequencies above, you can use circuits that work on the same principle as the circuit in Fig. 18.32. As shown in Fig. 18.40, a simple differential amplifier is used as a comparator in such circuits.
Positive feedback in the Schmitt trigger circuit is provided by directly connecting the amplifier output to its input, i.e., the resistance of the resistor in the voltage divider is chosen equal to zero. According to formula (18.16), such a scheme should have resulted in an infinitely long period of oscillation, but this is not entirely true. When deriving this equation, it was assumed that the amplifier used as a comparator has an infinitely large gain, i.e. that the circuit switching process occurs when the input voltage difference is equal to zero. In this case, the switching threshold of the circuit will be equal to the output voltage, and the voltage on capacitor C will reach this value only after a very long time.
Rice. 18.40 Multivibrator based on a differential amplifier.
The differential amplifier circuit on the basis of which the generator is made in Fig. 18.40, has a fairly low gain. For this reason, the circuit will switch even before the difference between the amplifier's input signals reaches zero. If, for example, such a scheme is implemented as shown in Fig. 18.41, based on a linear amplifier manufactured using ESL technology (for example, based on an integrated circuit, the difference in input signals at which the circuit switches will be approximately. When the output voltage amplitude is about typical for circuits made on the basis of ESL technology, the pulse period the generated signal is equal to
The considered circuit allows you to generate a pulse voltage with a frequency of up to
A similar generator can also be made based on TTL circuits. A ready-made Schmitt trigger chip (for example, 7414 or 74132) is suitable for these purposes, since it already has internal positive feedback. The corresponding connection of such a microcircuit is shown in Fig. 18.42. Since the input current of the TTL element must flow through the Schmitt trigger resistor, its resistance should not exceed 470 Ohms. This is necessary for confident switching of the circuit at the lower threshold. The minimum value of this resistance is determined by the output load capacity of the logic element and is equal to about 100 Ohms. The Schmitt trigger thresholds are 0.8 and 1.6 V. For an output signal amplitude of about 3 V, typical for TTL-type ICs, the pulse frequency of the generated signal is
The maximum achievable frequency value is about 10 MHz.
The highest generation frequencies are achieved when using special multivibrator circuits with emitter connections (for example, microcircuits or The circuit diagram of such a multivibrator is shown in Fig. 18.43. In addition, these integrated circuits are equipped with additional final stages made on the basis of TTL or ESL circuits.
Let's consider the principle of operation of the circuit. Let us assume that the amplitude of alternating voltages at all points of the circuit does not exceed the value When the transistor is closed, the voltage at its collector is almost equal to the supply voltage. The voltage at the emitter of the transistor is the Emitter Current
Rice. 18.41. Multivibrator based on a linear amplifier made using ESL technology.
Rice. 18.42. Multivibrator based on a Schmitt trigger, made using TTL technology. Frequency
Rice. 18.43. Multivibrator with emitter connections.
transistor is equal In order for a signal of the desired amplitude to be released at the resistor, its resistance must be Then in the considered state of the circuit, the voltage at the emitter of the transistor will be equal to . During the time when the transistor is closed, the current of the left source according to the circuit flows through capacitor C. As a result, the voltage at the emitter of the transistor decreases at a rate
Transistor T opens when the voltage at its emitter decreases to the value. In this case, the voltage at the base of the transistor decreases by 0.5 V and the transistor closes, and the voltage at its collector increases to the value Due to the presence of an emitter follower on the transistor, the voltage at the collector of the transistor increases with increasing voltage also the transistor base voltage. As a result, the voltage at the emitter of the transistor increases abruptly to this value. This voltage jump through capacitor C is transmitted to the emitter of the transistor so that the voltage at this point increases abruptly from to
During the time when the transistor is closed, the current flowing through capacitor C causes the voltage at the emitter of the transistor to decrease at a rate
The transistor remains off until its emitter potential drops from value to value For a transistor this time is
