|
Arabic
Bengali
Chinese
English
French
German
Hebrew
Hindi
Italian
Japanese
Korean
Malay
Polish
Portuguese
Spanish
Turkish
Ukrainian
Vietnamese|
BOOKS AND ARTICLES
Self-oscillating multivibrator. Radio - for beginners
Directory / Radio - for beginners A diagram of one of the variants of a self-oscillating multivibrator is shown in fig. 1, a. It should remind you of the well-known two-transistor symmetrical multivibrator circuit.
But here the function of the active elements of the multivibrator is performed by logical elements 2I-NOT, included by inverters. Thanks to two positive feedback circuits - the output of the element DD1.2 through the capacitor C1 with the input DD1.1 and the output of the element DD1.1 through the capacitor C2 with the input DD1.2, the device is self-excited and generates electrical impulses. The repetition rate of the generated pulses depends on the values of the specified capacitors and resistors R1 and R2. What are electrical impulses? If the constant voltage is jumpy and changes its value at regular (in a particular case) intervals, taking alternately a high level, then a low one, then this type of signal is commonly called a pulse sequence or a pulse sequence. Those segments of this sequence, when the voltage takes a high level, are called high-level impulses; pauses between them are segments with a low level of tension. However, with the same reason we can talk about low-level impulses; in this case the pauses will be high. In general, the duration of the pulses may not be equal to the duration of the pauses between them. The ratio of these durations is estimated by such a parameter as the duty cycle, which shows how many times the period of the sequence is greater than the pulse duration. The moment of occurrence of an impulse of both a high level and a low level is commonly called the front of the impulse, and the moment of termination is the decay of the impulse. It is clear that for a high-level pulse, the front is a positive (or plus) voltage drop - from low to high, and the decline is a negative (negative) voltage drop when the level changes from high to low. It is also understood that the edge of the high level pulse is the fall of the low level pulse and vice versa. To mount the multivibrator on a packet panel, you just need to connect these capacitors and resistors to the corresponding pins of the DD1 chip (Fig. 1, b). Check the installation - for errors - and especially carefully check the polarity of the inclusion of oxide capacitors. Connect a power source to the breadboard, and a voltmeter to the output of the second logic element. What does the voltmeter needle show? DC voltage intermittently, about 30 times per minute, rapidly increasing to a high level and also rapidly decreasing to a low level. The multivibrator therefore generates pulses with a repetition rate of about 0,5 Hz. Then connect a voltmeter in parallel with the output of the first element. You will see that the arrow will also record the transitions of the logical element from the zero state to the one state, and vice versa, with the same frequency as in the previous case. This means that electrical impulses can also be taken from this output, but with respect to the impulses at the output of the second element, they will be shifted in phase by 180 °. What experiments can be done with our multivibrator? First of all, try to simultaneously increase the capacitance of both capacitors, for example, twice, by connecting the same capacitor in parallel to each of them, and then replace them with capacitors with a capacity of 100 ... .200 microfarads. In the first case, the pulse repetition rate will decrease, in the second it will increase. You can change the capacitance of only one capacitor, for example C1. This will change not only the frequency, but also the ratio of the duration of the pulses and pauses between them, however, according to the circuit design, the multivibrator will remain symmetrical. Capacitors can be 1 ... 5 microfarads. Then the frequency of the generated pulses will increase to approximately 500.. .1000 Hz. These are already oscillations of the sound frequency, and the needle of the voltmeter, due to its inertia, cannot react to them. In order to verify the operation of the multivibrator in this case, you need to connect headphones through a capacitor with a capacity of 0,01 ... 0,015 μF to its output - you will hear a tonal sound in them. By replacing now one of the fixed resistors with a variable of the same value, you can smoothly change the frequency of the generated pulses within certain limits, which means the tone of the sound in the phones. It is possible that the multivibrator you have assembled is unstable, it is not always excited after replacing parts, with a slightly reduced power supply voltage. The reason for this is some criticality of the resistor values at the input of logic elements due to the peculiarities of the emitter input of TTL microcircuits. The essence of these features is as follows. The resistor at the input of the logic element, which forms one of the arms of the multivibrator, is included in the emitter circuit of the input transistor of the microcircuit element. The emitter current creates a voltage drop across this resistor, turning off the transistor. With a relatively large resistance of the resistor (more than 2,2 ... 2,6 kOhm), the voltage drop across it turns out to be so significant that the transistor practically does not respond to the input signal. And vice versa, with a low resistance of the resistor (no more than 600.. .700 Ohm), the input transistor of the element is always open to saturation and, therefore, turns out to be uncontrollable by input signals. Thus, for the reliable operation of the multivibrator of this variant, the resistance of the input resistors of the logic elements must be within 800 Ohm ... 2,2 kOhm. By appropriate selection of these resistors, stable operation of the multivibrator can be achieved. In addition, it must be remembered that the operation of the multivibrator is affected by the spread of microcircuit parameters, the instability of the power supply voltage, and significant changes in ambient temperature. I must say that the diagrams often depict a symmetrical multivibrator as shown in Fig. 10, c. More stable in operation is a multivibrator based on three logic elements without resistors in their input circuit, assembled, for example, according to the circuit in Fig. 2, a. All elements are connected by inverters and connected in series. The time-setting circuit that determines the generation frequency is formed by the capacitor C1 and the resistor R1. Mount the details of this version of the self-oscillating multivibrator on the same breadboard panel (Fig. 2, b). On it, place the details of the multivibrator operation indicator shown on the panel on the right. The indicator transistor VT1 (Fig. 2, c), powered by the same source as the microcircuit, operates in the switching mode - like an electronic key. When the element DD1.3 of the multivibrator is in a single state (the voltage at its output corresponds to a high level), the transistor is open and the incandescent lamp HL1 in its collector circuit is on. When the element goes to the zero state, the lamp goes out. By the glow of the signal lamp, you will judge the frequency and duration of the generated pulses. However, it is also possible to indicate the state of any of the elements of the multivibrator using a DC voltmeter, as was done in experiments with the first multivibrator. After checking the installation, turn on the power. The multivibrator will immediately begin to generate electrical impulses, which will be indicated by a periodically flashing signal lamp. Calculate how many flashes there will be per minute. It should be about 60. If so, then the multivibrator pulse frequency is 1 Hz.
Connect a second capacitor of the same capacity in parallel with capacitor C1. The pulse frequency should decrease by about half. The same change in the pulse frequency can be achieved by increasing the resistance of the resistor. Check this, and then replace the resistor with a variable with a nominal resistance of 1,5 ... 1,8 kOhm. Now, using only this resistor, you can smoothly change the frequency of the multivibrator within 0,5 ... 20 Hz. The highest frequency will be in the case when the resistor is completely removed from the circuit, i.e., pins 8 and 1 of the microcircuit will be closed. And if the capacitance of the capacitor is 1 uF? In this case, only a variable resistor will be able to change the frequency of the multivibrator from about 300 Hz to 10 kHz. To make sure that the multivibrator works at such a frequency, the indicator light will have to be replaced with acoustic headphones (or a capsule from them). What is the principle of operation of such a variant of a self-oscillating multivibrator? Let's return to its schematic diagram (Fig. 2, a). After turning on the power, one of the logical elements will take one of two possible states faster than others and thereby affect the state of the remaining elements. Let's assume that element DD1.2 was the first to be in the single state. A high-level signal from its output through an uncharged capacitor C1 is transmitted to the input of element DD1.1, as a result of which this element is set to zero. The element DD1.3 is in the same state, since there is a high voltage level at its inputs. This electrical state of the device is unstable, since the voltage at the input of the element DD1.1 at this time gradually decreases as the capacitor C1 is charged through the resistor R1 and the output circuit of the element DD1.3. As soon as the voltage at the input of the DD1.1 element becomes equal to the threshold, this element will switch to a single state, and the DD1.2 element will switch to zero. Now the capacitor C1 will begin to recharge through the output of the element DD1.2 (at its output at this time the voltage is low) and the resistor R1 from the output of the element DD1.3. Soon the voltage at the input of the first element of the multivibrator will exceed the threshold and all elements will switch to opposite states. This is how electrical impulses are formed at the output of our multivibrator - output 8 of the DD1.3 element. However, the generated pulses can also be taken from the output of the 6-output element DD1.2 of the multivibrator Now, having understood the operation of a three-element multivibrator, exclude element DD1.3 from it and switch the right (according to the diagram) output of the resistor to the output of the first element, as shown in Fig. 3. The multivibrator has become a two-element. By connecting a light indicator to its output, you will make sure that the frequency of the generated pulses remains the same - 1 Hz. As in previous versions of the multivibrator, it will change when parts of other ratings are installed.
How does this version of the pulse generator work? Basically the same as the three-element. When, for example, the element DD1.1 is in a single state, and the element DD1.2 is in zero state, the capacitor C1 is charged through the resistor R1 and the output of the second element. As soon as the voltage at the input of the first element reaches the threshold, both elements switch to opposite states and the capacitor begins to recharge through the output circuit of the second element, the resistor and the output circuit of the first. When the voltage at the input of the first element drops to the threshold, the elements will switch back to the opposite state. It must be said that among the K155LLZ microcircuits there are such instances whose logical elements do not work stably enough in a two-element multivibrator. In such cases, it is necessary to include a resistor with a resistance of 1,2 ... 2 kOhm (R2, shown in Fig. 3 by a dashed line) between the input of the first element and the common wire of the device. It creates a constant voltage close to the threshold voltage at the input of the element, which facilitates the start-up and operating conditions of the multivibrator as a whole. Such variants of the multivibrator are widely used in digital technology to generate pulses of various frequencies and durations.
Traffic noise delays the growth of chicks
06.05.2024 Wireless speaker Samsung Music Frame HW-LS60D
06.05.2024 A New Way to Control and Manipulate Optical Signals
05.05.2024
▪ Graphene will be used in a quantum computer ▪ AMD Radeon Pro W6600X graphics card ▪ Developed a chip that can withstand the cold of space
▪ site section Parameters of radio components. Article selection ▪ article Power has changed! Popular expression ▪ article Which of the sex symbols most of his life could not experience an orgasm? Detailed answer ▪ article Personnel of x-ray departments. Standard instruction on labor protection ▪ article Pathfinder. Encyclopedia of radio electronics and electrical engineering
Home page | Library | Articles | Website map | Site Reviews
www.diagram.com.ua |
See other articles Section 
Latest news of science and technology, new electronics:
All languages of this page