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Practical acquaintance with the digital microcircuit. Radio - for beginners
Directory / Radio - for beginners In a variety of instruments and devices of digital technology, designed by radio amateurs, the K155LAZ chip is most widely used. We believe that practical acquaintance with microcircuits of this series should begin with it. The appearance and symbolic graphic designation of this microcircuit is shown in fig. 1. Structurally, it is a rectangular plastic case with 14 plate leads (some microcircuits of this series have 16 or even 24 leads) located along both long sides of the case. On top of the case there is a conditional key - a small round mark indicating the location of pin 1. The rest of the pins are counted from it. If you look at the microcircuit from above - from the side of the marking, then you need to count the conclusions counterclockwise, and if from below - then clockwise. This rule applies to all microcircuits, and not only the K155 series. What is the K155LAZ microcircuit structurally? It consists of four logical elements 2I-NOT (number 2 indicates the number of inputs of each element), powered by a common external DC voltage source.
Each of its logical elements works independently. It is not difficult to select elements by the pin numbers indicated on the graphic circuit designation of the microcircuit. So, input pins 1, 2 and output pin 3 refer to one of its elements, for example, the first, input 4, 5 and output 6 - to the second element, etc. Not shown in Fig. 1, b conclusions 7 and 14 of the microcircuit are used to supply power to all elements. It is not customary to depict these conclusions on the diagram so as not to clutter it up with power lines, and also because the elements are usually not located together on the circuit diagram of the device, as in Fig. 1b, a separately in different areas. The power supply chains of the elements remain common. Moreover, for the K.155LAZ microcircuit, output 14 must be connected to the positive, and output 7 to the negative poles of the power source. The K155LAZ microcircuit, like all other microcircuits of this series, is designed to be powered from a 5 V direct current source. You can also use a battery of galvanic cells with a voltage lower by 0,5 V, for example, a 3336 battery. decrease more, which, of course, will affect the operating mode of the microcircuit, and with a certain discharge of the battery, the microcircuit will generally stop working normally. Therefore, it is desirable to use a power supply that provides a stable voltage of 5 V. Such a power supply can be assembled, for example, according to the one shown in fig. 2 scheme. In it, the constant current source GB1 is two 3336 batteries connected in series. Power is supplied to the microcircuit through a voltage regulator formed by a zener diode VD1, a ballast resistor R3 and a regulating transistor VT1. The capacitance of the oxide capacitor C1 can be 20 ... 50 microfarads, and the ceramic or mica capacitor C2 - 0,033 ... 0,047 microfarads. How does the voltage regulator of such a microcircuit power supply work? Resistor R3 and zener diode VD1 form a battery voltage divider GB1. The voltage acting on the zener diode is equal to its stabilization voltage (for the KS168A zener diode it is 6,8 V). The voltage removed from the zener diode is fed through the trimmer resistor R2 to the base of the transistor VT1, and it opens. The greater the voltage at the base of this transistor (and hence the greater the base current), the more open it is, the greater the voltage at the output of the stabilizer and the current through its load. The voltage at the output of the unit, equal to 5 V, set the tuning (or variable) resistor R2 using a control DC voltmeter. The stabilizer will maintain such a voltage on the load practically unchanged when the voltage of the GB1 battery drops to 7 ... 7,5 V. Capacitor C1 smooths out ripples in the power supply circuit of the microcircuit at a low, and C2 at a high frequency of electrical oscillations, protecting the microcircuit from the influence of various electrical interferences on its operation. Resistor R1 is necessary so that even when the microcircuit is turned off, the stabilizer does not remain without load. The mock-up panel (Fig. 3, a), necessary for conducting experiments, checking the performance of simple devices and devices, can be made of fiberglass, getinaks or other sheet insulating material with a thickness of 1,5 ... 2 mm. In extreme cases, well-glued plywood, hardboard and even hard cardboard will do. The approximate dimensions of the panel are 120x80 mm. Strengthen pre-tinned copper conductors 1,2 ... 1,5 mm thick along its long sides - these will be the power lines. Over the entire remaining area, every 10 mm, drill holes with a diameter of 0,8 ... 1 mm, into which, as necessary, you will insert pieces of tinned wire (or narrow strips of tin), curved like loops - they will be temporary reference points for the leads of resistors, capacitors, mounting conductors. From below, at the corners of the panel, attach low legs-stands and proceed with the experiments. Place the microcircuit anywhere on the breadboard with the pins down, after bending their narrow ends so that they fit snugly against the panel. With segments of the mounting wire, connect output 14 of the microcircuit to the positive, and output 7 to the negative (common) power lines (Fig. 3, b). In order not to overheat the microcircuit during soldering, the power of the soldering iron should not exceed 40 W, and the duration of soldering the leads should not exceed 2 s.
After checking the reliability and correctness of the soldering, and also making sure that there is no short circuit between the pins of the microcircuit, connect the power source to the lines. Using a DC voltmeter with a relative input resistance of at least 5 kOhm / V (avometer), measure the voltage at all logic outputs of the elements. To do this, connect the negative probe of the voltmeter to a common line, and alternately touch the input terminals 1, 2, 4, 5, 9, 10, 12, 13, and then the output terminals 3, 6, 8, 11 with the positive one. When the power supply voltage is 5 V the voltmeter should show about 1,4 V at the input terminals of the elements, and about 0,3 V at the output. If this is not the case, then the microcircuit is faulty. An experimental check of the logic of the action of the elements of the 2I-NOT microcircuit can be started with any of them, for example, from the first - DD1.1 with conclusions 1-3 (Fig. 4). First, connect one of the input terminals, for example, terminal 2, to a common negative line, and terminal 1 to positive, but through a resistor with a resistance of 1 ... 1,5 kOhm (in Fig. 4, a-Rl). Connect the voltmeter PU3 to the output terminal 1.1 of the DD1 element. What does the voltmeter needle show? A voltage equal to approximately 3,5 ... 4 V, i.e. corresponding to a high level. Then measure the voltage at input pin 1 with a voltmeter. And here, as you can see, there is also a high voltage level. Hence the conclusion: when one of the inputs of the 2I-NOT element has a high voltage level, and the other one has a low voltage level, the output will have a high voltage level. In other words, the element is in a single state. Now connect the input terminal 2 of the element through a resistor with a resistance of 1 ... 1,5 kOhm with a positive line and at the same time with a wire jumper with a common one (Fig. 4, b). Measure the voltage at the output terminal. On it, as in the previous case, there will be a high voltage level. Following the arrow of the avometer, remove the wire jumper so that a high voltage level appears at the second input of the element. What does the voltmeter detect at the output of the element? The voltage is about 0,3 V, corresponding to a low level. The element, therefore, switched from a single state to a zero state. With the same wire jumper, close the first input to the common line. At the same time, a high voltage level will immediately appear at the output. And if any of the input terminals is periodically closed to a common line, as if simulating the supply of a low-level voltage to it? With the same repetition rate, electrical impulses will appear at the output of the element and the arrow of the voltmeter connected to it will oscillate. Check it out experimentally. What do the experiments say? They confirm the logic of the 2I-NOT element, previously tested on its electrical counterpart: when a high-level voltage is applied to both inputs, a low-level voltage appears at the output of the element, or, in other words, the element switches from a single state to zero. Another experience: disconnect both input terminals of the element from other parts and conductors. What is the output now? Low voltage. This is as it should be, because not connecting the input pins is tantamount to applying a high voltage level to them and, therefore, setting the element to zero. Do not forget about this feature of logic elements in the future! The next experiment is to check the operation of the same 2I-NOT logic element when it is turned on by the inverter, i.e., as a NOT element. Close both input terminals together and connect them to the positive power line through a resistor with a resistance of 1 .... 1.5 kΩ (Fig. 8, c). What does the voltmeter connected to the output of the element show? Low voltage. Without disconnecting the resistor from this line, close the combined input to the negative line (shown by dashed arrows) and at the same time monitor the reaction of the voltmeter. It will show a high voltage level. This way you make sure that the output of the inverter is always opposite to the input. Carry out similar experiments with other logical elements of the K155LAZ chip and draw the appropriate conclusions. Let's interrupt the experiments for a while to answer the question: what is inside the logical element 2I-NOT? Until now, we have considered a logic element as a kind of "black box" with two inputs and one output. Now let's, as if looking inside the element, get acquainted with its electronic "stuffing" (Fig. 5). It consists of four npn transistors, three diodes and five resistors. The connection between transistors is direct. Resistor Ri, shown by dashed lines, symbolizes the load connected to the output of the element. Such electronic devices of digital technology are called transistor-transistor logic chips, or, for short, TTL. This reflects the fact that the input logic operations (or, as they often say, the input logic) are performed by a multi-emitter transistor (the first letter J), amplification and signal inversion are also transistors (the second letter T).
The input transistor VT1, connected according to the common base circuit, is two-emitter. Moreover, the emitters are connected to a common power wire through diodes VD1, VD2 - they protect the transistor from accidental negative polarity voltage on the emitters. Transistor VT2 forms an amplifier with two loads: emitter (resistor R3) and collector (resistor R2). The antiphase signals taken from them (opposite in level: if the voltage level is high on the collector, low on the emitter) are fed to the bases of the output transistors VT3 and VT4. Thus, the output transistors during operation are always in opposite states - one is closed, and the second is open at this time. If there is a low-level voltage element at one or both inputs (for example, when they are connected to a common wire), the transistor VT1 will be open and saturated, the transistors VT2 and VT4 will be closed, and the transistor VT3 will be open and through it, the diode VD3 and the load RH will flow - element in single state. In the same case, when a high voltage level is applied to both inputs, the transistor VT1 will close, and the transistors VT2 and VT4 will open and thereby close the transistor VT3. In this case, the current through the load will practically stop, since the element will take on a zero state. The low voltage level at the output of the logic element is equal to the voltage at the collector of the open transistor VT4 and does not exceed 0,4 V. The high voltage level at the output of the logic element (when the transistor VT4 is closed) is less than the voltage of the power source by the value of the voltage drop across the transistor VT3 and diode VD3 - not less than 2,4 V. In fact, the voltage of the low and high logic levels at the output of the element depends on the load resistance and may differ slightly from that indicated above. The transition of an element from a single state to zero occurs abruptly when its input voltage passes through a value of about 1,2 V, called the threshold.
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