Getting to know the digital circuit

In the second part of the article, it was told about the conventional graphic symbols of logical elements and about the functions performed by these elements.

To explain the principle of operation, contact circuits were given that perform the logical functions AND, OR, NOT and AND-NOT. Now you can start a practical acquaintance with the K155 series microcircuits.

Appearance and design

The basic element of the 155th series is the K155LA3 chip. It is a plastic case with 14 pins, on the upper side of which there is a marking and a key indicating the first pin of the microcircuit.

The key is a small round label. If you look at the microcircuit from above (from the side of the case), then the counting of the conclusions should be carried out counterclockwise, and if from below, then clockwise.

A drawing of the microcircuit housing is shown in Figure 1. Such a housing is called DIP-14, which in English means a plastic housing with a two-row pin arrangement. Many microcircuits have a larger number of pins and therefore packages can be DIP-16, DIP-20, DIP-24 and even DIP-40.

Figure 1. DIP-14 package.

What's in this box

The DIP-14 package of the K155LA3 chip contains 4 2I-NOT elements independent of each other. The only thing that unites them is only common power pins: the 14th pin of the microcircuit is the + of the power source, and pin 7 is the negative pole of the source.

In order not to clutter up the circuits with unnecessary elements, power lines, as a rule, are not shown. This is also not done because each of the four 2I-NOT elements can be located in different places in the circuit. Usually, they simply write on the diagrams: “Connect + 5V to terminals 14 DD1, DD2, DD3 ... DDN. -5V lead to pins 07 DD1, DD2, DD3…DDN.». separately located elements are designated as DD1.1, DD1.2, DD1.3, DD1.4. Figure 2 shows that the K155LA3 chip consists of four 2I-NOT elements. As already mentioned in the second part of the article, input terminals are located on the left, outputs are on the right.

The foreign analogue of K155LA3 is the SN7400 chip and it can be safely used for all the experiments described below. To be more precise, the entire series of K155 microcircuits is an analogue of the foreign SN74 series, so sellers on the radio markets offer it.

Figure 2. K155LA3 chip pinout.

To conduct experiments with a microcircuit, you will need a voltage of 5V. The easiest way to make such a source is by using the K142EN5A stabilizer microcircuit or its imported version, which is called 7805. In this case, it is not at all necessary to wind the transformer, solder the bridge, and install capacitors. After all, there will always be some Chinese 12V AC adapter, to which it is enough to connect the 7805, as shown in Figure 3.

Figure 3. A simple power supply for experiments.

To conduct experiments with a microcircuit, you will need to make a small breadboard. It is a piece of getinaks, fiberglass or other similar insulating material with dimensions of 100 * 70 mm. Even simple plywood or thick cardboard is suitable for such purposes.

Along the long sides of the board, tinned conductors should be strengthened, about 1.5 mm thick, through which power will be supplied to the microcircuits (power rails). Holes with a diameter of no more than 1 mm should be drilled between the conductors over the entire area of ​​the breadboard.

When conducting experiments, it will be possible to insert segments of tinned wire into them, to which capacitors, resistors and other radio components will be soldered. Low legs should be made at the corners of the board, this will make it possible to place the wires from below. The design of the breadboard is shown in Figure 4.

Figure 4. Breadboard.

After the breadboard is ready, you can start experimenting. To do this, you should install at least one K155LA3 chip on it: solder pins 14 and 7 to the power buses, and bend the rest of the pins so that they are adjacent to the board.

Before starting experiments, you should check the soldering reliability, the correct connection of the supply voltage (connecting the supply voltage in reverse polarity can damage the microcircuit), and also check if there is a short circuit between adjacent terminals. After this check, you can turn on the power and start the experiments.

For measurements, it is best suited, the input resistance of which is at least 10Kom / V. This requirement is fully satisfied by any tester, even a cheap Chinese one.

Why is arrow better? Because, by observing the fluctuations of the arrow, one can notice voltage pulses, of course, of a sufficiently low frequency. A digital multimeter does not have this capability. All measurements must be carried out relative to the "minus" of the power source.

After the power is turned on, measure the voltage at all pins of the microcircuit: at input pins 1 and 2, 4 and 5, 9 and 10, 12 and 13, the voltage should be 1.4V. And at the output pins 3, 6, 8, 11 about 0.3V. If all voltages are within the specified limits, then the microcircuit is working.

Figure 5. Simple experiments with a logic element.

Checking the operation of the logical element 2I-NOT can be started, for example, from the first element. Its input terminals are 1 and 2, and the output is 3. In order to apply a logic zero signal to the input, it is enough to simply connect this input to the negative (common) wire of the power source. If it is required to apply a logical unit to the input, then this input should be connected to the + 5V bus, but not directly, but through a limiting resistor with a resistance of 1 ... 1.5 KΩ.

Suppose that we connected input 2 to a common wire, thereby applying a logical zero to it, and a logical unit was applied to input 1, as just indicated through the limiting resistor R1. This connection is shown in Figure 5a. If, with such a connection, the voltage at the output of the element is measured, then the voltmeter will show 3.5 ... 4.5V, which corresponds to a logical unit. A logical unit will give a measurement of the voltage at pin 1.

This completely coincides with what was shown in the second part of the article using the example of a relay-contact circuit 2I-NOT. Based on the results of the measurements, we can draw the following conclusion: when one of the inputs of the 2I-NOT element has a high level, and the other one has a low level, a high level is necessarily present at the output.

Next, we will do the following experiment - we will apply a unit to both inputs at once, as shown in Figure 5b, but one of the inputs, for example 2, will be connected to a common wire using a wire jumper. (For such purposes, it is best to use an ordinary sewing needle soldered to a flexible wire). If we now measure the voltage at the output of the element, then, as in the previous case, there will be a logical unit.

Without interrupting the measurement, remove the wire jumper - the voltmeter will show a high level at the output of the element. This is fully consistent with the logic of the 2I-NOT element, which can be seen by referring to the contact diagram in the second part of the article, as well as looking at the truth table shown there.

If now with this jumper we periodically close any of the inputs to a common wire, simulating the supply of a low and high level, then using a voltmeter at the output you can detect voltage pulses - the arrow will oscillate in time with the touches of the microcircuit input jumper.

From the experiments carried out, the following conclusions can be drawn: a low-level voltage at the output will appear only if there is a high level at both inputs, that is, the 2I condition is met at the inputs. If at least one of the inputs has a logical zero, there is a logical unit at the output, it can be repeated that the logic of the microcircuit is fully consistent with the logic of the 2I-NOT contact circuit considered in.

Here it is appropriate to do another experiment. Its meaning is to turn off all input pins, just leave them in the "air" and measure the output voltage of the element. What is going to be there? That's right, there will be a logic zero voltage. This suggests that the unconnected inputs of logic elements are equivalent to inputs with a logical one applied to them. This feature should not be forgotten, although unused inputs, as a rule, are recommended to be connected somewhere.

Figure 5c shows how the 2I-NOT logic element can simply be turned into an inverter. To do this, it is enough to connect both of its inputs together. (Even if there are four or eight inputs, such a connection is quite acceptable).

To make sure that the output signal has a value opposite to the input signal, it is enough to connect the inputs with a wire jumper to a common wire, that is, apply a logical zero to the input. In this case, the voltmeter connected to the output of the element will show a logical unit. If the jumper is opened, then a low level voltage will appear at the output, which is just the opposite of the input.

This experience suggests that the operation of the inverter is completely equivalent to the operation of the NOT contact circuit discussed in the second part of the article. These are, in general, the wonderful properties of the 2I-NOT microcircuit. To answer the question of how all this happens, one should consider the electrical circuit of the 2I-NOT element.

The internal structure of the element 2I-NOT

Until now, we have considered a logical element at the level of its graphic designation, taking it, as they say in mathematics, for a “black box”: without going into details of the internal structure of the element, we have studied its response to input signals. Now it's time to study the internal structure of our logic element, which is shown in Figure 6.

Figure 6. The electrical circuit of the logic element 2I-NOT.

The circuit contains four npn transistors, three diodes and five resistors. There is a direct connection between the transistors (without coupling capacitors), which allows them to work with constant voltages. The output load of the microcircuit is conditionally shown as a resistor Rn. In fact, this is most often an input or several inputs of the same digital microcircuits.

The first transistor is multi-emitter. It is he who performs the input logical operation 2I, and the following transistors perform amplification and inversion of the signal. Microcircuits made according to a similar scheme are called transistor-transistor logic, abbreviated as TTL.

This abbreviation reflects the fact that the input logic operations and the subsequent amplification and inversion are performed by transistor circuit elements. In addition to TTL, there is also diode-transistor logic (DTL), the input logic stages of which are made on diodes, located, of course, inside the microcircuit.

Figure 7

At the inputs of the 2I-NOT logic element, diodes VD1 and VD2 are installed between the emitters of the input transistor and the common wire. Their purpose is to protect the input from a voltage of negative polarity, which may arise as a result of self-induction of the mounting elements when the circuit operates at high frequencies, or simply applied by mistake from external sources.

The input transistor VT1 is connected according to a common base circuit, and its load is the transistor VT2, which has two loads. In the emitter, this is the resistor R3, and in the collector R2. Thus, a phase inverter is obtained for the output stage on transistors VT3 and VT4, which makes them work in antiphase: when VT3 is closed, VT4 is open and vice versa.

Let's assume that both inputs of the 2I-NOT element are low. To do this, simply connect these inputs to a common wire. In this case, the transistor VT1 will be open, which will lead to the closing of transistors VT2 and VT4. The transistor VT3 will be in the open state and through it and the diode VD3 current flows to the load - at the output of the element, a high level state (logical unit).

In the event that a logic unit is applied to both inputs, the transistor VT1 will close, which will lead to the opening of transistors VT2 and VT4. Due to their opening, the transistor VT3 will close and the current through the load will stop. At the output of the element, a zero state or a low level voltage is set.

The low-level voltage is due to the voltage drop at the collector-emitter junction of the open transistor VT4 and, according to the specifications, does not exceed 0.4V.

The high-level voltage at the output of the element is less than the supply voltage by the amount of voltage drop across the open transistor VT3 and the diode VD3 in the case when the transistor VT4 is closed. The high level voltage at the output of the element depends on the load, but should not be less than 2.4V.

If a very slowly changing voltage, varying from 0 ... 5V, is applied to the inputs of the element, connected together, then it can be seen that the transition of the element from a high level to a low level occurs abruptly. This transition is performed at the moment when the voltage at the inputs reaches a level of approximately 1.2V. Such a voltage for the 155th series of microcircuits is called threshold.

Boris Alaldyshkin

Article continued:

EBook -

The K155LA3 microcircuit, like its imported counterpart SN7400 (or simply -7400, without SN), contains four logical elements (gates) 2I - NOT. The K155LA3 and 7400 microcircuits are analogues with a complete pinout match and very close operating parameters. Power is supplied through terminals 7 (minus) and 14 (plus), with a stabilized voltage from 4.75 to 5.25 volts.

Chips K155LA3 and 7400 are based on TTL, therefore - a voltage of 7 volts is for them absolutely maximum. If this value is exceeded, the device burns out very quickly.
The layout of the outputs and inputs of logic elements (pinout) K155LA3 looks like this.

The figure below shows the electronic circuit of a separate element 2I-NOT of the K155LA3 microcircuit.

Parameters K155LA3.

1 Rated supply voltage 5 V
2 Low level output voltage less than 0.4 V
3 High-level output voltage of at least 2.4 V
4 Low level input current -1.6 mA or less
5 High level input current 0.04 mA or less
6 Input breakdown current no more than 1 mA
7 Short circuit current -18...-55 mA
8 Current consumption at a low output voltage level, not more than 22 mA
9 Current consumption at a high output voltage level, not more than 8 mA
10 Consumed static power per logic element no more than 19.7 mW
11 Propagation delay time when turned on no more than 15 ns
12 Propagation delay time when switching off no more than 22 ns

Scheme of the generator of rectangular pulses on K155LA3.

It is very easy to assemble a square-wave generator on the K155LA3. To do this, you can use any two of its elements. The diagram might look like this.

Pulses are taken between 6 and 7 (minus power) pins of the microcircuit.
For this generator, the frequency (f) in hertz can be calculated using the formula f = 1/2 (R1 * C1). Values ​​are substituted in Ohms and Farads.

The use of any materials on this page is allowed if there is a link to the site

From 08/10/2019 to 09/07/2019 technical break.
We will resume receiving parcels from 09/08/2019.

Reception of microcircuits (MS) 155, 172, 555, 565 series, prices

This page presents microcircuits of the 155 series and similar ones in black and brown plastic cases. Our company has been accepting microcircuits of other series at high prices from individuals on an ongoing basis for more than 6 years. You can reliably and safely for you.

It is worth noting that the price for series 155 and others is calculated by the weight of the microcircuits when the parts arrive at our office for evaluation by specialists. We are often asked the same question: I have about 50 grams of KM capacitors, 200-400 grams of 155 series chips and a few other parts. Can you send them in a parcel?

Answer to all: Yes, you can. Send as much as you can. The calculation will always be made in full. Chips of 565,555,155 series with a yellow (gold-plated) substrate-plate inside have the highest price. If you want to get the maximum benefit from the sale, then you need to bite through each MC and look for the presence of a yellow substrate plate, since in 155,555 series there are often empty microcircuits with a white substrate inside, instead of the desired, gold-plated substrate. The photos below will show this.

The price of microcircuits of these series directly depends on the year of manufacture, manufacturer and acceptance conditions (military, civil, and so on).

Also, MS 155, 172, 176, 555, 565 series and other similar series must be cut off the boards before being sent in a parcel by Russian Post and only in this form, without the boards themselves, sent to our company. Since sending on boards leads to an increase in the cost of the package due to the greater weight and if only the data of the microcircuits on the boards are sent in the package. If there are few boards with these microcircuits (MC), up to 5-7 units (boards), then send the MC on the boards as is, along with other radio components and components.

Often there are boards where there are part of microcircuits with yellow leads in a ceramic case and part of microcircuits of the 155 series and the like in a black plastic case. Such boards can be sent as is, without removing parts from the boards.

In this case, the calculation will be made after our specialists dismantle the MS from the boards. Ceramics (white, pink), 133, 134 series and the like will be counted by the piece, MS in a black plastic case will be weighed and the MS data markings will be inspected. The price of this will not change downwards.

For more information on microcircuits, see the following pages:

Photos and prices for microcircuits

Appearance	Marking/Price	Appearance	Marking/Price
	K155LA2 Price: up to 4000 rubles / kg.		KR140UD8B Price: up to 1000 rubles/kg.
	K155IE7 partial yellow leads Price: up to 4500 rubles / kg.		K155LI5 Price: up to 1500 rubles / kg.
	K157UD1 Price: up to 4000 rubles / kg.		K155LE6 Price: up to 800 rubles/kg.
	K118UN1V Price: up to 3800 rubles/kg.		K1LB194 Price: up to 1500 rubles / kg.
	K174UR11 Price: up to 4000 rubles / kg.		KM155TM5 Price: up to 2200 rub./kg.
	KR531KP7 Price: up to 4000 rubles / kg.		KS1804IR1 Price: up to 2300 rub./kg.
	K555IP8 Price: up to 4100 rubles/kg.		KR537RU2 Price: up to 850 rubles/kg.
	KR565RU7 Price: up to 6500 rub./kg.		K561RU2 Price: up to 700 rubles/kg.
	KR590KN2 Price: up to 3000 rubles/kg.		KR1021XA4 Price: up to 2750 rubles/kg.
	KR1533IR23 Price: up to 4000 rubles / kg.		Chips-blend Price: up to 5000 rub./kg.
	KR565RU1 without partial yellow legs Price: up to 5500 rubles/kg.		KR565RU1 with partial yellow legs Price: up to 4500 rubles / kg.
	K155KP1 Price: up to 2000 rub./kg.		K155ID3 Price: up to 700 rubles/kg.
	K174XA16 Price: up to 3400 rubles/kg.		KR580YK80 Price: up to 500 rubles/kg.
	KR573RF5 Price: up to 2500 rubles/kg.		KR537RU8 Price: up to 3700 rub./kg.
	K555IP3 Price: up to 4000 rubles / kg.		KR572PV2 Price: up to 500 rubles/kg.
	K561IR6A Price: up to 2900 rub./kg.		K145IK11P Price: up to 500 rubles/kg.
	K589IR12 Price: up to 3100 rubles/kg.		KR581RU3 Price: up to 500 rubles/kg.

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Chip K155LA3 is, in fact, the basic element of the 155th series of integrated circuits. Externally, it is made in a 14-pin DIP package, on the outside of which there is a marking and a key that allows you to determine the beginning of the pin numbering (when viewed from above, from a dot and counterclockwise).

In the functional structure of the K155LA3 microcircuit, there are 4 independent logic elements. Only one thing unites them, and these are the power lines (common pin - 7, pin 14 - positive power pole) As a rule, microcircuit power contacts are not shown on circuit diagrams.

Each individual 2I-NOT element microchips K155LA3 in the diagram they denote DD1.1, DD1.2, DD1.3, DD1.4. On the right side of the elements are the outputs, on the left side are the inputs. The analogue of the domestic K155LA3 microcircuit is the foreign SN7400 microcircuit, and the entire K155 series is similar to the foreign SN74.

Truth table chip K155LA3

Experiments with the K155LA3 chip

On the breadboard, install the K155LA3 chip to the terminals, connect the power supply (pin 7 minus, pin 14 plus 5 volts). To perform measurements, it is better to use a pointer voltmeter with a resistance of more than 10 kOhm per volt. Ask why you need to use an arrow? Because, by the movement of the arrow, you can determine the presence of low-frequency pulses.

After applying voltage, measure the voltage on all legs of K155LA3. With a working microcircuit, the voltage at the output legs (3, 6, 8 and 11) should be about 0.3 volts, and at the terminals (1, 2, 4, 5, 9, 10, 12, and 13) in the region of 1.4 IN.

To study the functioning of the logical element 2I-NOT of the K155LA3 microcircuit, we take the first element. As mentioned above, its inputs are pins 1 and 2, and the output is 3. The logical 1 signal will be the plus of the power source through a current-limiting resistor of 1.5 kOhm, and we will take logical 0 from the power minus.

First experience (Fig. 1): Let's apply a logical 0 to leg 2 (connect it to the minus of the power supply), and to leg 1 a logical unit (plus the power supply through a 1.5 kΩ resistor). Let's measure the voltage at output 3, it should be about 3.5 V (voltage log. 1)

Conclusion one: If one of the inputs is log.0, and the other is log.1, then the output of K155LA3 will definitely be log.1

Experience of the second (Fig. 2): Now we will apply log.1 to both inputs 1 and 2 and in addition to one of the inputs (let it be 2) we will connect a jumper, the second end of which will be connected to the power supply minus. We apply power to the circuit and measure the voltage at the output.

It should be equal to log.1. Now we remove the jumper, and the voltmeter needle will indicate a voltage of no more than 0.4 volts, which corresponds to the log level. 0. By installing and removing the jumper, you can observe how the voltmeter needle “jumps”, indicating changes in the signal at the output of the K155LA3 microcircuit.

Conclusion two: Signal log. 0 at the output of the element 2I-NOT will be only if there is a log.1 level at both of its inputs

It should be noted that the unconnected inputs of the 2I-NOT element ("hanging in the air"), leads to a low logic level at the K155LA3 input.

Third experience (Fig. 3): If you connect both inputs 1 and 2, then the logical element NOT (inverter) will turn out from the 2I-NOT element. By applying log.0 to the input, the output will be log.1 and vice versa.

Every radio amateur has a k155la3 chip somewhere “littered around”. But often they cannot find a serious application for them, since in many books and magazines there are only schemes for flashing lights, toys, etc. with this detail. This article will consider circuits using the k155la3 chip.
First, consider the characteristics of the radio component.
1. The most important thing is nutrition. It is supplied to 7 (-) and 14 (+) legs and amounts to 4.5 - 5 V. More than 5.5 V should not be applied to the microcircuit (it starts to overheat and burns out).
2. Next, you need to determine the purpose of the part. It consists of 4 elements, 2 and not (two inputs). That is, if you apply 1 to one input and 0 to the other, then the output will be 1.
3. Consider the pinout of the microcircuit:

To simplify the diagram, separate elements of the part are depicted on it:

4. Consider the location of the legs relative to the key:

It is necessary to solder the microcircuit very carefully, without heating it (you can burn it).

Here are the circuits using the k155la3 chip:

1. Voltage stabilizer (can be used as a phone charger from the car's cigarette lighter).
Here is the diagram:


Up to 23 volts can be applied to the input. Instead of the P213 transistor, you can put a KT814, but then you have to install a radiator, since it can overheat under heavy load.
Printed circuit board:

Another option for a voltage stabilizer (powerful):


2. Car battery charge indicator.
Here is the diagram:

3. Tester of any transistors.
Here is the diagram:

Instead of diodes D9, you can put d18, d10.
Buttons SA1 and SA2 have switches for testing forward and reverse transistors.

4. Two options for the rodent repeller.
Here is the first diagram:


C1 - 2200 uF, C2 - 4.7 uF, C3 - 47 - 100 uF, R1-R2 - 430 Ohm, R3 - 1 kohm, V1 - KT315, V2 - KT361. You can also put transistors of the MP series. Dynamic head - 8 ... 10 ohms. Power supply 5V.

Second option:

C1 - 2200 uF, C2 - 4.7 uF, C3 - 47 - 200 uF, R1-R2 - 430 Ohm, R3 - 1 kohm, R4 - 4.7 ohm, R5 - 220 Ohm, V1 - KT361 (MP 26, MP 42, kt 203, etc.), V2 - GT404 (KT815, KT817), V3 - GT402 (KT814, KT816, P213). Dynamic head 8...10 ohm.
Power supply 5V.