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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Intercom for 100 subscribers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer The system from the base unit, located in the room of the duty officer at the entrance of the multi-apartment residential building, and the subscriber units installed in the apartments, provides a duplex loud-speaking communication between the duty officer and the residents. Its peculiarity is that all blocks are connected in parallel to only one pair of wires. This favorably distinguishes the proposed device from conventional switchboards, during the installation of which it is necessary to lay a multi-core cable with separate pairs of wires for each subscriber. The idea of a two-wire (linear) intercom is not new. Home voice communication systems based on this principle have been produced abroad for a long time. A brief description and a simplified diagram of one of them, CD2000, are available in [1]. However, the base unit of this system is very complex and contains many microcircuits that have no domestic analogues. The proposed device operates on the same principle, but is much simpler and is built on widespread cheap elements. Intercoms of industrial production provide, as a rule, the opportunity not only to negotiate, but also to remotely control the electromagnetic lock on the entrance door. However, in amateur conditions it is difficult to make a device that is not susceptible to damage by hooligans. In the proposed version, the base unit is not designed to be installed in a public place, it must be constantly monitored by a person on duty. After contacting the desired apartment and finding out whether it is possible to let the visitor through, the attendant on duty will open the door manually. Up to one hundred subscriber units can be connected to one base unit. (More precisely, it is possible to assign up to one hundred different numbers to subscribers. With what maximum number the intercom will work reliably depends on the specific conditions: the length and route of the cable, the level of external interference. - Note, ed.). To establish communication with any of them, it is enough for the duty officer to dial the two-digit number assigned to the called subscriber on the keyboard of the control panel. After two seconds, the unit that received the call will automatically switch from passive to operational state, the LED indicator on it will light up, and a call tone will sound for 2 seconds. Since then, voice communication has been established. At the end of the conversation, the duty officer presses the "Hang up" button on the remote control - the connection is terminated, the system returns to its original state. BASE UNIT The main part of the basic block diagram is shown in fig. 1, and the diagram of its node (and the entire system) power supply - in fig. 2. At the moment the +12 V voltage is turned on, the C3R1 circuit generates a pulse that resets the triggers on the elements DD1.1, DD1.2 and DD2.1. On the trigger DD6.1, the initial setting signal is supplied through the VD5 diode from pin 3 of the DDI.2 element. Thanks to the feedback circuits, after a fraction of a second, triggers DD2.2, DD6.2 will also go to their original state. Since the transistor VT3 is closed, there is no supply voltage on the conversational node (on transistors VT1, VT2, VT4-VT7) and on the subscriber units.
While dialing the number of the subscriber unit, the binary codes of its digits are sent to pins 3-6 of the XP1 plug, and each code is accompanied by a pulse on pin 2. The first of the pulses changes the state of the trigger DD2.1 to the opposite. As a result, the level drop at pin 1 of the trigger, differentiated by the C9R14 circuit, writes the code of the tens digit of the subscriber's number to the counter DD4 (serving in this case as a regular register).
The second pulse on pin 2 of the XP1 plug again changes the state of the trigger DD2.1. Now the level drop at trigger pin 2, differentiated by the C8R15 circuit, has the desired polarity. As a result, the digit code of the units of the number is entered into the register DD3, and the trigger from the elements DD1.1, DD1.2 goes into the log state. 0 at pin 3 of the element DD1.2. Transistor VT3 opens, supplying power to the conversational node of the base unit. Through the resistor R16, the supply voltage enters the line connecting the base unit with the subscriber units. After approximately 2 s (the delay depends on the time constant of the R10C7 circuit), necessary to charge the capacitors in the subscriber units, the state of the trigger DD2.2 changes. Now on its output 12 is a log. 0. This removes the blocking of the clock generator on the elements DD5.1, DD5.2 and counting the pulses of the counter DD7. The generator pulse repetition period is 12 ms. The counter outputs (pins 6, 11, 14 DD7) are connected to the address inputs of the multiplexer DD8. Voltage diagrams at the characteristic points of the base unit during dialing number 25 are shown in fig. 3.
The parallel code of the subscriber number, supplied from the outputs of the registers DD3 and DD4 to the inputs X0-X7 of the DD8 multiplexer, is converted into a serial one at the output 3 of the latter. Thanks to the clock pulses at the input S of the multiplexer, the duration of its output pulses corresponding to the log. 1 code is equal to 6 ms. A single vibrator (DD6.2 trigger with R30C15 feedback circuit) generates 0,5 ms clock pulses from clock pulses. The output pulses of the single vibrator and multiplexer DD8 logically adds the element DD5.3, at its output a sequence is formed in which the log. 0 correspond to short (0,5 ms), and log. 1 - long (6 ms) pulses. Through the element DD5.4, the resistor R12 and the diode VD3, this sequence enters the base of the transistor VT2 and then to the communication line. The amplitude of the code message at the output of the base unit is approximately 2 V. After the eighth clock pulse, a log appears. 1 at pin 2 of the counter DD7 and the input R of the trigger DD2.2 connected to it. This returns the trigger to its original state. Log. About on its output 12 is replaced by a log. 1, which stops the transmission of the code, blocking the clock generator and resetting the counter DD7. With a delay of approximately 1 s, depending on the time constant of the R19C10 circuit and necessary to charge the capacitors in the subscriber unit turned on by the call, the DD6.1 single vibrator is started, the pulse of which for 2 s (depends on the time constant of the R28C12 circuit) turns on the tone call signal generator on elements DD1.3, DD1.4. The signal goes to the input 6 of the element DD5.4 and then to the communication line similarly to the code of the subscriber's number. The tone call can be repeated by restarting the DD6.1 single vibrator with a log pulse. 1 applied to pin 9 of the XP1 plug. To terminate the connection, a log pulse is given. 1 in the circuit for setting the unit to its initial state - to pin 1 of the XP1 plug, after which you can dial another number or turn off the power with the SB 1 switch (Fig. 2). Circuits R13VD1, R29VD9 and diode VD4 are necessary for the rapid discharge of capacitors C7, C12, C10. The conversation node is based on the scheme of an imported loud-speaking telephone set published in [2]. The signal from the output of the microphone amplifier on the transistor VT1 enters the communication line through the capacitor C5, the diode VD2 and the transistor VT2. Since the variable component of the voltage at the emitter of the transistor VT2 is identical, but out of phase with the signal transmitted to the line, you can choose such a position of the trimming resistor R18 engine that this signal will be suppressed on it and at the input of the receiving amplifier (base of the transistor VT4). At the same time, the signal that came from the communication line, not having its own similarity at the emitter of the transistor VT2, will pass to the input of the amplifier only slightly weakened. Thus, it is possible to eliminate the "local effect" - listening to the signal of one's own microphone in the loudspeaker, leading to self-excitation. The receiving amplifier is assembled on transistors VT4-VT7. Trimmer resistor R27 - volume control. Since the dialing and tone call code packets generated in the logical nodes of the base unit have a large amplitude, they are inevitably heard in the dynamic head BA1. The same signals in antiphase are fed to the base of the transistor VT4 through the diodes VD6-VD8 and resistors R21, R22, so the tuning resistor R22 can achieve an acceptable listening attenuation, sufficient only for auditory call control. REMOTE CONTROL The XP1 plug of the base unit is connected by a ten-wire cable to the one of the same name installed on the control panel board, the diagram of which is shown in fig. 4.
When you press the buttons SB3 - SB11 (numbers 1 - 9), the corresponding binary code generated by the diode encoder (VD1 - VD5, VD7 - VD10, VD12 - VD18) enters pins 3-7 of the XP1 plug. The logical sum of all bits of the code, formed using the diodes VD6, VD11, VD16 and VD19, serves as a recording pulse. It enters pin 2 of the XP1 plug through capacitor C3. Capacitor C2 protects against bounce of button contacts and interference, resistor R8 is necessary to discharge the capacitors in the pauses between button presses. -7 forks XP1 remain zero, and the signal coming from the button serves as a recording pulse. Buttons SB1 and SB11 give commands to clear and repeat the call. The HL2 LED lights up when the device is in active mode. Voltage is supplied to it from pin 10 of the XP1 plug through the resistor R4 of the base unit (see Fig. 1). The base of the transistor VT1 that controls the HL1 LED is fed through the R1C1 circuit with write pulses. Therefore, when you press any "digital" button, this LED flashes briefly. SUBSCRIPTION UNIT The diagram of the subscriber unit is shown in fig. 5. All its nodes are fed by the constant component of the voltage coming through the communication line, separated by an electronic filter on transistors VT3 and VT5 [2]. When the supply voltage appears, the R15C10 circuit generates a pulse that sets the DD1.2 trigger and the DD3 register to its initial state (at the output 13 DD1.2 - log. 1, at all outputs DD3 - log. 0). The power supply circuit of the conversational node is broken by a closed transistor VT7.
The voltage diagrams at the characteristic points of the subscriber unit when receiving code number 25 are shown in fig. 6. From the communication line, the pulse sequence through the amplifier-shaper on the transistor VT2 is fed to the input of the nine-bit shift register formed by the D-flip-flop DD1.2 and the DD3 microcircuit. Shift pulses are generated by a single vibrator on a DD1.1 trigger with a feedback circuit R13C8. The edge of each of the code pulses triggers the one-shot. Writing to the register and data shift occur at the end of the pulse generated by the single vibrator. Since its duration (4 ms) is chosen as intermediate between short (0,5 ms) and long (6 ms) code pulses, each time the value corresponding to the next code bit is entered into the register [3, 4].
As a result, after the eighth code pulse, the first eight bits of the shift register contain the code of the called subscriber's number converted into parallel form, and the last, ninth bit contains the log. 1 (in the initial state, it was in the first bit of the register - trigger DD1.2). This log 1, entering the input 2 element DD2.1, prohibits further shift of information in the register, which otherwise could occur, for example, under the influence of interference. Next, the unit for comparing the received number with the one assigned to this subscriber unit comes into action. It consists of diodes VD6 - VD13 and elements DD2.3 and DD2.4 [4]. The block number is assigned by setting, according to the table, each diode to one of two possible positions - "0" or "1". Shown in fig. 3 solid lines, the position of the diodes corresponds to the number 25.
At the output of the element DD2.4 log. 1 appears if and only if the values of all digits of the received code matched the specified ones and the reception is over (log. 3 from output 1 of register DD2 was received through the VD3 diode in the comparison node). When the match is fixed, the opened transistor VT7 turns on the conversational node of the subscriber unit, according to the scheme, it almost does not differ from that used in the base one. When you hear a tone call signal coming over the communication line, you can start a conversation. By shunting the collector-emitter section of the transistor VT1 with the jumper XT7, you can turn on the conversational node without waiting for the appropriate code combination to arrive. This is useful when setting up and testing the system. The receiving amplifier of the conversational node of the subscriber unit can be made not on transistors, but on a microcircuit according to one of the circuits shown in Fig. 7-9. In this case, transistors VT6, VT8 - VT10 (Fig. 5) and related elements are excluded. The simplest amplifier is on the TDA7050 chip (Fig. 7). However, this microcircuit is low-voltage, the voltage between its terminals 8 and 5 should not exceed 6 V. The excess extinguishes the zener diode VD1. Instead, for this microcircuit, a separate integrated voltage regulator for 5 ... 6 V from the K1157, K1170 series or their foreign analogues can be provided.
If TDA7050 or other low-voltage microcircuits (for example, KR174UN23, KR174UN23) are used in all subscriber units, you can get rid of unnecessary parts (zener diodes or integrated stabilizers) by reducing the supply voltage of the entire system. To do this, it is enough to install the KR1EN2B chip as DA142 (see Fig. 5). Since a decrease in the supply voltage of logical nodes will entail a change in the duration of all time intervals formed in the system, the values of the elements of the time-setting circuits may have to be re-selected. It is possible to replace the transistor receiving amplifier with a microcircuit one, assembled according to one of the above schemes, in the base unit. The input of the amplifier is connected to the right according to the scheme (see Fig. 1) plate of the capacitor C11, the power output of the DA1 microcircuit (Fig. 7-9) is connected directly or through a zener diode to the +12 V circuit, and the common output is connected to the common wire of the base unit. DETAILS As microcircuits DD3, DD4 (see Fig. 1), in addition to K561IE11, K561IE14, K561TMZ, K176IRZ can be used. In position DD7, the same chip will be replaced by K561IE14, K561IE10, K176IE1, K176IE2. All replacements are made taking into account differences in the purpose of the conclusions. Transistors of the KT6114 and KT6115 series are analogues of the imported S8050 and S8550, respectively. Their characteristics: Ik \u1,5d 1A; Pk = 21 W; h85E = 160...XNUMX (group A), 120...200 (group B), 160...300 (group C). In the base and subscriber units, instead of them, you can install more common transistors of the KT814, KT815 or KT816, KT817 series. Less powerful ones are also suitable: n-p-n - KT645A, p-n-p - KT209 with any letter index. It is desirable to select a pair of transistors with close coefficients h21E. The KT645A transistor (VT2 in the base unit) will replace the KT3107B or KTZ107G-KT3107L with a coefficient h21E of at least 150, but with such a replacement and an accidental long circuit of the communication line, a low-power transistor may fail. VT3 (see Fig. 1) - any silicon transistor of the p-n-p structure with a permissible collector current of at least 100 mA. All KT3102G transistors can be replaced by any low-power silicon structures np-p with h21E at least 50. However, it is still desirable to use KT1 series transistors as VT1 (see Fig. 5 and 3102). All diodes (with the exception of those installed in the power unit, Fig. 2) are any low-power silicon ones, for example, 1N4148 or the KD522, KD521 series. Diodes of the KD13, KD16, KD2, KD243, KD209 series are suitable as VD226 - VD105 (see Fig. 221). Here you can also use the ready-made diode bridge KTs407A or the KTs402, KTs405 series. The voltage on the secondary winding of the transformer T1 is 15 ... 20 V at a load current of 100 mA. Microphones ВМ1 (see Fig. 1 and 5) - any electret, widely used in imported telephone sets and tape recorders. Dynamic heads 1GDSH - 11 were chosen for subscriber units due to their smaller dimensions compared to the 1GDSH - 10 installed in the base unit. All heads can be replaced by others with a power of at least 0,25 W with a voice coil resistance of 8 ... 50 Ohm. Power switch SA1 (see Fig. 2) - PKN41.1.2. Control panel buttons (see Fig. 4) - PKN - 125 or similar imported ones. Pushbuttons with conductive rubber contacts are not suitable. Oxide capacitors of the power supply unit (see Fig. 2) - K50 - 16 or K50 - 35. In other nodes, similar oxide capacitors or more reliable K53 - 19 can be used. Non-polar capacitors - any ceramic, for example, K10 - 17 or K10 - 62 Fixed resistors - MLT or C2 - 23, tuning - SDR - 38a. DESIGN Using bushings and screws, the base unit board is attached to the bottom of its case, and the control panel board is attached to the inside of the case cover at such a distance from it that the SB1 - SB12 buttons can be pressed through specially provided holes in the cover. As already noted, the XP1 plugs of both boards are connected with a ten-wire cable or bundle. The case must have holes for access with a screwdriver to the trimming resistors R18 and R27 with the lid closed. It is important to choose the right place for installing the BM1 microphone and the BA1 dynamic head in order to ensure the least acoustic coupling between them. The acoustic axis of the microphone must be perpendicular to the axis of the head. Therefore, if the dynamic head is located on the front panel of the unit, the microphone should be installed on the side. The microphone is placed in a piece of metal or plastic pipe with an inner diameter 2-3 mm larger than the outer diameter of the microphone. The gap is filled with sound-absorbing material, for example, porous rubber. The quality of sound insulation can be checked by covering the hole in the housing in front of the microphone with your finger. If it is bad, the amplifier will immediately self-excite. Holes in any walls of the unit body, except for the one where the microphone is installed, contribute to the increase in stability. The larger the total area of these holes, the better. To radically eliminate acoustic feedback, you can completely abandon the dynamic head, replacing it with headphones. Subscriber units are easily placed, for example, in loudspeaker housings for a radio broadcasting network. Everything said above about the elimination of self-excitation applies to them as well. FORMING The final adjustment of the system is carried out after the completion of its installation in the house and connection to the communication line of all subscriber units. All conversational nodes are configured in the same way. First of all, the trimmer resistor R27 (see Fig. 1) or R18 (see Fig. 5) sets the volume at the excitation threshold. It should be borne in mind that the sound waves emitted by the dynamic head reach the microphone in various ways, including being reflected from the hands of the traffic controller. By tapping on the microphone, they achieve balance with the help of R18 (see Fig. 1) or R14 (see Fig. 5) - the least audibility of "one's own" microphone. Now you can increase the volume by setting it back to a little less than the excitation threshold. It may take several attempts to get the best result. Since the overall stability of the system depends on the gains and acoustic feedback in both the subscriber and base units, a compromise must be found. By reducing the volume in the base unit, you can increase it in the subscriber units, and vice versa. If necessary, changing the values of the elements R25, C14 and R30, C15 (see Fig. 1), set the required duration of the pulses of the code message. The duration of the single vibrator pulses in the subscriber units is regulated by a selection of the values of the resistor R13 and capacitor C8 (see Fig. 5). Lastly, by pressing the SB12 "Repeat" button, the trimming resistor R22 of the base unit sets the volume for listening to the ringing signal. Literature
Author: E.Pletnev, Kharkov, Ukraine
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