Without claiming any special discoveries in circuit design, I propose a rather elegant version of an intercom that can also be used as an intercom in a private house or country house. The design is simple and accessible to self-made even for a novice radio amateur. In standby mode, this device does not consume battery power, but is always ready for use: just press the switch button and - please speak! Anyone working on the second set will certainly hear you.
Germanium transistors of the MP series were chosen as the element base. Available, cheap and reliable, they have parameters sufficient for use in this intercom. In addition, it is worth recalling: to “bring to life” a silicon transistor, about 0.6 V is required. For a push-pull output stage, this number should be doubled. In germanium semiconductor triodes, the consumption is half as much, which cannot but affect the maximum achievable range of the output signal.
Among the advantages of the proposed development is de-energizing the corresponding amplifier in receive mode with the SB1 button while simultaneously connecting the SA1 loudspeaker directly to the communication line. The signal coming from another device (the second set) is almost all connected to the speaker, the impedance of the voice coil of which is at least three times less than the input impedance of the amplifier - thanks to resistor R3. This circuit solution allows, unlike analogues with complex and expensive multi-contact microswitches, to get by with only one (and not at all scarce) button of the KM2-1 type, domestically produced.
When you press SB1 (operation in transmit mode), the amplifier is connected to the power supply battery, and the speaker, disconnected from the communication line, is now used as a kind of microphone. The output signal goes into the communication line through closed contacts 1′ and 2′. The purpose of R3 is to stabilize the operation of the input stage assembled on VT1 when changing temperature, supply voltage, or changing the transistor. The parameters of the R6C5 chain are selected to improve the quality of speech signal transmission such that the second stage on VT2 provides greater gain at high audio frequencies than at lower ones.
It is possible to install C3 in the circuit and on the printed circuit board to limit the bandwidth of the amplifier and prevent it from self-excitation at high frequencies. However, the assembly of several original copies showed that the device works well even without the above-mentioned capacitor. However, if excitement suddenly arises in the form of a thin squeak or users consider the sound timbre to be too high, then the situation can easily be corrected by returning C3 to its intended place.
The output stage, assembled on transistors VT4 and VT5, is a push-pull emitter follower. In terms of voltage, it does not amplify anything, but it is capable of delivering significant current to the load, which is especially valuable when working on a speaker with a low voice coil resistance.
Diode VD1 is connected between the bases of transistors VT4 and VT5. Its purpose is to reduce possible nonlinear distortions in the output push-pull stage.
Now about the radio components that are needed to assemble this device. Resistors are suitable for any type, with a power of 0.125 or 0.25 W. Capacitors C2, C4 - C8 electrolytic K50-6, K50-16 or K50-35. But with C1 everything is somewhat more complicated. After all, it is affected by voltages of different polarities, so for reliability this capacitor must be non-electrolytic. In the original versions, for example, K73-17 is successfully used, designed to operate in alternating, pulsating and direct current circuits. It is also possible to use a monolithic ceramic capacitor of the KM type with a capacity of 0.68-1 μF.
The device is not too critical in the choice of semiconductor devices for its installation. In particular, a diode from the D9, D18, D20 or D311 series is suitable for VD1. And with any letter index. As pnp transistors VT2 - VT4 are suitable for types MP14, MP16, MP39-MP42, MP25. For VT5 any of the semiconductor triodes are acceptable n-p-n series MP36-MP38, MP10 (the letter index at the end of the name in this case does not matter).
But it is advisable to use low-noise transistor VT1. Otherwise, the fluctuation background from the first stage, appearing in the form of a characteristic hiss, will be comparable to a weak input signal and, after appropriate amplification on VT2-VT5, will be reproduced by the speaker in the second set of equipment.
But here too there is plenty to choose from. In addition to the low-noise transistor MP39B indicated in the diagram, MP27 (P27) or MP28 (P28) will do.
The printed circuit boards for both devices are the same. Foil “common wire”, forming closed loop, serves as a kind of screen that additionally protects the amplifier from interference. The installation is quite tight, so the resistors and diode are installed first, then the capacitors come, and the transistors come last. To prevent accidental contacts, polyvinyl chloride tubes of suitable diameter are placed on the metal housings of electrolytic capacitors. And in order to avoid short circuits through possible solder leaks, use a knife to clean the narrowest gaps between the tracks, especially near solder joints.
Setting up the device comes down to selecting resistors R1, R4, R9, which ensure the proper operation of the amplifier. In particular, they ensure that the voltage on the collectors VT1, VT2 and emitters VT4, VT5 is half of Upit. It is advisable to do this operation more precisely for the output stage, and in the preliminary stages 1.5-volt deviations are allowed, both downward and upward. Moreover, if the voltage turns out to be low, then the corresponding resistor should be increased, and in the opposite case, on the contrary, decreased.
It is more convenient to solder the above resistors from the foil side while debugging the circuit. Many transistors can fail if power is applied to the collector and emitter without the base connected.
For a final check of the amplifier, connect a loudspeaker to pins 2 and 3 of the connector. Place the dynamic head (to avoid self-excitation of the entire device due to acoustic feedback) 3-4 meters from the device. Press the button and speak into the microphone to ensure that the transmission quality is acceptable. At the same time, determine the optimal distance between your mouth and the microphone. Remember: at a short distance the sound is too low and somewhat distorted, and at a large distance it is quiet.
Each device is powered by a Krona galvanic battery. But it is possible to use 7 disk batteries with a capacity of at least 0.1 Ah or a power supply (preferably stabilized) type BP-02B2, which is supplied with some portable domestic tape recorders. When using batteries, you cannot do without charger, which can also be homemade (for example, made according to a development published in the magazine “Modelist-Constructor” No. 9, 1998).
In conclusion, here are some tips addressed to beginners in the field of electrical and radio engineering. A sharp increase in current consumption and strong heating of VT4, VT5 can be caused by a malfunction or incorrect connection of VT3, VT4 (VT5), a low value of resistance R9, breakdown or violation of polarity, short-circuited terminals when installing C4 or C8. It happens that the fault is due to a malfunction or incorrect connection of the semiconductor diode VD1, as well as an error in the polarity of the power supply.
If the protective paint on resistor R7 begins to darken or smoke, it means that the capacitor Sat is broken, shorted during installation, or turned on incorrectly. It is also worth making sure that the power supply polarity of the assembled device is correct.
One more common mistake for beginners - when instead of a 470 kOhm resistor they mistakenly solder in a 470 Ohm one, that is, with a value that is 1000 times less than required. Be attentive to symbols on radio components!
Further checking of the mounted device should be carried out from output to input. Using tweezers, connecting the bases of transistors VT3, VT2, VT1 alternately to the “common wire” (doing this for a short time in this circuit is not dangerous), a “capricious” (if any) cascade is identified - by the absence of a proper crackling sound in the dynamics and an increase in voltage on the collector. Any faults found are corrected.
As a rule, an important problem for beginning radio amateurs is making a housing for their homemade products. They solve it in different ways. Good results are obtained, for example, with a U-shaped plastic bracket that forms 3 walls of the case itself. Then the printed circuit board fits well between the sidewalls, being at the same time the back of the device or part of it. Having protrusions on its opposite sides, and special cutouts in the plastic bracket, the board is securely fixed in the case due to its elasticity.
The blank for the base bracket of the body of each of the intercom devices is cut out of 2-mm thermoplastic (for example, polyvinyl chloride) and then bent over a hot metal rod to give the required U-shape. The SG-5 connector, the KM2-1 switch button, as well as the 0.25GD-10 (0.5GDSh-1) or 0.25GDSh-19 (0.5GDSh-2) dynamic head are secured using the holes drilled for them. The cutouts for the board in the sidewalls, as well as for the diffuser on the front part of the bracket, are made with a jigsaw.
Those who wish to experiment can try to increase the output power of each of the manufactured devices by replacing the VT4, VT5 transistors in them with a complementary pair GT402-GT404, capable of “swinging” more modern speakers. But this will require a corresponding increase in the capacitance of C8, a decrease in the resistance of R8 and an adjustment of the value of the resistor R9.
You can connect not two, but three devices to a communication line. True, the dialogue between two subscribers will be broadcast to a third subscriber, who may not participate in this conversation. In some cases, such use of equipment is not a disadvantage at all.
If there is a separate power source in each of the devices, a two-wire line connecting only contacts 2 and 3 of connectors is sufficient.
A. LISOV, Ivanovo
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Schematic diagram The PU is shown in Fig. 2. The amplifier is assembled on an operational amplifier (op-amp). This is a medium precision op-amp with built-in correction and output protection against short circuit in the load.
Let's consider the operation of the amplifier. The signal from the carbon microphone VM1 with an amplitude of 30...60 mV is amplified by the op-amp to a voltage of 1 V. The op-amp gain is set by resistors R5 and R4 and is selected equal to 20...30 (Ku=R5/R4=240k/9.1k=26, 3).
These values of the gain of a given op-amp and the amplitude of the input signal from the microphone were obtained from experimental data and are optimal. The longest communication range is ensured by the maximum amplitude of the signal in the line, at which there is no distortion. When a signal with an amplitude of 150 mV was applied to the input of the amplifier, a signal with an amplitude of 3.5 V was obtained at the output of the control unit. As the input signal increased further, noticeable distortion began. Increasing the op-amp gain more than 30 is impractical, because the probability of self-excitation of the amplifier increases.
The input signal level is set by resistor R1, which determines the current passing through the carbon microphone. A decrease in resistance causes an increase in current through the carbon microphone, which means an increase in the input voltage taken from the microphone and supplied to the op-amp.
If an MKE-3 electret microphone or an electrodynamic DEMSh microphone is used, then resistor R1 can be eliminated and the switching circuit for the microphone used can be used.
A voltage divider consisting of resistors R2 and R3 allows for unipolar power supply. These resistors should, if possible, be of the same value, otherwise signal distortion at the output of the op-amp cannot be ruled out. Their choice will be correct if the voltage measured at pin 6 of the op-amp is equal to half the supply voltage.
Resistor R6 is balanced, necessary to ensure duplex communication. It performs the function of a resistor Ra or Rb (Fig. 1).
Resistor R7 allows you to adjust to different line resistances and the resistance of the telephone capsule, and therefore eliminate the local effect when the signal from your microphone drowns out the signal coming to your phone from the interlocutor. If there are several lines and subscribers, it makes sense to make resistor R7 variable and bring it out for operational adjustment on the case.
To call another subscriber, just press the S1 “Call” button. In this case, the feedback formed by capacitor C2 turns the op-amp into an RC oscillator. The amplitude of the signal in the line during a call is from 3.5 to 4.5 V, the repetition rate of rectangular pulses is 1 kHz. The power released in the telephone capsule of the interlocutor is at least 150 mW. This is enough to hear the call.
Rice. 2
A little about the design and details of the PU. The printed circuit board (Fig. 3) for the amplifier is made of single-sided foil-coated fiberglass 1.5 mm thick.
SP3-1b is used as a tuning resistor R7 in the amplifier; it can be replaced with SP-4 or a variable resistor, for example, SP3-41. All other resistors are MLT-0.125 W. Oxide capacitor C1 - K56-12 (or K50-35); . C3 - K50-35; capacitor C2 - MBM. Instead of a microcircuit, one made in a rectangular plastic case is suitable. Switch S1 - PKN2-1V, switch S2 - P2K. Telephone capsule - resistance 50...60 Ohm, microphone - carbon, electrodynamic (DEMS), electret (MKE-3). Power source - battery "Krona", "Korund", "Nika".
Now about the setup. The first thing to do is to check that the pins of the DA1 microcircuit are correctly soldered (if you look at it from the side of the legs, then opposite the key-metal protrusion there will be the first leg of the microcircuit and then clockwise - the second, third, etc.). If you are not satisfied with the quality of the connection, you will have to deal with the control panel more thoroughly. You will need an audio frequency generator, an oscilloscope and an avometer. Then we can recommend the following algorithm of actions. Check whether there is a voltage at pin 6 of the microcircuit equal to half the supply voltage. If necessary, set the desired mode by more accurately selecting resistors R2 and R3.
Having connected the oscilloscope first to the microphone and then to the PU output, measure the signal amplitude in each case while talking in front of the microphone. If the signal from the microphone is significantly less than 50 mV, change the microphone. If there are no other microphones at hand, and the signal from this one no longer develops with any selection of R1, try increasing the gain of the op-amp by increasing the resistance of resistor R5 or decreasing R4.
When observing a signal from a microphone using an oscilloscope, many harmonics of different frequencies and amplitudes are visible; it is difficult to determine and measure the true amplitude of the signal. Therefore, it is better to temporarily turn off the microphone and instead apply a sinusoidal signal with a frequency of 1000 Hz from the generator. Using an oscilloscope, measure the signal amplitude at the input (pin C1, left in the diagram) and output (pin 6 of the op-amp) of the amplifier, determine the gain and, if it turns out to be less than 20, select resistors R4 and R5.
Wired communication using an intercom between the rooms of an apartment or house is far from a luxury, but a rather useful invention of our time. To ensure such communication, it is not at all necessary to purchase a home telephone exchange or DECT radiotelephones.
If you need a simple intercom, then making a low-frequency amplifier and laying a couple of thin wires between rooms will not be difficult. In addition, apartment owners who have disconnected from the wired broadcast line can use the “noodles” already installed in the walls of their houses.
So, all that remains is to make an electronic device that will ensure the transmission of sound signals through wires. It is very convenient to use the LM386 chip, a universal low-frequency amplifier, as the basic element of such a device.
This inexpensive microcircuit is widely used and allows, with a minimum of external “piping” elements, to produce a low-frequency amplifier with the required parameters.
The intercom offered to your attention allows you to organize the transmission of a voice message (for example, “Food is served”) or provide a conversation in the so-called simplex mode, when one of the subscribers speaks and the second listens, and vice versa.
A schematic diagram of one of the two subscriber stations is shown in the first figure. In the position of push-button switch SB1 shown in the diagram, the device is de-energized.
When you press the SB1 button and hold it, a supply voltage of 9 V is applied to the circuit and the signal from the electret microphone VM1, amplified by the DA1 microcircuit, is supplied via a pair of wires to the second subscriber station (Fig. 2), the circuit of which is similar to that shown in Fig. 1. Via The normally closed contact of switch SB1 of the second station sends the signal to loudspeaker BA1.
Having completed the transmission of the message or finished the phrase in the conversation, the subscriber of the 1st station releases the SB1 button. If after this the same button is pressed and held by the subscriber of the 2nd station, then the signal from him will be heard by the 1st subscriber.
To power each subscriber station, it is convenient to use small-sized transformer power supplies, which are inserted into an AC outlet or a Krona battery. Current consumption from the power supply occurs only when the SB1 button is pressed, and the quiescent current of the DA1 chip is no more than 10 mA. With an average volume of sound emitted by a loudspeaker having a winding resistance of 8 - 16 Ohms, current. consumed by the amplifier from a 9 V source does not exceed 50 - 70 mA.
The optimal volume of the sound signal is set when setting up the intercom. To do this, select the resistance of resistor R2. At R2=1.2 kOhm, the voltage gain of the low-frequency amplifier is about 50.
By decreasing the resistance of this resistor, you can increase the gain (the maximum value of 200 is obtained when R2 = 0), and by increasing its resistance, you can reduce the gain down to the minimum (20 in the absence of the R2-C3 chain)
Based on materials from the article “Jednoduchy intercom - domaci telefon”, published in the journal Prakticka elektronika.”
This electrical diagram was assembled and tested in laboratory conditions with a communication channel connected to an external telephone exchange. The dial-up virtual number for connecting the circuit was kindly provided by the Mango Office company.
Surely each of us has at least once encountered such a situation when we need to provide two-way communication to two objects located close to each other. For example, two neighboring garden houses or apartments. You can, of course, use a mobile or regular wired phone from your provider, but this, alas, requires costs, since our communication is not free. Moreover, it does not work everywhere and not always. But there is a way out of the situation. For example, assemble it yourself. Duplex means that during a conversation you do not need to press any receive/transmit buttons, that is, the principle of operation is the same as that of a regular landline telephone, only the connection is free! And you can chat with a friend living on your staircase at least for days at a time. Moreover, the length of the wire connecting the intercom can reach 100 meters or more; the length can be increased using a thicker cable. The device consists of two identical amplifiers:
They are located at communication points and are each powered from its own source. The switch supplies power only during calls. The amplifier itself consists of four stages and has a good sensitivity of 2 mV and an output power of 0.5 W with a load resistance of 20 Ohms. With a supply voltage of 12 volts, the circuit consumes 80 mA.
The amplifier is assembled on a printed circuit board made of textolite. The board drawing, specially developed by me for this circuit, can be downloaded (link to the print).
Transistors KT315, GT404, GT402 can be taken with any letter indices. KT315 can be replaced with KT312 or KT306. The trimming resistor for setting up intercoms should be somewhere around 47 or 33 kOhm. Speaker impedance 8-10 ohms. The devices are placed in housings and it is advisable to remove the speaker from the microphone at a distance of more than 20 cm, otherwise you will hear a squeak - an acoustic connection. The device does not require adjustment and works immediately after assembly. The only thing is that the trimmer resistors will have to match the sound quality. In this case, I used the case from an old phone to house one device.
Often in the practice of a novice radio amateur there is a need to assemble a simple wired intercom, say, for summer cottage, so that you can carry on a conversation from the room with those who are in the kitchen, in the bathhouse, utility unit or with neighbors in the country. To solve this problem, two device options are offered - for two and three subscribers.
Each of the intercoms is assembled from available parts, requires virtually no setup and is capable of providing duplex communication over a distance of up to 200 m. In operation, they closely resemble ordinary telephones, since the main part in them is a working handset.
Of course, ideally, it would be nice to use a damaged telephone set with a lever switch on which the handset rests, but if this is not available, any case with a toggle switch installed on it will be quite suitable - it will have to be switched manually.
Before moving on to getting acquainted with the options for the proposed devices, let's consider the operation of a ring signal generator or simply a call generator (GV). Its circuit diagram is shown in Fig. 1.
The generator is an asymmetrical multivibrator made of transistors of different structures. It is connected to the power source and load by three wires through the “Output”, “Common”, and “+” terminals.
The generator frequency is unstable and depends on the supply voltage, load resistance and resistor R2. With the ratings indicated in the diagram, it is in the range of 500...2000 Hz. The sound volume depends on the resistance of resistor R1 - the higher it is, the louder the sound. However, if the resistance is too high (more than 1 kOhm), the generator’s oscillations may fail.
The assembled generator should be checked and adjusted together with the power source (3...12 V battery GB1) and the telephone capsule, which will be used in the real device. The setup consists of selecting resistors R1 and R2 in order to obtain a loud and distinct sound.
Let's tell you more about the operation of the multivibrator. After turning on the power, transistors VT1 and VT2 are closed, since there is zero potential at the base of transistor VT1. Capacitor C1 begins to charge through resistor R2 and a chain of series-connected elements R1.BF1. This process proceeds linearly until the voltage on capacitor C1 exceeds the opening threshold of transistor VT1.
As soon as transistor VT1 starts to open, VT2 also opens. At the “Exit” point positive voltage appears. Through resistor R1, it is added to the voltage on capacitor C1 and supplied to the base of transistor VT1. And that, in turn, opens even more, and opens VT2 even more. An avalanche-like process occurs, leading to the fact that transistors VT1 and VT2 enter saturation, and the full battery voltage is applied to the telephone capsule BF1 through the open transistor VT2.
This state is unstable and will continue while capacitor C1 is recharged through resistor R1. Once the capacitor is recharged, it will not be able to provide sufficient base current to transistor VT1 to maintain saturation mode. VT1 will begin to close, closing VT2 as well. Positive voltage at the “Output” point will decrease, thereby reducing the voltage at the base of VT1 - it closes even more, dragging VT2 with it.
An avalanche-like process occurs again, as a result of which the transistors are completely closed. The base of VT1 is under negative voltage provided by capacitor C1, which acquired it during the recharging process. This voltage is not kept constant, but due to the current through resistor R2 it smoothly goes to zero and then, reaching a positive value sufficient to open VT1, causes a new cycle.
Thus, the multivibrator periodically connects the telephone capsule to the battery, allowing sound to be emitted. It should be noted that the current consumed from the battery is also modulated by the frequency of the generator, and if a second telephone capsule is connected in series with the battery, it will also emit sound.
O. Khovaiko, Moscow.
