The circuit diagram of 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 inappropriate, 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 electret microphone MKE-3 or an electrodynamic DEMSh 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 distortion of the signal at the output of the op-amp is possible. 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.
The device consists of a pre-amplifier on VT1 and a power amplifier assembled on the K174UN7 IC. Switching between reception and transmission modes occurs using switch S1; only one subscriber has the switch, with which you can connect speakers one by one to the input or output of the amplifier. A simplified diagram of the second version of the intercom is shown in the following figure.
In this case, amplifiers M and switches S are installed in each loudspeaker. Switching from reception to transmission can be done for each subscriber. When switch S is pressed, the loudspeaker is used as a microphone and is connected to the preamplifier input via coupling capacitor C1. The pre-amplifier is assembled on transistor VT1. Variable resistor R1 determines the feedback level and sensitivity at the input of the cascade. From the output of the pre-amplifier, through variable resistor R5, the signal is supplied to a power amplifier assembled on the DA1 chip.
Using resistor R5, the output power is regulated. From the output of the amplifier, the signal enters the line and, through the pressed S2 button, goes to the loudspeaker, which in this case is used for its intended purpose. It is necessary to pay attention to the fact that the line can be either two-wire or single-wire, if grounding is used as the second wire. Grounding can be water pipes, heating pipes, or simply a metal rod driven into the ground.
The intercom is powered from a network nine-volt power source or galvanic elements. A correctly assembled device begins to work immediately; if necessary, you can adjust the sensitivity using resistor R1, and use resistor R5 to adjust the output power. I used miniature loudspeakers in which the step-down transformers were removed, instead of volume controls, buttons S1, 2 of the P2K type without fixing were installed. Author: Valery Ivanov.
The wired intercom is designed for groups of speleologists to communicate with each other and the base camp while exploring deep karst caves.
Extreme conditions during the route: branched passages, lack of daylight, high humidity and low temperatures require precise coordination of the work of search groups.
Radio communication does not work in such conditions.
The communication line is laid with a single-core, grounded, or two-core insulated wire. Signal losses in such a line are very high and measures have been taken in the intercom circuit to reduce them, increasing the output power of the transceiver device and the signal voltage in the line.
To illuminate the workplace when the intercom is connected to the line, an ultra-bright LED is additionally installed in the case, powered together with the amplifier from a single current source.
The calling tone is easy to hear in cramped cave conditions.
The intercom circuit is based on a low-frequency amplifier of the DA1 analog microcircuit type K174 UN14 (Fig. 1), which has a low current consumption and high gain with an output power sufficient for simultaneous connection of several stations to the line.
The intercom is equipped with “Receive-transmit” and “Call” buttons, the flashlight LED is turned on by a separate microswitch. The power battery is located in the speleologist's jacket and is connected to the intercom with a flexible wire; connection to the line is made with two crocodile clips; the line wire is pre-stripped.
Technical data of the intercom:
Supply voltage 4.5-12 Volts.
Output power 1-4.5 Watt.
Gain more than 40 dB.
The signal voltage in the line is 15 Volts.
Current consumption: rest/load 10/50 mA
Reception range more than 2 km
Weight without batteries 60 g.
Electrodynamic head BA-1 type TRI-50 with a resistance of 50 Ohms of small size allows it to be used in reception and transmission modes. The DA2 chip stabilizes the current in the power circuit of the ultra-bright HL3 LED, regardless of the battery supply voltage, the feedback voltage is removed from the divider composed of the HL2 LED and the current-limiting resistor R7 and is supplied to the input (2) of the DA2 chip.
The supply voltage is supplied to the amplifier when the SB2 button is pressed into the transmit mode. In standby mode, the amplifier does not consume current.
The matching transformer at the output of the amplifier transmits the signal to the line in a ratio of 1: 10, and during reception it matches the high resistance of the line with the low resistance of the BA1 head, which eliminates signal loss during reception and transmission.
The HL1 indicator indicates the mode of signal transmission to the line, and the HL2 indicator indicates the presence of a signal in the line during reception and transmission.
The line is galvanically separated from the intercom by transformer T1 and capacitor C9.
Capacitors C7, C8 in the power circuit eliminate self-excitation of the amplifier when the battery is completely discharged.
The call signal appears when circuit SB1.1 - “Call” is closed, capacitor C2 is connected to the output of the amplifier, generating conditions are created with a frequency of 600-1200 Hz, in the initial state the capacitor is grounded and eliminates self-excitation of the amplifier through the power supply circuits.
A filter consisting of capacitors C1, C2, C3 and resistors R2, R3 cuts off the upper frequencies of the audio range, improving speech intelligibility.
The second contacts of the SB1.2 button close the power circuit.
Circuit C6.R6 eliminates self-excitation of the amplifier at low frequencies.
Negative feedback from the output (4) of the amplifier through the separating resistor R4, capacitor C4, is supplied to the inverting input (2) of the DA1 microcircuit, which allows not only to reduce signal distortion, but also to regulate the signal level in the line with resistor R4.
The matching transformer T1 is connected by the primary, low-resistance winding W1 to the separating capacitor C5, and when receiving it is loaded onto the electrodynamic head BA1.
The secondary high-resistance winding W2 is connected to line X3X4 through an isolation transformer T1.
With a single-wire line, the second terminal of transformer T1 is grounded. The HL2 LED allows you to visually determine the signal level in the line and the operating status of the amplifier.
The circuit does not contain scarce and homemade radio components: MLT type resistors, SP capacitors KM, K50-3T, SANA.
An analogue of the K174UN14 microcircuit is TDA 2003, the DA2 stabilizer is 7805.
The author’s version has an electrodynamic head BA1 type TRI-50 with a diameter of 28 mm with a waterproof diffuser with a power of 250 mW; other heads of small dimensions and winding resistance from 30 to 150 Ohms can be used.
Buttons SB1,SB2 - KM 2-1, MTZ switch.
White LED L200CW 8KB-12D or NSPW500BS with a supply voltage of 3.6 volts and a current of 20 mA with a light intensity of 3.6 cd.
The HL1 and HL2 line power indicators are complete
red or green LEDs. It is better to install the power battery from NI-CA batteries or use a flat battery of the KBS type.
The matching transformer is used from a radio broadcast speaker or from a pocket radio.
When a correctly assembled intercom circuit is connected to a power source, it enters operating mode during transmission and a sound signal appears on the line. The line indicator indicates the operation of the amplifier. To check the volume and sound quality, you should temporarily connect a broadcast speaker to pins X3, X4 through a 100 Ohm resistor.
When installing the device in a housing, do not place the input and output circuits of the amplifier close to each other. Connect to the buttons with a short wire, preferably in the screen.
The buttons for switching operating modes of the intercom are conveniently located at the ends of the case, the electrodynamic head is glued to the inner front side of the case, the power and transmission LEDs are attached to the upper part of the case, and the flashlight is attached to the end side.
Ground the input and output circuits at different points on the board.
The photo shows the printed circuit board in expanded form - a version with two channels, one in reserve. The designations of the parts are conventional and differ from the diagram in Figure 1.
Literature:
1. Application of K174 series microcircuits in low-frequency amplifiers. Radio No. 12 1994. p.12.
2. Intercom based on telephone handsets. Radio No. 10 1994 p.20.
3. Sh.A.Myachin. 180 analog microcircuits p.59, 79.
Sometimes situations arise when communication is required between just two apartments, garages, cottages and other objects for various purposes. In this case, using a telephone switch is impractical, so an intercom for two subscribers can solve the problem.
Such systems have a significant limitation in line resistance, reaching 1-2 kOhm. The copper wire used with a diameter of 0.5 mm provides a communication range of several tens to several hundred meters, and when using an amplifier - up to 5-10 km. If you increase the length of the lines or the cross-section of the wire, the inductance will begin to increase and the capacitance of the line will increase, which will cause significant attenuation during signal transmission.
Principle of operation
The main components of intercoms are two remote controls installed at sites and a two-wire communication line connecting these remote controls. Each remote control is a communication device with an amplifier and a dynamic head. The last element can be dual purpose. During the transmission of messages, the dynamic head acts as a microphone, and during reception it is used for its intended purpose - to convert an electrical signal of audio frequency into sound.
In most intercoms, the signal amplified by the head travels from one device to the dynamic head of another via a direct communication line. Due to the low resistance of the head, losses occur in the communication lines: the sound volume begins to fall as the distance increases. Therefore, the operation of these systems is limited by distance depending on the scheme used.
It is quite possible to avoid losses on the line if the output signal from one remote control is supplied not to the dynamic head, but to the amplifier of another device, which has a significantly higher resistance. It is this connection that allows signal reception and transmission to reach several kilometers, without any significant losses. A significant advantage of such intercoms is the ability to be powered from a low voltage source.
Schematic diagram of two-way communication
The circuit diagram proposed for consideration includes two remote controls A1 and A2 and two communication lines connecting the sockets of the XS1 and XS2 remote controls to each other. Since the remote control amplifiers have the same circuits, only one of them will be considered - from device A1.
Transistors VT2, VT3 and VT4 were used for the audio amplifier. The negative feedback voltage is supplied from the collector VT4 to the base VT2 through resistor R8. Feedback helps stabilize the operating mode of the transistors and the cascade gain. Its action reduces sound distortion.
When the transmit-receive switch SB1 is in the closed position, the input signal from the communication line is supplied to the emitter circuit VT2 through C1. Thanks to the small capacitance of capacitor C1, the characteristics of the head are equalized for use as a microphone. Capacitor C2 protects the amplifier input from high-frequency interference, and resistor R2 maintains the component of the emitter current VT2 at a constant value.
The cascade on VT1 is an electronic switch that supplies voltage to the first stage of the amplifier. This key is located in the load circuit (R3) of transistor VT2. In the indicated diagram, switches SB1 are in standby mode, and in the closed position. At this time, the devices' current consumption from the power source is very small. Therefore, the remote controls do not require separate power switches.
After pressing the SB1 button, the BA1 dynamic head is connected to the amplifier input. In this case, the wire connected to the XS2 socket will be connected to the amplifier output. Next, from G1, power is supplied through R10 to the input of the amplifier of the second device along its line. Transistor VT1 opens in the second remote control, supply voltage is supplied to VT2 and the amplifier of the second remote control is turned on. At the same time, the amplifier is also turned on in the first remote control due to the opening of the transistor VT1 by the current flowing through the dynamic head BA1 along the base circuit. During a conversation, the voltage in front of the head increases, which is generated in its voice coil and enters the communication line through capacitor C5. Next, the signal weakened in the communication line is amplified, after which it goes to the dynamic head.
Thus, when you press the SB1 button, both remote controls are turned on simultaneously. However, in the transmitting apparatus, the amplifier acts as a microphone and its current consumption is only about 3.5 mA. In receiving equipment, it performs its direct function, consuming approximately 100 mA at the loudest sound. The conversation between subscribers is carried out one by one. The button is pressed after receiving the message and released when the transmission is complete.
The intercom circuit is simplified by the absence of a volume control. Therefore, in order to prevent significant sound distortion, certain rules must be followed. If the line is short, up to 2 km, then you should talk quietly, keeping a distance of 40-50 cm from the remote control. In the case when the devices are located at a maximum distance of 5-10 km, it is recommended to talk loudly, keeping a distance of 10-20 cm from the remote control.
Intercom installation
To mount amplifier parts, one-sided foil-coated PCB is used as a board. The installation itself can be performed not only by printed methods, but also by a hinged method, when special copper pins are attached to the pins of the parts on the board.
The remote control body is made of steel, 0.5 mm thick. The board is attached to the rear wall of the case so that the switch button protrudes outward.
In order to finally install the intercom for two subscribers, it is necessary to determine the location of the sockets XS1 and XS2. Instead, you can use a small-sized connector from a tape recorder. All that remains is to secure the dynamic head, install the power source and check the functionality of the device.
Intercom - diagram
The personal computer has long and deeply penetrated our everyday life, becoming as necessary a device as a television. Many have already replaced their PC more than once or upgraded it. Among other things, once purchased inexpensive computer “speakers” (active speakers), at some stage ceased to meet the user’s requirements or were replaced by more expensive ones with the sound quality of a good music center.
The same story happened with my Genius SP-E120 “speakers” - they went to the back shelf, but came in handy when I needed to make a simple intercom for one subscriber.
Figure 1 shows a diagram of the Genius SP-E120 active speaker system.
As can be seen from the figure, the diagram is quite consistent with the price of this equipment. Weak ULF on a chip TEA2025, switched on according to a standard scheme, a passive volume control, and two speakers, one of which (SP1) is in the main unit, and the second in the additional one. There, in the main block, there is a board with ULF and a transformer power supply. The additional block is almost empty - there is only the SP2 speaker.
The sound quality is very mediocre, but for working as an intercom - an intercom, it is more than satisfactory.
The idea is to leave the main unit indoors and take the additional unit outside. Also add a couple of electret microphones and a blocking switch to prevent self-excitation due to acoustic feedback.
The intercom circuit diagram is shown in Figure 2.
The external unit is the second speaker (which is empty), in addition you need to install an M1 electret microphone. Connection to the main unit via a cable in which one core is shielded (which carries the signal from the microphone).
S1 - “receive/transmit” switch. There is only one, and it is located only in the main block. The diagram shows it in the “reception” position, that is, when we are listening to a guest.
Power is supplied to microphone M1 through resistor R1. Since the signal level at the output of the electret microphone was not enough to supply an active speaker to the ULF input, there is an additional ULF on transistor VT1.
And so, the guest says, the signal from the microphone via cable goes first to the ULF on VT1, and then to the left channel of the speaker. From the output of the left speaker channel through S1.2, the amplified signal is supplied to the speaker SP1, located in the main unit. And we hear the guest.
To answer the guest you need to press S1. Now our speaker SP1 turns off, but speaker SP2, located in the external unit, turns on.
The signal from microphone M2 goes to the preliminary ULF on VT2 and from it to the input of the right channel of the active speaker. From the output of the right speaker channel, through S1.1, the amplified signal goes to speaker SP2, located in the external unit.
The guest hears what we tell him.
No ringing device is provided here, since there was already a regular apartment bell, and before starting a conversation the guest presses its button.
I want to admit, I initially planned to make a duplex system, since the amplifier is stereophonic, but, alas, the acoustic feedback was overcome only by the S1 switch, which turned the system into a simplex one. But that's not bad either.
I’m sure a similar device can be made from any inexpensive and no longer needed active speaker for a personal computer.
