Adapter for AVU. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Telephony

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With a shortage of telephone lines, high-frequency multiplexing equipment (AVU) is sometimes used. Unlike a blocker, which disconnects one of the subscribers from the line while another one occupies it, AVU allows two subscribers to use one line simultaneously and independently of each other. The proposed device allows you to connect to such lines modern telephone sets with push-button dialing, faxes, caller IDs.

When using the AVU system, the telephone set (TA) of one of the subscribers is connected to the line in the usual way, and its owner, as a rule, does not know about the existence of the second channel, which is implemented due to frequency division multiplexing. In the room where the second subscriber is located, the HF-A block is located. and at the telephone exchange, this line is connected to the HF-C unit. The signals of the device of the second subscriber are transmitted at a carrier frequency of 28 kHz using amplitude modulation (AM), and the signals of the station - at a frequency of 64 kHz. The HF-A unit is connected to the line through an isolation transformer. It is powered by a 220 V network. It is possible to connect to the "+B" and "-B" terminals of a backup battery of eight galvanic cells 373.

Block HF-A. which was developed for devices with a mechanical dialer, has at least two significant drawbacks: low output voltage (only 10 V) and the need for a three-wire connection of the SLT (L1 - signal, L2 - common, Zv - separate call output). Because of this, various problems arise. The fact is that modern TAs with an electronic dialer, as a rule, do not have a separate call output. Often they cannot work at a line voltage of 10 V (the standard for domestic lines is 60 V, for foreign lines it is 45 V). As a result, connecting two or more SLTs to the block is very difficult. In addition, the full-fledged work of modern TA. cordless telephones and devices that are now common, such as modems, fax machines. AONs, answering machines, etc. are generally impossible.

To eliminate these shortcomings and limitations, without interfering with standard equipment, the adapter, the diagram of which is shown in Fig. 1. The main components of this device are a bipolar stabilized power supply ± 15 V. a passive low-pass filter, a Schmitt trigger with a powerful output and an executive relay. The green LED HL1 is designed to indicate the on state of the adapter, and the red HL2 glows when the SLT is off-hook or when the line is occupied by some other device.

(click to enlarge)

If the TA tube is laid on the lever, a voltage of -3 V is supplied to the inverting input of the comparator DA5 through resistors R8 - R15 (in relation to the common wire, for which the contact of the X4 connector is conditionally taken). This voltage is below the threshold set by the trimmer R9 on the non-inverting input DA3. therefore, the output transistor of the comparator is closed, the HL2 indicator is off, relay K1 is de-energized. The SLT is connected to the ringing output of the HF-A block through the relay contacts and is ready to receive a call signal.

When the handset is lifted, a direct current flows through the conversational node TA. The resistance of the conversational node is approximately an order of magnitude less than the resistance of the resistor R5. As a result, the voltage at the inverting input of the comparator changes polarity to positive, the output transistor of the comparator opens, the HL2 LED lights up. relay K1 is activated. With its contacts, the relay disconnects the bell output from the TA and connects the signal output of the HF-A block to it. This is a line seizing mode that allows you to dial a number, carry on a conversation, exchange data by fax or modem.

Thus, the three-wire line of the HF-A unit is converted into a standard two-wire line, to which, as usual, one or more SLTs or other devices can be connected. The order of the signals remains the same. as in lines without AVU.

In the line occupation mode of the HF-A unit, its power supply and one of the stabilized adapter sources are connected in series with respect to the SLT. Therefore, the line voltage for the SLT is 25 V. Practice has shown that this is enough for the full operation of both various SLTs and the devices listed above. Such a low voltage in combination with a low ringing signal voltage (about 50 V) makes it possible to use without risk any imported SLTs that do not have special adaptation to domestic telephone networks.

LPF R6C3R7C4R8C5 attenuates dialing and ringing pulses tenfold. This is necessary so that in the standby mode the ringing signal does not lead to the operation of the relay, and in the line seizure mode, the dialing pulses do not lead to its release. Trimmer resistor R9 allows you to set a threshold that will provide a clear separation of these two states. Positive feedback through resistor R11. converting the DA3 comparator into a Schmitt trigger. reduces its sensitivity to low-pass filter output voltage ripple, preventing false positives.

Resistors R3 and R4 are designed to reliably start stabilizers (as a rule, they are not needed when using imported microcircuits).

The printed circuit board of the adapter is shown in fig. 2. It is made of one-sided foil fiberglass with a thickness of 1.5 mm. The secondary winding of the transformer T1 must provide a voltage of 15 V at a current of 0,3 A. Relay K1 - RES-15. passport RS4.591.004. It can be replaced by a relay of the same type with passport RS4.591.001. but at the same time, instead of the resistor R10, you need to install a jumper.

(click to enlarge)

When assembling the adapter, it should be noted that stabilizer microcircuits are found with different pinouts. For imported analogues of these microcircuits (78L15 - DA1 and 79L15 - DA2), the pinouts correspond to those indicated in fig. 1.

Resistor R9 - SPZ-386. the rest - ML T. Capacitors - K50-35. KM and K73-17. All resistors and capacitors can be derated within ±20%.

The adapter board is placed in any suitably sized case, which is located close to the RF-A unit. The total length of the wires connecting the unit with the adapter and the adapter with the SLT must not exceed 25 m.

Set up the adapter in two steps. First, make sure it works. After connecting the power, check the voltage at the inputs and outputs of the stabilizers. Then, setting the engine trimmer R9 in the middle position, close the contacts X1 and X2. In this case, the relay should work, and the HL2 LED should light up.

At the second stage of the settings, the response threshold is selected. The adapter is connected to the HF-A unit and the TA is connected. First, a rough selection is made. If the HL2 LED does not light up when the TA tube is removed, turn the R9 slider counterclockwise (down the diagram) until HL2 starts to light up reliably (the relay makes a weak click). If the HL2 LED does not go out when the tube is lowered onto the TA lever, turn the R9 slider clockwise (up in the diagram) until HL2 starts to go out approximately 0,5 s after the tube is lowered (the relay also makes a weak click).

Then, an accurate selection of the threshold is made. If numbers such as 8, 9, 0 are not dialed, and after that a beep appears in the handset, turn the R9 slider counterclockwise, checking the reliability of dialing the number "0". If the HL2 LED lights up during a call and clicks the relay, the R9 slider is slightly rotated clockwise until this effect disappears.

Author: A.Brazhnikov, Penza

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