Telephone line splitter. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Telephone line splitter. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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I offer a device that allows you to connect an additional subscriber to the telephone line.

To communicate with him, the method of redialing the number is used, i.e. after dialing the number and hearing the call control tone, you need to hang up and immediately redial the number.

In addition, the device contains additional blocks that allow you to take into account the talk time and control long distance calls.

(click to enlarge)

The device consists of three functionally completed blocks: a basic one and two service ones - a call time recording unit and a long distance access analysis unit.

Consider the operation of the base unit (Fig. 1).

When an inductor call is received, a pulse voltage with an amplitude of 1 V and a frequency of 120 Hz appears at the output of the diode bridge VD25 (point A). Pulse duration - 1s, pause - 4s. Comparator D1.1 converts this voltage into a series of rectangular pulses of the MOS IC level.

From the output D1.1, pulses are fed to single vibrators D2.1, 02.2. Single vibrator D2.1 has a time constant of 5 s, i.e. the period of inductive sendings. Single vibrator D2.2 converts pulses from the output of the comparator into a single pulse with a duration of 1 s.

In the incoming call waiting mode, both subscribers are connected to the telephone line through public keys DA1...DA4. Opens the keys logic level "1" coming from the output of the element D1.2 through the transistor VT2 and diodes VD13, VD14.

When the first inductor message arrives, the single vibrator D2.1 is triggered, as a result of which a logic level "1" appears at its output, and a logic "1.2" appears at the output of the inverter D0. Level "1" through the diodes VD2, VD3 enters the inputs R triggers D6.1, D6.2, resetting them to zero and thereby disconnecting both subscribers from the telephone line. At this moment, a determination is made to which subscriber - the main or additional - the call is addressed.

If the call goes to the main subscriber, the inductor sending from the output of the one-shot D2.2 through the delay circuit D1.3, D1.4 and the public key VT1 goes to the counter D4. After counting five pulses, the counter is self-blocking at input V. From output 5 (pin 1) D4, the logical "1" through the diode VD8 goes to input S (pin 14) of trigger D6.1, sets it to "1" and thereby opens the keys DA1 , DA2 by connecting to the telephone line of the main subscriber. Simultaneously with the output of the inverter D3.1, the level of logical "0" breaks the pulse at the output D2.1.

When the main subscriber goes off-hook, current begins to flow in the telephone circuit. At the same time, a voltage of about 11 V drops across the resistor R8 relative to the common wire, which corresponds to the passport data of the level of logical "1" MOS IC. Resistors R11, R12 do not have any effect on the operation of the phone, and their value can vary within a small range to select the voltage at which the logic is clearly triggered. The voltage from the resistor R11 is supplied through the diodes VD15, VD16 to the input S D6.1 and the input R D6.2. As a result, the main subscriber is constantly connected to the telephone line, and the additional subscriber is disconnected. In principle, the VD15 diode can be excluded from the circuit, because the input SD6.1 receives the level of logic "1" from the output 5 D4. For more reliable blocking of the telephone set of an additional subscriber, the keys VT2 ... VT4 are included in the scheme. Key VT2 turns off the output of the inverter D1.2 when the handset is picked up, and key VT3 additionally "plants" resistor R12 to the ground.

At the end of the conversation by the main subscriber, when he hangs up on the telephone, the drop in the level of logical "1" on the resistor R11 triggers the pulse shaper on element D3.4. Logic level "1" from the output of the shaper resets the counters D4, D5, preparing the device for the next call. Counter D5 does not affect the operation of the circuit when calling the main subscriber. It is necessary to control the VT1 key and save the result of counting the first series of inductor parcels when dialing to an additional subscriber. Since one pulse comes from the output of the one-shot D2.1 to the input C D5 when dialing to the main subscriber, then there is a level "1" on the basis of the transistor VT1, and the transistor is open.

Incoming call mode of additional subscriber. The call to an additional party is selected by redialing the number. At the same time, without waiting for the arrival of the fifth ringback control signal, it is necessary to press the end call lever. As in the first case, both subscribers are disconnected to determine the destination of the call. Counter D4 will have time to count 2...4 pulses. After redialing the number, the very first inductor message switches the counter D5 to the state corresponding to two counted pulses, the key VT1 closes without passing pulses to the counting input D4. 5 seconds after pressing the release lever, the inputs of element D3.2 will have logical "0" and "1" levels. At the output of this element, a logical level "1" appears, which starts the one-shot D2.3, which turns on the additional subscriber's telephone for 1 ... 2 minutes.

To prevent simultaneous receipt of logical "1" levels at the inputs of trigger D6.2 from the outputs of one-shot D2.1, D2.3, the key on the transistor VT9, when picking up the handset by any subscriber, "puts" the D2.1 output through resistor R19 to ground. If within 1 ... 2 min the tube was not removed, the decay of the positive pulse from the output of the one-shot D2.3 starts the reset pulse shaper D3.3. If the additional subscriber picked up the phone, the RS-flip-flops are blocked by the logic level "1" from the resistor R12: D6.1 - in the state of logic "0", and D6.2 - "1".

After the end of the conversation, the decline of the positive pulse starts the reset pulse shaper D3.3.

Outgoing call to the main or additional subscribers. When the subscriber picks up the handset, the logic level "1" from the resistor R11 or R12 is set to the appropriate states of the triggers D6.1, D6.2. Blocking the telephone set of another subscriber occurs as described above. For clearer operation of VT1 nodes; D4, D5 set the delay circuit for a time of about 10ms.

The logical level "1" coming from the resistors R11, R12, you can start the clock for recording the talk time of both the additional and the main subscribers.

Authors: A.Kizlyuk, .Ilyin; Publication: N. Bolshakov, rf.atnn.ru

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