Intercom. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Intercom. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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Descriptions of intercoms have been repeatedly published in amateur radio literature. However, I offer my version of a simple device for negotiating between two ordinary telephones (SLT), which is easy to repeat for beginner radio amateurs. Communication via a two-wire line is provided at a distance of up to 1 km, if its resistance does not exceed 500 ohms. The devices can be located both in different rooms of the same house, and in different houses or, for example, in the house and at the gate. The device is reliable in operation and allows round-the-clock operation.

The scheme of the intercom is shown in fig. 1. When you pick up the handset of one of the SLTs, a rumble will be heard in it, and a ringing signal will sound on the second SLT. When you pick up the handset and on the second SLT, the rumble and ringing signal stop and you can talk.

Rice. 1. Scheme of intercom (click to enlarge)

Devices can be of any type - TA68, TAN66, TAN70, DD-2240. SLTs without dialers, used in some enterprises for internal communication, will also work. The speech path is symmetrical, so there is no interference from pickups, and with good TA, audibility is good. It is advisable to replace the carbon microphones in them with piezoceramic MPK-101-II-U with a built-in amplifier.

Transformer T1 - unified TAN1-220-50. Its two secondary windings at 28 V are connected in series to obtain an alternating voltage of 56 V, another of the same is used directly. The remaining secondary windings of this transformer are left free. Another network transformer with a power of about 20 W with suitable secondary windings will do.

The voltage from the windings II and III of the transformer T1, rectified by the diode bridge VD1-VD4, is supplied to the voltage regulator on the transistor VT1 and the zener diode VD5. This zener diode and transistor are mounted on heat sinks with a cooling surface area of 50 cm². The output voltage of the stabilizer is 56 V.

When you remove the handset on one of the SLTs, this device receives power through the relay windings K1 or K2 (they do not pass the variable component of the conversational current into the power source), and a voltage of about 6 V is released on the zener diode VD10. Through the resistor R2, the HL2 LED and the emitting diode of the optocoupler U1 current flows, as a result of which the resistance of the photoresistor of the optocoupler decreases sharply and the triac VS1 opens, connecting the winding IV of the transformer T1 with the winding I of the transformer T2.

The voltage from winding II of transformer T2 is used as a ringing voltage. Through separating capacitors C4 and C6, it enters the TA connected to the XS1 socket, and through the capacitors C5 and C7 - to the one connected to the XS2 socket.

When the tube is removed on the second TA, the current will flow through the windings of the second relay and it will work. Since the first relay has already worked (this happened at the time of removing its tube), the closed contacts of both relays will bypass the HL2 LED and the emitting diode of the optocoupler U1. The resistance of the photoresistor of the optocoupler will increase and the triac VS1 will stop opening. The ringing signal will stop. Now you can have a conversation. At the end of it, when the handset of one of the SLTs is hung up, the ringing signal will sound again until the second subscriber is also on-hook.

Relays K1 and K2 - on-load tap-changers (passport R4.535.381D) with two windings with a resistance of 500 ohms each, containing 6200 turns of PEL wire with a diameter of 0,11 mm each. These relays have been redesigned - contacts have been removed from them and, as shown in fig. 2, a reed switch 1 is inserted into the gap between the armature 2 and the magnetic circuit 4 of the coil 3, wrapped with PVC insulating tape. First, the anchor is removed, a recess is made in it for the reed switch and installed back. The position of the reed switch is selected so that it works reliably when the TA tube is removed.

Intercom
Rice. 2. Reworked relay

Transformer T2 is made on the magnetic circuit Ш20х25. Winding I consists of 600 turns of wire PEV-2 0,35, and winding II - 1500 turns of wire PEV-2 0,17.

The intercom is assembled in a plastic case with dimensions of 220x80x150 mm, its weight does not exceed 2 kg.

Sometimes one of the SLTs is installed in such a place that the call always comes from it (for example, at the gate). A full-fledged apparatus here can be replaced by a handset, in which the MPK-101-M-U microphone capsule, the TK-67-UT-I telephone capsule and a switch are installed and connected in series. Capsule MPK-101-II-y is quite sensitive and "catches" a conversation at a distance of 1...1,5 m.

Author: S. Babyn

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