In some cases, there is a need to put the phone on one of two parallel telephone sets and move on to another, to continue the conversation. If you do not use a special device, the conversation will be terminated and the subscriber will have to call back again.
There is a simple diagram of the hold line, which is represented in Fig. 1.4-11. The prefix can be placed in the phone, its principle of operation is as follows. When the tube TA1 put on the apparatus, the button CN hold down. The thyristor VS 1 opens, causing the illumination of the led HL1. In case the receiver of the telephone line is typically held at a voltage of 10-20 V, in turn, open the thyristor VS1, given HL1 and resistance R1 denominated in 2-3 ohms is holding the line at voltages over 30 V. thus disconnection of subscribers does not occur.
After that, when the tube is withdrawn on the phone TA2, the line voltage again drops to 10-20 In that will result in a reduction of the current through the thyristor VS1. If this current is less than the holding current of the thyristor, it closes, translating circuit to its original state. Phone TA2 is operated in the usual manner.
Resistance R1, R2 may have to pick up when the device is used on various types of PBX, but if the physics are clear it does not seem particularly difficult.
Such a device can be equipped with every phone.
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