Refinement of the calling device of the phone. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Refinement of the calling device of the phone. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Telephony

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The issue of reworking the calling device of telephones with improvised means for working on the blocker is being considered.

It just so happened that domestic telephone communications chose a way to save on their own users. And, despite the increase in tariffs for long-distance telephone conversations, the introduction of per-minute payment for local calls, the invention of various additional tariffs for fax machines and caller ID, the quality of communication with you has not improved. And for many (especially residents of large cities) the phone is connected according to a paired scheme, or as it is called through a blocker. In the future, it is about them that will be discussed.

So, to begin with, let's consider the principle of operation of a conventional line without a blocker. A line is approaching the subscriber, on which, in the "standby" mode, a voltage of 60V is maintained at a current strength of about 40 mA. When ringing, the voltage fluctuates from 60V to 120V with a frequency of about 25 hertz. It is because of this that the pendulum swings in the mechanical bells of ordinary telephones. This system is already old, but no one is going to change it, and there is no point in this.

Everything would have been fine, but in order to save (in this case twice) on the number of wires, domestic telephone communications put blockers into operation, which affected the catastrophic inconvenience of system users. These devices are widely used. The difference from the usual line is that the line from the PBX is also suitable for users, but one for two. The polarity of the supply voltage in which it changes with a certain frequency, and when using the phone or calling, remains in the polarity that will correspond to this subscriber.

In order to ensure call selectivity and exclude the possibility of eavesdropping, subscribers are connected according to the following scheme:

Refinement of the phone ringer, connection diagram on paired lines

Refinement of the calling device of the phone. Blocker circuit

(click to enlarge)

Diagram of a caller for Panasonic phones

The ringing signal voltage when connecting the phone in a paired circuit is lowered, and it is not enough to turn on the microcircuit. You can fix this if you reduce the resistance of the resistor R1 to 3,3k and the capacitance of the capacitor C2 to 4,7uF. I've been using this mod for a long time now. So far, no customers have complained. At the same time, the quality characteristics of the phones do not deteriorate.

Publication: cxem.net

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