Calling an extension phone. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony
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It often happens that several telephone sets are connected to one telephone line, and the ringing signal arrives at the same time on all devices. Therefore, it is impossible to determine which subscriber is being called. The proposed device allows you to transfer a call to an extension telephone with a simple push of a button.
The scheme for connecting telephone sets ТА1, ТА2 and calling devices (subscriber sets) A1, A2 to the telephone line is shown in Fig. 1.
Several devices and subscriber sets can be connected to a telephone line. The scheme from [1] was taken as the basis of the device. which is greatly simplified.
Diagram of parallel phone call device is shown in Fig.2.
(click to enlarge)
In standby mode, the device practically does not consume current from the telephone line, since the BF1 piezoelectric emitter connected to the line has an infinitely high resistance to direct current. The telephone line voltage is usually 60 V. It has a certain polarity, which must be taken into account when connecting the subscriber set to the line.
When a call signal is received, the subscriber picks up the handset, current begins to flow in the telephone line, and the voltage in the line drops to 10 ... 20 V (depending on the parameters of the telephone set and on its distance from the telephone exchange). To forward a call to another subscriber, you need to press the SB1 "Call" button. At the same time, through the closing contacts of the button, voltage is supplied to the parametric voltage regulator R1-VD1, and from it to the call generator on the DD1 chip and transistors VT1, VT2. The first generator on the logic elements DD1.1, DD1.2 generates pulses with a frequency of about 20 Hz, and the second generator on the elements DD1.3, DD1.4 - pulses with a frequency of about 2 kHz. A modulated two-frequency signal is applied to the base of the transistor VT1.
Transistors VT1 and VT2 periodically close the telephone line. thus forming ringing voltage pulses, and the other subscriber (or several) hears a sound signal in the emitters BF1, which are equipped with telephone sets. By the agreed number of signals given or by their nature, other subscribers will understand who should pick up the phone. The selection of resistors R2. R3, you can change the frequency of the ringing signal, which will distinguish subscribers from each other. When talking on the phone, weak unintelligible speech can be heard from the emitters, which serves as a signal that the telephone line is busy. At the time of making a call to the bridged subscriber, the calling subscriber hears a call forwarding tone on his handset.
Details. Transistors KT503E can be replaced with KT604A, KT605A. KT940A and others, designed for an allowable emitter-collector voltage of at least 80 V. Button SB1 - type KM1-1. Piezo emitter ZP-1 can be changed to ZP-2. ZL-3. ZP-18, ZP-19.
The device is assembled in a plastic case, made in the form of a stand for a telephone set. Most of the elements of the device are located on the printed circuit board. The call button is located on the side surface of the case, there is also a piezo emitter.
The connection of such a device to the telephone line must be made in accordance with the polarity, which is a disadvantage of this scheme. Figure 3 shows how to include a bridge rectifier in a subscriber set in order to connect the device to a telephone line in arbitrary polarity.
As a bridge diode assembly VD1, you can use the assembly KTS405. KTS409, KTS417 with any letters, as well as imported RS201. RS202. DB101. DB102. BR305, BR31.
This device can be used to call parallel subscribers and in the absence of an incoming call from the city telephone exchange. It is only necessary to agree on the number of ringing signals for different subscribers.
Literature
- P. Bialiatsky. Parallel phone call device. Radio amateur, 1997. No. 7, p.15.
Author: A. Evseev, Tula.
See other articles Section Telephony.
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