Prefix-selector for the phone. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Prefix-selector for 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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When operating a telephone set, sometimes it becomes necessary to select the number of ringing signals (rings) coming to the phone. The scheme shown in the figure makes it quite easy to implement the "incoming" selection of call packets from the PBX.

Unlike the annunciator [1], the device is powered by a telephone line and almost 100% suppresses the necessary calls.

The attachment includes:

- input node for receiving and processing call packages to DD1.1, R1...R3, C1, C2, C4, VD1, VD2...VD5;
- counter of the number of calls - DD2;
- key for connecting the phone to the line - DA1, VD13...VD16;
- tube lifting unit on VT1, VT2, R8, R14, VD6, C5, DD1.2,DD1.3;
- batteries 60B-VD17, VD18, R13, C8; 8B-VD19.VD20.R15, C9;
- call selector switch - SA1.

In the initial state, the handset is on, there is no voltage drop on R8, as a result of which VT2 is closed, and the open state of VT1.3 is maintained through the elements DD1.2, DD1 (logical "10" at output 1.2 DD1). The DA1 key is closed, and the VD13...VD16 bridge disconnects the phone from the telephone line. In this state, a voltage of 40 ... 60 V is supplied to the upper output of the telephone set through the circuit VD18, R13, C8, VD17. Suppose switch SA1 is connected to pin 10 of counter DD2. When a call comes in from the PBX, it does not go directly to the phone, but is processed by DD1.1 and sequentially switches the counter DD2.

(click to enlarge)

After the fourth call, a high level from output 10 DD2 through VD21, R7 opens the transistor VT2 and thus through DD1.3, DD1.2, VT1 opens the key DA1. The phone is connected to the line through the open bridge VD13...VD16 and starts ringing from the fifth ringing. At the same time, the input node DD3 is locked through VD1.1.

By changing the position of SA1, you can adjust the number of missed calls. When SA1 is connected to pin 3 of DD2, the phone is permanently connected to the line. The operation of the tube lifting unit is similar to that described in [2]. When off-hook, the voltage drop across R8 opens VT2 and through DD1.3, DD1.2 closes VT1. The voltage from the line through R9, R10, VD8 opens the DA1 key, the VD13 ... VD16 bridge and connects the phone to the line. The process of picking up the handset must be considered in dynamics, since the capacitance C8 is discharged through an open phone quite quickly, and if the DA1 key does not have time to open before the end of the C8 discharge, the phone is not connected to the line. The necessary time for opening DA1 is provided by the VD10, R12, C7 chain, which charges very quickly and discharges in about 1 s. This is usually sufficient to securely connect the telephone to the line. Further, the line voltage itself through the open phone maintains the required voltage on resistor R8, which exceeds 0,8 V, which keeps VT2 open. While dialing the chain VD6, C5, as well as VD10, R12, C7, and VD8, VD9, C6 do not allow you to disconnect the phone from the line.

If the handset is hung up for more than 1 s, the circuit returns to its original state. This device can be used as a "night" phone, when the first 5...6 calls are ignored, and only the subscriber who knows that the handset will be picked up after 6...7 rings will be able to get through.

Another possible case of using a prefix-selector is the establishment of call incoming priorities in the operation of two parallel devices located in different rooms. In this case, one of the devices is connected through a prefix-selector. When a call is received, only one unit rings. If the handset has not been picked up within a certain number of rings, the second device also starts ringing. It turns out a kind of prefix "director-secretary". You can use such a prefix on any type of automatic telephone exchange with a nominal voltage of station batteries of 48 ... 60 V and an invariable polarity when establishing a connection.

In addition to the described set-top box, the authors develop and manufacture paired line adapters, parallel "pirate" connection protection blocks, number splitters, intercity and parallel telephone blockers. All devices are powered by a telephone line.

Literature

1. Evseev A. Prefixes for telephone sets. To help the radio amateur: Collection. Issue 112. - M.: Patriot, 1991.-S.28-36.
2. Drik A., Balakhnichev I. Micro-ATS. Radio amateur.- 1996.- No. 6.-C.20-21.

Authors: A.Drik, I.Balakhnichev; Publication: N. Bolshakov, rf.atnn.ru

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