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Digital Call control and call transfer circuit for telephone extension line

The same line phone and an extension is quite common. Small commercial transfer switches and a simple phone call is used to implement the call-control call functions. The author describes publications based on recent electronic test equipment a few units, found that both need to re-cable, and the function is too simple.
This article describes the application of microcontroller and telephone line carrier digital encoding and decoding technology, a host on the same telephone line and extension and a transfer of the call (see Figure 1), using the telephone keypad to achieve call transfer between the main station, from distance restrictions, do not change and external pull-up connection, installation is extremely simple.
Main features:
1. With the line calls ringing and transfer the same line. When there is an outside line into the main, ext simultaneous ringing, pick up any phone can be with the outside line. If the other person is not looking for the side of the phone can be used to transfer call control to transfer calls. Call transfer device number is set to host # # 1, extension number is set to transfer call control # # 2. Call Transfer, simply click on the corresponding number on the telephone keypad to hear called the main (sub) machine can transfer calls hang up after ringing device. At this time 60 seconds outside line will remain connected, if called main (sub) machine is not answered within 60 seconds, then release an outside line, automatically hang up. Called the main (sub) mechanism of respiratory control ring, the outside of the calling and called parties hear the sound transfer. Called the main (sub) machine hook, press any number key on the keyboard phone transfer call control to terminate the ring, you can talk with the outside. Answer in the transfer process, the Lord, each extension can be transferred between the answer an outside line.
2. Ext fax. Sometimes, a phone and then have a fax machine, need to receive faxes during a call, you can also press the Lord, call transfer between extensions method calls to the fax machine.
3. Group call feature. In the telephone keypad, press # # 0, all transferred calls ring simultaneously, to achieve the main, extension of the group call feature.
4. Other. The call transfer can be combined with any type of telephone used in parallel, without affecting its normal use and Caller ID.
Circuit (see Figure 2):
Call Forwarding with the phone line is controlled by microcomputer, telephone interface, DTMF decoder, and so on. One phone line LINE, TEL, and then a telephone, D1 ~ D4 constitute a telephone line polarity conversion circuit, R0, Q2, R2, Q1, R1 constitute Abstract Electrical Road, R6, R4, C1, Q3, R5 constitute Phone line and ring back to the circuit, ZD1, R7, R8, ZQ2 constitute the same line telephone hook detection circuit, R12, Q4, SPK constitute a ring generation circuit, C7, C8, R13, U2, C9, R18, C6 etc. DTMF decoding circuit, U1 is SCM, R11, LED indicating circuit for the switch to call.
How it works:
Call control with the line call-3V battery installed, U1 to work. When the phone is not the same line and hook, the phone line polarity power supply by D1 ~ D4 ZD1 transform to the positive terminal, the voltage of 48V or so, ZD1 breakdown conduction, HOOK-DET is high, ZD2 protection U1, U1 to sleep, when power consumption is small.
When the telephone line and hook with the work, E-side voltage to 24V the following, ZD1 not breakdown, HOOK-DET is low, wake U1, so that in normal working condition, PWR output low, the U2 to work. When the same line phone type DTMF code, press the "0 to 9 ,*,#" key, the DTMF signal on the line by C6, C7, C8, R13, R14, R15, R16, R17 to U2, by U2 decoding, U1 read data, and with their numbers compared to the state, if K1 dial to "1", and enter the number for # # 1, enters the same line Call Transfer Call Forwarding state, such as the input is the other number, the same line telephone does not respond; if K1 dial to "2", the processing 2, the response "# # 2 ";"## 0" as the group calls, call transfer all online receives call this number were to turn. Response to a call, HOOK-CTL output high to turn on Q2, Q1 also will be conducting telephone line is off hook state, while D0 through R6, C1, Q3, R5 line out to the ring back tone, TONEQ by R12 , Q4, SPK issued a ringing, LED light. At this point the same telephone line can hold on for the others to another telephone extension on the same line to answer calls. When another telephone hook with the line, and press the "*" key, U1 turn call out the state received signal in order to let the normal call, that is, low output D0 HOOK-CTL stop back tones, TONEQ stop hair tones.
To avoid the prolonged occupation of the telephone line, telephone line switch with the call is in turn call the state, 60 seconds, still no one to answer the call will automatically hang up closing line, U1 to sleep.

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Digital Call control and call transfer circuit for telephone extension line