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Single User JFET Series Telephone Connection

2016-02-12 20:17  
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A telephone line or telephone circuit (or just line or circuit within the industry) is a single-user circuit on a telephone communication system. Typically this refers to the physical wire or other signaling medium connecting the user's telephone apparatus to the telecommunications network, and usually also implies a single telephone number for billing purposes reserved for that user.Here is a simple method of series connection into thetelephone set suitable for PABX or short line applications. When the telephone handset is lifted, Cl is charged via D1 to (Vz1 -0.7) V and DF320  power on reset occurs. When the first keyed digit is recognized, Ml goes to logic 1, muting the telephone network by switching on the low on resistance JFET  Ql, and maximizing the line-loop current for impulsing. Impulsing occurs through DP switching Q2, and hence Q3 turns off. Rapid discharge of Cl through Zl is prevented during line break by blocking diode Dl.When dialing is complete, the circuit returns to the static standby condition, and Ql is switched off. The circuit reset, during a line interruption by the cradle switch, is for the parallel connection mode.