Posted on Nov 23, 2012 12397
Here is a simple method of series connection into the telephone 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.