Telephone Line Tester Circuit

  
The telephone-line tester consists of nothing more than a meter (that`s used to measure line voltage in the on- and off-hook state), three resistors (one of which is variable), a pushbutton switch, and a modular telephone connector.
Telephone Line Tester Circuit - schematic

When the circuit is connected to the telephone line, a meter reading of 5 to 10 V (when SI is pressed) indicates that the line is okay.




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