Telephone Line Tester

The telephone-line tester consists of nothing more than a meter (thats 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. When the circuit is connected to the telephone line, a meter reading of 5 to 10 V (when S1 is pressed) indicates that the line is okay.

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Telephone Line Tester - Circuitos de Electronica