Telephone Line In Use Indicator 2

This circuit functions as a line-current sensor and can be connected in series with either of the phone lines. For the circuit to indicate an in-use status for all phones on a single line, it must be connected in series with the phone line before, or ahead of all phones on the line. Since the power for the circuit is supplied by the phone company, a circuit could be added to each phone as an off-hook indicator.

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