Telephone in-use indicator

  
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.
Telephone in-use indicator  - schematic

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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