Telephone In-Use Indicator
By Bill Bowden
Telephone In-Use Indicator circuit
LED should illuminate when circuit is not connected to phone line, or when phone is off-hool. If LED fails to extinguish, check for 3 to 4 volts across the 680K resistor and check polarity of connections to phone line.
1606 22 August 2007
This circuit is able to handle nine independent telephones (using a single telephone line pair) located at nine different locations.
Audio from a telephone line can be obtained using a transformer and capacitor to isolate the line from external equipment.
The circuit presented here can be used for connecting two telephones in parallel and also as a 2-line intercom.