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.
1430 22 August 2007
The circuit presented here can be used for connecting two telephones in parallel and also as a 2-line intercom.
This ring generator will ring a telephone once every 10 seconds.
Audio from a telephone line can be obtained using a transformer and capacitor to isolate the line from external equipment.