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.
1578 22 August 2007
The telephone ring generator shown below generates the needed high voltage from a simple switching mode power supply.
Audio from a telephone line can be obtained using a transformer and capacitor to isolate the line from external equipment.