Telephone In-Use Indicator
Telephone circuits

Telephone In-Use Indicator

By Bill Bowden


Circuit to illuminate an LED when any phone extension is off-hook. LED should illuminate when circuit is off-hook.

Telephone In-Use Indicator circuit
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.

1811 22 August 2007

See also:

Telephone Audio Interface
Audio from a telephone line can be obtained using a transformer and capacitor to isolate the line from external equipment.
Telephone Ring Generator Using Small Power Transformer
This ring generator will ring a telephone once every 10 seconds.
Telephone Ring Generator Using Switching Supply
The telephone ring generator shown below generates the needed high voltage from a simple switching mode power supply.

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