Phone In Use Indicator

With this circuit
mounted in or near every phone in the house, it will allow users to know
if the phone is being used and not to pick up the phone. When a phone
is taken off hook, the voltage across the tip and ring terminals drops
to 10 volts or less. This will cause the FET
(2N4360) to turn on and also turn on the transistor (2N2222). When the
transistor turns on it will allow current to flow through the LED and make it light. A blinking LED could be used to make the effect better.

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