This circuit provides a steady logical "high" indication when the telephone is off its hook. Operation is as follows:  During the off-hook condition, DC telephone line voltage flows through clamping diodes CR1 and CR2, which develop about 1.5 volts across resistor R1. This voltage is provided to the input of optical coupler U1 via current-limiting resistor R2, turning on U1's internal light-emitting diode. Under these conditions, U1's NPN output transistor turns on, providing the +5 to +15 volt logic "high" level at its emitter, pin 4. Pin 4 of the coupler may be connected to subsequent indicator circuitry, such as a transistor-controlled lamp or audible device.

If a logical "low" level is desired for off-hook indication, then R3 may be placed between pin 5 and the +5 to +15 volt supply, and pin 4 grounded. In this case, the output is taken from pin 5. For either configuration, note that a square wave will be present at the circuit's output during ringing.

In the off-hook condition, DC loop current, supplied by the telephone company central office, flows through the telephone. The loop operates in a constant current mode, that is, it will maintain a certain DC current, usually about 20 milliamperes, to the telephone. The current mode allows more than one phone, such as multiple extensions, to be placed in series without a reduction of the energy in the circuit. The loop's capability to deliver the desired current (compliance) is limited by its maximum voltage, usually about 90 volts AC for ringing and 48 VDC for off-hook conditions (dialing and speech).

Another method to indicate the off hook condition for one or more telephones connected to a single line is described in the article "Phone In-Line Use Indicators."


Under no circumstances may a user connect this or any other non-approved device to a telephone line without obtaining specific, prior permission from their telephone company.

Under no circumstances may any external voltage be connected to telephone lines. The optical coupler in this circuit provides galvanic isolation between the telephone line and user-defined circuitry to satisfy this requirement. The fuse provides fire protection in the event of a voltage surge from lightning or other abnormal conditions. Do not defeat either of these provisions.

January 27, 2000

Source document dated February 17, 1991

Text and images ©2000 by Arthur Harrison

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