Telephone Free Indicator

Depending on local
regulations and the telephone company you happen to be connected to, the
voltage on a free telephone line can be anything between 42 and 60
volts. As it happens, that’s sufficient to make a diac conduct and act
like a kind of zener diode maintaining a voltage of 38 V or so. The
current required for this action causes the green high-efficiency LED
in the circuit to light. Line voltages higher than about 50 V may
require R1 to be changed from 10 kΩ to a slightly higher value. When the
receiver is lifted, the line voltage drops to less than 15 V (typically
12 V) causing the diac to block and the LED to go out.

Telephone Free Indicator Circuit

Telephone Free Indicator Circuit Diagram

The circuit diagram indicates + and – with the phone lines. However,
in a number of countries the line polarity is reversed when a call is
established. To make sure the circuit can still function under these
circumstances, a bridge rectifier may be added as indicated by the dashed
outlines. The bridge will make the circuit independent of any polarity
changes on the phone line and may consist of four discrete diodes, say,
1N4002’s or similar. Finally, note that this circuit is not BABT approved for connection to the public switched telephone network (PSTN) in the UK.

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