LED to indicate when telephone is engaged - has LED and transistor circuit in series with line to indicate telephone or extension usage on small plans

摘要

The LED indicator is for use in telephone instruments. It is particularly for parallel connected instruments, to indicate which is in use of which exchange line is in use, or for boss/secretary units, Plan 7, to indicate who is usin the exchange line. The LED is lit by the line current which is in excess of 10 mA and less than 100 mA. The circuit comprises an LED (L) connected in correct polarity to be lit by any line current flowing when the telephone is in use. A zener diode (D) is connected across in opposite polarity to prevent back biassing the LED, as overvoltage protection in conjunction with series resistors (R1, R2) in series with the LED (L), and to provide continuity in the event of LED failure. A transistor (T) is connected between the series resistors (R1, R2) the LED and the line continuation to limit the current through the LED to the required 20 mA. The circuit causes little voltage drop in the telephone loop circuit and little speech impairment.