telephone line use indicator by AikyaReddy1



TelepHone line in-Use indicaTor
   T.K. Hareendran
                                                             Fig. 1 shows the circuit of the
                                                         telephone line indicator. When the

        his circuit gives a visual indica-               telephone is not in use (the handset is                 s.c. dwiv
        tion of whether a telephone line                 on hook), the line voltage is 48-60V.
        is in use or not, helping two or                 This voltage passes through the sim-
more people sharing the line to avoid                    ple polarity guard built around the
interrupting each other’s calls. As the                  bridge rectifier comprising diodes D1      the line is not in use.
circuit connects in parallel with the                    through D4 and fed to the remaining             When the telephone line is in use
telephone line, it can also detect phone                 circuit. Zener diode ZD1 breaks down       (the handset is off-hook), the line volt-
tapping/misuse attempts. The circuit                     and transistor T1 conducts. As a result,   age drops to a level below the zener
draws negligible current from the tel-                   transistor T2 does not conduct and         voltage of ZD1. Transistor T1 cuts
ephone line for its operation.                           LED1 does not glow, indicating that        off and transistor T2 conducts. As
                                                                                                                               a result, LED1
                                                                                                                               lights up to in-
                                                                                                                               dicate a busy/in
                                                                                                                               the circuit on a
                                                                                                    Fig. 2: Pin configurations general-purpose
                                                                                                    of MPSA42 and LED
                                                                                                                               PCB and en-
                                                                                                                               close in a suit-
                                                                                                    able cabinet. Refer Fig. 2 for the pin
                                                                                                    configuration of transistor MPSA42
Fig. 1: Telephone line indicator circuit
                                                                                                    and LED. 

1 3 2 • M ay 2 0 1 1 • e l e c t ro n i c s f o r yo u                                                                  w w w. e f y M ag . co M

To top