Phone in use indicator 5 Electronic Circuit

From CircuitsArchive

 >>-----------------+------+---->> phone line
                    |      |
                    |      o
                    |       /
                    |      /   momentary switch
                    |      |
                    |      /
                    |      \   1200 Ohm
                    |      /
                  -----____|
                   / \
                  /___\  SCR
                    |
                    /
                    \  600 Ohm
                    /
                    |     ^^
                  -----  //
                   / \  //
                  /___\    LED
                    |
                    |
 >>-----------------+--------->> phone line
 

Sources

• Terri Barber barber[at]beowulf.ucsd.edu