PURPOSE: To automatically release on-hook holding and to remove the invalid holding of a line by adding an on-hook holding timer, an alarm oscillation circuit and a timer output circuit to a telephone set.
CONSTITUTION: When a hook switch 1 turned from off-hook holding to on-hook, an on-hook holding timer 9 starts to be driven by a hook switch state detecting part 5 and an off-hook holding control part 8. Even when the on-hook holding state is continued, the output terminal O1 of the timer 9 is turned to 'H' after the passage of T1min, an alarm oscillation circuit 10 is driven and an alarm sound is generated through a loudspeaking circuit 14. The output terminal O1 is turned to 'L' after the passage of T2sec, the alarm sound is stopped, and after the passage of T3sec, a pulse is generated on the output terminal O2 of the timer 9 and the output of a holding button detecting part 6 is turned to 'L' by a 1st output of a timer output circuit 11. Simultaneously, the output of a loudspeaking button detecting part 7 is turned to 'L' by a 2nd output of the circuit 11. Thereby, the circuit 14 and a holding circuit part 16 are turned to a waiting state released at its operation through a state control part 12.
YAMAGUCHI TAKESHI
NAGATA ATSUSHI
NEC SHIZUOKA LTD