Current source and off-hook detector
US4742538A · kind A · utility
Assignee
Inventor
Key dates
| Filing date | Jan 29, 1987 |
| Grant date | May 3, 1988 |
| Priority date | — |
| Expiry date | Jan 29, 2007 |
Classification
- Technology area (CPC H)Electricity
- CPC primaryH04M3/54
- WIPO fieldTelecommunications
- WIPO sectorElectrical engineering
Abstract
The invention relates to circuitry for providing and monitoring the status of telephones, especially those used in conjunction with advanced telephone answering machines, such as is also disclosed herein. The voltage on the extension line leading to the telephone set (termination device) is monitored to determine whether the telephone set is on-hook or off-hook. Also, a tone decoder monitors DTMF signals on the extension line. The telephone set, when off-hook, is normally connected to the subscriber line. However, if a predetermined digit is keyed into the telephone set, the telephone set will be disconnected from the subscriber telephone line and connected to a selectively actuable current source. The telephone set, being powered by the current source and being disconnected from the subscriber line, can then be used for the control and monitoring of the telephone answering machine. Also, the current source is protected against the application of the subscriber line voltage.
Source: USPTO / EPO open patent data. Objective bibliographic and citation counts.