Two-telephone intercom

Details Two-telephone intercom Two-telephone intercom

1990-09-04

1991-08-13

Schreyer, Stafford D.

Telephonic communications

Private or single line system

Doorbell system

379176, H04M 902, H04M 1902

Patent

active

050402072

ABSTRACT:
A two-telephone intercom has a simple station-call signalling system. In the preferred embodiment, each telephone is provided with its own respective signalling unit comprising (a) a dc voltage source, (b) a voltage-sensitive threshold element (preferably, a zener diode), and (c) a signalling device, e.g., a beeper and/or flashing light. Whenever one of the telephones is activated (i.e., taken off hook), the signalling device associated with the other telephone becomes operative and remains operative until the other telephone is answered (i.e., activated) or until the first telephone is hung up. Three embodiments of the signalling-unit circuits are disclosed.

REFERENCES:
patent: 2855461 (1958-10-01), Bernstein
patent: 4001506 (1977-01-01), Brandstatter
patent: 4096358 (1978-06-01), Bidlack et al.
patent: 4196317 (1980-04-01), Bartelink
patent: 4588859 (1986-05-01), Liberman
patent: 4636585 (1987-01-01), Brandstatter
patent: 4636589 (1987-01-01), Lee
patent: 4821319 (1989-04-01), Middleton et al.

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