Ring detection circuit for a telephone answering device

Claim 1

For use in a tape cassette or like telephone answering device in which metallic components that are electrically connected to the device electrical ground are accessible to the user, thereby requiring dc isolation of the telephone line from the device power source and electrical ground, a ring responsive circuit comprising: a relay having a coil and a set of contacts, an audio transformer having a primary and a secondary, the primary being connected in series with said relay contacts across the telephone line, said transformer being able to couple audio from the telephone line, a neon bulb and circuit connected in parallel with said relay contacts so as to capacitively connect the transformer primary across the telephone line when said relay contacts are open, said neon bulb turning on for a portion of each cycle of the ring signal, conduction of said neon bulb resulting in the production of fast rise time transients having high frequency components which are effectively coupled by said transformer, a source of power, a transistor, a first capacitor connected to be charged from said source of power via said transistor, the secondary of said transformer being effectively connected to the base of said transistor so that the coupled high frequency components momentarily turn on said transistor so as to cause concomitant charging of said first capacitor from said power source, and bistable switching means, latched in response to the charging of said first capacitor, for connecting said source of power to electrical components of said telephone answering device, including said relay coil, the resultant closure of said relay contacts directly connecting said transformer primary to the telephone line to seize the same, audio thereafter being coupled between said telephone line and said answering device via said transformer.