Telephonic communications – Multi-line or key substation system with selective switching... – With exclusion or priority feature
Patent
1994-12-14
1996-09-24
Chin, Stephen
Telephonic communications
Multi-line or key substation system with selective switching...
With exclusion or priority feature
379168, 379184, 379380, 379194, H04M 100, H04M 160, H04M 1300, H04M 300
Patent
active
055598730
ABSTRACT:
A simple and inexpensive telephone exclusion or privacy device. A conference feature is and am indicator light showing when the line is in use are also provided. The basic exclusion or privacy circuit includes only eleven components, thereby providing a very low cost device. The circuit includes a diode bridge with a Silicon Controlled Rectifier (SCR) connected across the midpoint of the bridge circuit. A resistor in series with the SCR limits the amount of current which the SCR can draw from the telephone line. A Zener diode in series with a resistor is connected between the anode and the gate of the SCR and a resistor in series with a capacitor is connected from the gate of the SCR to the second side of the bridge circuit i.e. to the point where the cathode of the SCR is connected. The breakdown voltage of the Zener diode is chosen so that at normal out of use line voltage (nominally 48 volts) the SCR will be placed into a conducting state and thus when someone picks up the handset they can access a line which includes the device. However, if a line is in use and someone picks up another handset which includes one of these devices, the line voltage will only be twelve volts and the SCR will not be turned on, thereby presenting the second handset with an open circuit.
REFERENCES:
patent: 3875342 (1975-04-01), Stern
patent: 4243844 (1981-01-01), Waldman
patent: 4332983 (1982-06-01), Vakili
patent: 4941166 (1990-07-01), Waldman et al.
Larson Gaylan I.
Wood Samuel F.
Chin Stephen
Loomis Paul
Notify Corporation
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