Line security device

Details Line security device Line security device

1975-06-13

1977-08-02

Claffy, Kathleen H.

179 17B, 179 99, H04M 170

Patent

active

040397599

ABSTRACT:
A line security device adapted for use in a conventional telephone instrument including a Zener diode tied by its cathode to the high, or the Tip, lead within the instrument, the anode of the Zener diode controlling, across a resistor, the base of a first NPN-PNP transistor pair which, in the manner of a silicon controlled rectifier, or SCR, controls the all or nothing conduction of a second composite pair. The controlling base of the first pair is tied across a capacitor to the low, or Ring, lead of the instrument and the emitter of the PNP transistor thereof drives the base of the second composite pair which is in circuit with the high, or Tip, signal lead. The Zener diode is selected to break down on high line signal levels corresponding to the instances when the instrument is off the line, e.g., prior to the closure of the hook switch of the instrument, and will not pass the lower line to turn on the first transistor pair once another instrument is connected across the line. The configuration of the second composite pair thus provides an all or nothing switch in the instrument circuit including at its output regenerative feedback to maintain the first pair latched on. To preclude various noise spikes from appearing at the receiver, the receiver circuit includes a PNP transistor emitter connected and base biased to the Tip lead output of the second composite pair and further controlled at the base by the output of the first pair.

REFERENCES:
patent: 2774952 (1956-12-01), Batcheller
patent: 3155776 (1964-11-01), Kano et al.
patent: 3499121 (1970-03-01), McIntosh et al.
patent: 3514544 (1970-05-01), Chambers, Jr.
patent: 3636505 (1972-01-01), Poltonavage
patent: 3725601 (1973-04-01), Jetzt et al.
patent: 3860763 (1975-01-01), Sudoh et al.

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