GB1394148A - Push-button telphone subset - Google Patents

Abstract

1394148 Substation equipment INTERNATIONAL STANDARD ELECTRIC CORP 6 Sept 1973 [11 Sept 1972] 41881/73 Heading H4K A push button impulse transmitter, which utilizes a latching relay for producing loop disconnect pulses under the control of an electronic decoding circuit, is powered by a capacitor that is charged over the line via the impulsing contact when this and the hook switch contacts are closed. The capacitor is connected in series with the shunt combination of a resistor and a diode, the latter being reverse biased during charging and forward biased during discharging, i.e. during dialling loop interrupts, so as to present a low internal impedance to the load. An appropriately biased transistor may replace the diode. With hook switch contacts Gs1, Gs2 closed, the line loop is maintained via contact S1 in the push button key block. When any key is operated, S1 opens so as to transfer the line loop to the Zener diode stabilized diode bridge D1-D4. If impulsing contact IC is not closed at this time, transistor amplifier T7, T8 is effective to energize impulsing relay IR so that a charging circuit for storage capacitor C5 is made. A network consisting of capacitor CR, resistor RR and diode DR produces a resetting pulse for a MOST logic decoding circuit M and clock C is started. The operation of the push button also actuates contact S2 which causes a contact-bounce blanking network CB, RB3 and transistor amplifier T3, T4 to bring-up offnormal relay ONR. The latter short circuits the speech network at its contact ONC. About 800 msec. after the depression of the button, the logic finishes decoding the digit value and via amplifier T5, T6 causes impulsing relay IR to generate the appropriate impulse train. During breaks in the train, capacitor CS in its discharge path via diode DS provides the requisite power for the circuit components.