A unit for causing an auxiliary telephone to ring if a main telephone is not answered

摘要

959,641. Automatic exchange systems. TELEFONAKTIEBOLAGET L.M. ERICSSON. Dec. 12, 1962 [Dec. 12, 1961]. No. 47017/62. Heading H4K. In a unit for connecting an extension telephone to a line if a call is not answered at the main telephone, a capacitor is charged by rectified ringing current and is discharged through mechanically latching relays which connect the extension set to the line at the end of the second burst of ringing current, an electromagnetic device being incorporated to release the latched relays. As shown, rectifier L is connected across the bell of main telephone set A so that on receipt of ringing current relay R1 pulls in and capacitor C is charged. At the end of the burst of ringing current R1 releases and C discharges through relay R2 provided with mechanical latch h2 and connects relay R3 to C. Circuit values are selected so that R3 cannot operate until C has been recharged by the second burst of ringing current. R3 has a mechanical lath h3 and connects extension set B to line La, Lb. When the hand-set of A is next lifted relay R4 operates, releasing the latches h2, h3 of R2, R3, a break contact of R2 then shorting out R4 so that it does not present a high impedance to speech currents. Diode L1 is incorporated in the charging circuit of C to prevent C discharging through R1 at the end of the bursts of ringing current.