Patent GB-388869-A (GB)
Title: Improvements in or relating to electrical signalling circuits
IPC: H04Q1/30
Filed: 1931-11-18
Abstract: 388,869. Exchange systems. REID, F., 335, North Circular Road, Neasden, London. Nov. 18, 1931, No. 32040. [Class 40 (iv).] In an electrical signalling system employing direct current of a distinctive character, e.g. polarity or strength, for signalling purposes, the receiving devices responsive to the signals are connected up by alternating current superposed on the direct current throughout the signalling operation. The invention may be applied to remote control or telemetering systems but, as described, controls the lighting of a telephone kiosk from a manual or automatic exchange or the display of a call signal at a police or other telephone box. An alternating current relay is connected across the line or in series with the telephone bell, and when operated by A.C. (of different frequency, say 50 or 100 ~, from ordinary ringing current) connects up a polarized relay which responds to D.C. in one or other direction to connect or disconnect the lamp. The polarized relay may be adapted for connection across the lines or between one line and ground and in the latter case the other line may be used for supervisory signals. Arrangements are made for storing the signal at the transmitting end until a busy line becomes idle and for busying the line when a signal is being transmitted. A signal for calling the police may have preference over an ordinary call. To light lamp LP at the kiosk, the exchange attendant presses key ON to operate E whereupon F pulls up, disconnects the kiosk line from the exchange apparatus and connects up signalling potential which comprises 100 cycle A.C. derived from generator GNR and superposed on D.C. from the exchange battery EB. Chokes C1, C2 exclude the A.C. from exchange circuits. At the kiosk, FR responds to the A.C. and connects up POL which is operated by the D.C. to switch in the lamp LP. Relay POL is of the kind which remains in the position to which it was last set until it receives a pulse of opposite polarity. Key ON is only momentarily pressed, the signalling impulse being determined by the slow release period of F. When released, the key rises sufficiently to open its circuit but remains partially depressed to indicate the operation effected until key OFF is pressed to extinguish lamp LP. Key OFF operates H which connects up signalling potential with the polarity of the D.C. reversed so that POL extinguishes the lamp. Relays F, H ground the third wire P of the telephone circuit to prevent its seizure during signalling,.and busy ground on the third wire short-circuits F, H to prevent signalling during use of the telephone circuit, relay E or G being held by busy ground so that signalling is effected when the circuit becomes idle. If the exchange is manual, E or G is locked up over contacts of the line and cut-off relays. If it is necessary to summon a police officer, it is desirable to connect up the signalling potential even if the telephone line is busy. The potential may be directly connected up over a key or an automatic switch set by dialled impulses seizes the line whether idle or busy and connects signalling potential. The lighting circuit may be controlled by a mercury switch fixed or resiliently connected to the armature of the polarized relay, the switch being locked by the armature of the A.C. or polarized relay respectively. The control of a lighting circuit at the kiosk may be made entirely automatic by replacing the keys ON, OFF by clock-operated switches. When the polarized relay is connected between one line and ground, a relay in series with the lamp may apply an. acknowledging signal to the other line and a delayed alarm may be operated at the exchange if this signal is not received. A permanent loop may also be completed at the kiosk if the polarized relay and lamp circuit relay are not in corresponding condition.