GB881274A - Improvements in or relating to electrical apparatus for indicating the flow of an electric current in a telephone line - Google Patents

Abstract

881,274. Telephone systems. GENERAL ELECTRIC CO. Ltd. March 24, 1960 [April 10, 1959], No. 12203/59. Class 40 (4). An electric lamp 4, Fig. 1, connected in the collector circuit of a transistor 5 is lit when current flows in the line wire 1 of a telephone line. The lamp may be used to give a visual indication of a telephone call, or of a " busy " condition where two telephone instruments share the same line. A battery 6 supplies the collector current to operate lamp 4. In the absence of current flow in wire 1 the base 10 and emitter electrode 7 are at the same voltage, and the collector path is of high impedance so that insufficient current is supplied by battery 6 to light lamp 4. If current flows in the direction of arrow 3 an emitter/base voltage is developed across resistance 9 and collector current lights lamp 4. The apparatus may be made independent of the direction of flow of line current by inserting a full wave rectifier in series with the line wire 1 and the base/ emitter circuit of the transistor (Fig. 2, not shown). In another arrangement (Fig. 3, not shown), a single lamp responds to each direction of current flow by using two transistors 5 and resistors 9, one for each direction of current flow. Each embodiment responds to flow of alternating current operating a telephone bell, the visual indication being useful when a subscriber has a number of telephone instruments on his desk.