Patent GB-932774-A (GB)
Title: Improvements in or relating to mono-stable switch circuits
IPC: H04Q3/00
Filed: 1959-11-18
Abstract: 932,774. Automatic exchange systems. TELEFONAKTIEBOLAGET L. M. ERICSSON. Nov. 18, 1959 [Nov. 24, 1958], No. 39178/59. Class 40 (4). [Also in Group XL (c)]. In a monostable-state circuit (see Group XL (c) comprising two reciprocally coupled transistors, one of which is normally conducting and the other non-conducting, the application of a positive input pulse to a first terminal A causes the normally non-conducting transistor to conduct and deliver an output pulse at a second terminal B, the transistor thereafter returning to its non-conducting state unless a negative marking potential is applied to a third terminal C in which case the normally non-conducting transistor remains conducting indefinitely and a relay M1 in the circuit is energized. The Figure illustrates an arrangement for effecting a cyclical search in a telephone system and comprising a plurality of monostable-state circuits U1-U4 as described above. Thus on the application of an initiating pulse to the terminal A of circuit U1 a pulse is developed at terminal B which is applied to terminal A of circuit U2 and so on through the chain until circuit U4 is reached. In addition, a plurality of circuits X1- X4, each associated with a corresponding one of the circuits U1-U4, is provided for selective connection to a common circuit Y, the circuits ready for connection being indicated by the application of a negative potential of the corresponding terminal C. Thus on switching on, a positive pulse is applied to circuit U1 and the circuits U1 &c. start stepping forward until one is reached at which the terminal C has a negative potential, whereupon the stepping ceases, the corresponding relay M1 is operated and relay contacts 3, 4 are closed to connect the circuit Y to the corresponding one of the connection circuits X1-X4. This continues until such time as connection between Y and the corresponding one of the circuits X1-X4 is no longer required whereupon the potential of terminal C is changed and the circuit recommences stepping as before. As shown, the output B of circuit U4 is connected back to terminal A of circuit U1 so that a continuous cyclic operation is maintained. Once per cycle a negative pulse from U4 is applied to the base of transistor T11 to render the transistor conductive so that capacitor C12 can discharge through rectifier L11. When T11 switches off, C12 can charge once more, the charging current rendering transistor T12 conductive and hence permitting capacitor C13 to charge. Provided that pulses appear regularly at the base of T11 the charge will increase over several cycles until relay M11 operates. This closes contacts connecting the relay winding to a positive supply through contacts actuated by the relays M1 in the circuits U1-U4. Hence provided the cyclic operation continues relay M11 remains actuated by the pulses applied to transistor T11; alternatively, when stepping ceases due to a terminal C being marked, the relay M11 remains actuated due to the actuation of the corresponding relay M1. However, should the supply of pulses from the terminal B of circuit U4 otherwise fail, the relay M11 will release and contacts 15, 16 thereof will close to supply a fresh initiating pulse to terminal A of circuit U1.