Patent GB-551228-A (GB)
Title: Improvements in or relating to selective electrical circuits
IPC: H04B3/34
Filed: 1941-11-18
Abstract: 551,228. Selective control. HADFIELD, B. M. Nov. 18, 1941, No. 14798. [Class 40 (i)] [Also in Groups XXXIX and XL] In a selective circuit in which a number of inputs control corresponding outputs, the inputs and outputs are connected respectively to common points which are connected by an impedance or resistance of such a predetermined value in relation to the impedances or resistances of the output circuits that when one input predominates to a pre-determined extent, the other inputs are ineffective in their output circuits. The invention is applicable to arrangements in which the inputs are derived from a common source over different channels or from different sources over the same or different channels, in circumstances which gives rise to unwanted currents or voltages in some inputs, for example, in frequency selection by filter circuits including differentiation between speech and signal currents, in analyzing light and other radiant energy, or in detecting a moving object by a number of detectors so that it can be ascertained when the object is within pre-determined range of a particular detector. The basic circuit for two D.C. sources is shown in Fig. 1. Source E<SP>1</SP> has an output resistance R<SP>1</SP> the voltage across which operates a load RL<SP>1</SP> and source E<SP>2</SP> is connected to similar circuit elements. Similar poles of E<SP>1</SP> and E<SP>2</SP>, are connected together and by means of resistance r<SP>1</SP>, to the connecting point of R<SP>1</SP>, R2. Assuming that E<SP>1</SP> preponderates, the resistance of the circuits may be arranged so that the voltage across r<SP>1</SP>, equals and opposes E<SP>2</SP>, at a pre-determined ratio E<SP>1</SP>/E<SP>2</SP> so that there is no output across R<SP>2</SP>. When the ratio is exceeded, the voltage across R<SP>2</SP> is of opposite polarity to that due to E <SP>2</SP> alone, and in cases where RL <SP>2</SP> is not polarized its operation under these circumstances may be prevented by a series or shunt rectifier. The circuit may be extended to include more than two inputs and outputs, Fig. 2 (not shown), and dynamic response is increased . by a capacity C<SP>1</SP> in parallel with the resistance r<SP>1</SP>. Although the circuit is suitable for simple A.C. inputs, rectification is generally necessary and smoothing is effected by condenser (C<SP>1</SP> may serve this purpose). Half or full wave, voltage doubling or bridge rectifiers may be used. When the load is inductive, smoothing may be effected thereby. In the arrangement shown in Fig. 11, relays RL<SP>1</SP>, RL<SP>2</SP> are required to respond to different proportions of different colours present in source L<SP>5</SP> by using cells PC<SP>1</SP>, PC<SP>2</SP> having predominating responses to such colours. The outputs of the cells are connected to valves VA<SP>1</SP>, VA<SP>2</SP> operating as cathode followers to provide inputs E<SP>1</SP>, E<SP>2</SP> for the circuits described with reference to Fig. 1. In the frequency selective circuit shown in Fig. 12, tuned circuits K<SP>1</SP>, L<SP>1</SP>, K<SP>2</SP>, L<SP>2</SP> responding to frequencies F<SP>1</SP>, F<SP>2</SP> transmitted over a common line TL provide the inputs E<SP>1</SP>, E<SP>2</SP> which are rectified at M<SP>1</SP>, M<SP>2</SP>, the outputs across resistances R<SP>1</SP>, R<SP>2</SP> being smoothed by R<SP>3</SP>, C<SP>3</SP> and R<SP>4</SP>, C4.