GB1458499A - Logic systems - Google Patents

Abstract

1458499 Transistor switching circuits SQUARE D CO 9 Nov 1973 [9 Nov 1972] 51711/72 Heading H3T A logic system comprises an input switch having two transistors TRA, TRB, Fig. 2, with their emitter-collector paths connected in series across a first voltage supply, the emitter of the first being connected to the collector of the second to constitute an output of the switch; an output solid state switch connected in series with a load across a second voltage supply which is higher than the first; and a third transistor TR1 having its base connected to the output of the switch, and its output connected to a control circuit for the solid state switch. In one embodiment, Fig. 2, the solid state switch is a transistor TR3 which is coupled to the transistor TR1 via a further transistor TR2 such that TR3 can only be rendered conductive if TR1 is conductive. In a second embodiment, Fig. 3, the transistor TR1 constitutes an oscillator which is switched on and off according to the state of the input switch (not shown in the Figure). The output solid state switch is a triac TRC1 which is switched to its conductive state by the oscillator output. Both embodiments may include an inhibit input connected to the base circuit of transistor TR1, and also a light-emitting diode LED 1 connected so as to indicate the state of the control circuit signal to the output solid state device.