Load-switching relay circuit esp. for hospital-ward calling lamps

摘要

Neutral and phase lines (1, 2) of the mains supply a half-wave rectifying circuit (3-6) charging an electrolytic smoothing capacitor (7), e.g to 15 V. The relay (11) is energised by each operation of a push-button (10) isolated from the mains by a transformer (21).The prim. winding (20) normally presents a light load to an oscillatory circuit (16) so that a low-pass filter (18, 19) allows another rectifying circuit (17) to apply high-level voltage to the clock input (C) of a flip-flop (15) driving the relay-switching transistor (14). The pushbutton when pressed shorts out the sec.-side AC and lowers the flip-flop input voltage.