GB1098477A - Voltage indicating devices - Google Patents

Abstract

1,098,477. Keyed oscillators. ELECTROLOID Ltd. June 7, 1966 [June 15, 1965], No. 25194/65. Heading H3T. [Also in Divisions G1 and H2] A circuit arrangement for indicating when the voltage applied to terminals 50, 51 falls to a predetermined value comprises a Schmitt trigger circuit which includes transistors 56, 60 so arranged that with normal voltage at the terminals 50, 51 transistor 60 is conductive and transistor 56 non-conductive. When the voltage at the terminals 50, 51 falls to the predetermined low value, the voltage at the slider of a potentiometer 68 is insufficient to maintain transistor 60 conductive, thus causing the transistor 56 to become conductive. Conduction of this transistor causes the potential across resistor 55 to change, the change being fed over a capacitor 55 to a loud speaker coil 76 and a winding of a transformer 65. The voltage induced in the other transformer winding is fed, by way of a capacitor 64, to the base of the transistor 56 thereby setting up an oscillatory circuit the oscillation frequency being determined by capacitor 64, resistor 62 and, to some extent, by the tapped winding of the transformer 65. Furthermore, the voltage across this winding, rectified and smoothed by diode 74 and capacitor 73, acts to hold transistor 60 non-conductive in the event of a voltage rise from the predetermined value. In order to reset the circuit it must be disconnected from the applied voltage.