Voltage level detector circuit - has switching transistor connected to reference voltage from zener and input voltage to be monitored

摘要

A fail-safe voltage level detector circuit, for monitoring, for example, the cyclically varying voltage supplied to a flashing signal lamp, has a d.c. supply and includes a main switching transistor (TR2) the base of which is connected to a reference voltage, and the input to which is derived from the voltage to be monitored through an integrating capacitor (C1) and an input transistor (TR1) having a common emitter connection to the main switching transistor. The reference voltage is established by a potential divider which includes a zener diode (ZD) and two resistive networks (R1,R4,TR3 and R1,R4,TR4) which are connected to the zener diode upon switching on and off respectively of the switching transistor, to determine two different threshold voltages for voltages for the switching on and off of the transistor. A complete switching cycle of the transistor occurs only when the input signal to the detector circuit swings through a voltage range spanning upper and lower predetermined voltage levels, thereby providing correct voltage modulation of the signalling lamp.