When the voltage supplying the circuit falls below the zener value, the LED lights up, and if a low current buzzer is used, it is activated. The circuit is from a 1999 American publication.
This circuit was found in a 1970s General Electric documentation. Transistors are uncommon today....
This circuit, from a component manual, also applies to other components in the same family. The...
The sensor of this circuit is a common transistor and the trigger point is set on the trimpot or...