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 simple circuit can use any general-purpose transistor. The power supply must be isolated. The circuit...
This is a configuration much explored by assemblers, being found on this site in dozens of similar...
The frequency of this oscillator depends on T, which for the audio band can be a common output...