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.
The base of this circuit is one of the four Norton amplifiers existing in an LM3900 integrated circuit....
This simple CMOS metronome circuit can be mounted in a small box, providing both sound and light...
This circuit of an old American publication uses RF transistors to obtain unit voltage gain, but...