This circuit sounds when the input voltage drops below the value determined by the zener. The circuit is from a 90s American documentation.
An interesting option is the one that makes use of the MC3373 integrated circuit and which is...
We found this circuit in a Radio Electronics from February 1970. The frequency of 1,400 kHz in the...
This power step drains just 3.5 mA from the battery and can provide good playback on a 4-ohm...