We found this circuit in an American 1980s documentation. The circuit uses a piezoelectric transducer unusual in our day. The transistor is general purpose type.
If the input voltage falls below 12 V the LED glows up. Other voltages can be monitored by...
The diagram shows a tone control with the LM381 IC. The circuit is from a Radio Electronics from...
In other circuits we have seen a circuit for this application whose calculations have been...