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.
This circuit was found in a 90s Wels' Think Tank. The frequency depends on C2 and R3. The handset...
The circuit shown is from the 1994 Linear Applications Handbook by National Semiconductor. The...
This zero-crossing switch circuit triggers an SCR when the input voltage goes through zero. The...