This circuit was found I a 1965 Radio Schack documentation. The transistors are from that time, admitting equivalents. The transformer has 500 ohm primary and secondary depending on the speaker.
The operational amplifier for this circuit, found in a 1979 publication, may be the 741....
With a strong positive feedback (via a 10 M resistor) we can get the trigger setting for the...
Found in American documentation from the 90s, this CMOS circuit detects the source of a signal and can...