This circuit was found in a 1975 documentation. Current general-purpose transistors can be used, and the power is supplied with relatively high voltage. The gain is 30 dB.
This control for motors with current up to 1.5 A was obtained in a 1989 publication, but it can...
This circuit was found in a 90s Wels' Think Tank. The frequency depends on C2 and R3. The handset...
This circuit is from an old documentation but still current for assembly. Just use BC548 instead...