This high input impedance tone control circuit was found in American documentation from the 1980s. Equivalent operational amplifiers and FTEs can be used.
This Rytron transmitter was published in a 1953 book. The circuit can be implemented with a BC558...
Foto-Darlington for applications such as light sensor. In normal applications only the collector and...
This circuit was found in a 90s Popular Electronics. It generates tones given by the oscillators and...