Active three-tone filter (CB20808E)

This circuit was found in an American documentation and can be added to common amplifiers of any type. The transistor can be any general purpose.

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7MHz Transmitter (CB11873E)
This Rytron transmitter was published in a 1953 book. The circuit can be implemented with a BC558...
Phototransistor 2N5777 (CB16044E)
Foto-Darlington for applications such as light sensor. In normal applications only the collector and...
Two-tone generator (CB8446E)
This circuit was found in a 90s Popular Electronics. It generates tones given by the oscillators and...