The circuit shown was found in an American documentation from 1964. The circuit uses transistors of the time and the indicators are small lamps.
This circuit was found in an American audio documentation from 1968. The configuration can be...
This circuit was found in 1980s American documentation. The transducer is a piezoelectric capsule and the...
This circuit is from an English magazine from 1976. By replacing the transistor for a BC548, the...