Found in a 1964 Philips manual, this circuit uses only passive components. The relay is a high voltage sensitive type.
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...