This circuit detects changes in the active cycle of the input signal. The circuit was obtained in a 1966 EEE. Transistors are general purpose types of the time.
This sound relay or Vox was found in 70s RCA documentation. SCRs can be common types today and...
This two-door doorbell was found in a 1990s US publication. The circuit is still current as...
This sophisticated circuit using a liquid crystal display was found in Radio Electronics from the...